Attached vs. Detached ADUs in California: Shared Walls, Fire Separation, Entrances, Height, Energy and Coverage Rules

Attached or detached is the first real fork in an ADU project, and it is not mainly a style choice. The two configurations live under different fire rules, different energy rules, different height floors, and different construction realities — an attached ADU shares a rated wall, a roof joint, and the house’s energy classification as an “addition,” while a detached ADU stands on its own footings, digs its own trenches, and answers the full new-construction code. This guide works through both, rule by rule, and it is honest about the questions state law does not answer — privacy screening, resale premiums, and yard division among them — because a made-up rule is worse than none.

One finding frames everything below: state ADU law and the HCD ADU Handbook contain almost no attached-vs-detached construction rules. Grepping the March 2026 Handbook: “shared wall” 0 hits, “sound” 0, “screening” 0, “alley” 0, “resale” 0. What ADU law contributes is the definitions, the size and height floors, and the limits on what a city may demand. The assemblies themselves live in Title 24 — the California Residential Code (CRC), the California Building Code (CBC), and the California Energy Code. Citations below give the 2025 CRC numbering first: permits filed on or after January 1, 2026 are reviewed under the 2025 codes. This page is the Attached and Detached chapter of our California Accessory Dwelling Unit Handbook, and it builds on our detached ADU and attached ADU pages and our guide to ADU types.

The Two Configurations, Defined

What “attached” legally means

An attached ADU is a newly created dwelling structure that structurally abuts and connects to an existing or proposed primary dwelling or accessory structure, and extends beyond that structure’s building plane (HCD Handbook, citing Gov. Code section 66314(d)(3)). Its size is capped by statute: the total floor area of an attached ADU on a lot with an existing primary dwelling shall not exceed 50 percent of the existing primary dwelling (section 66314(d)(4)). Handbook examples: a unit on top of an attached garage, a new second story, a newly constructed basement unit. Everything in the attached half of this guide — the shared wall, the foundation interface, the roof tie-in — follows from that one word: abuts.

What “detached” legally means

A detached ADU is a separate structure. State law even guarantees one by category: section 66323(a)(2) entitles a lot with a single-family dwelling to one detached, newly constructed ADU of up to 800 sq ft of livable space at side and rear setbacks of no more than 4 feet, at the height floors described below. A detached unit shares nothing structural with the house — which removes every interface problem in the attached half of this guide, and replaces them with a full new foundation, full-length utility trenches, and newly-constructed-building energy compliance (Energy Code section 150.1, including the solar PV rule), where an attached ADU is treated as an addition (section 150.2(a)). The budget comparison between the two configurations is priced in our ADU cost guide.

The Shared Wall (Topic 377)

The wall an attached ADU shares with the house is not a partition — it is a dwelling-unit separation assembly, and its governing rule is the fire rule at CRC R302.3, covered in full in the next section: a fire-resistance-rated assembly running continuously from the foundation to the underside of the roof sheathing. Three construction consequences follow:

• Continuity is part of the rating. A vertical separation must extend to and be tight against the foundation, a complying horizontal assembly, the underside of the roof sheathing, or — under an uninhabitable attic — a ceiling of at least ⅞-inch Type X gypsum board with an attic draft stop above the assembly line and the ceiling’s supporting framing protected by at least ½-inch gypsum board (R302.3.3). Whatever supports the rated assembly must carry an equal or greater rating (R302.3.4).
• Penetrations are part of the wall. Pipes, ducts, and boxes through the assembly must be installed as tested or protected by an approved firestop system rated no less than the assembly (R302.4); unlisted steel electrical boxes are capped at 16 sq in. each and 100 sq in. per 100 sq ft of wall, with opposite-side boxes separated by at least 24 inches or otherwise protected (R302.4.2). Fireblocking is required at the line of dwelling-unit separation (R302.11). Pitfall: the shared wall rarely fails on its gypsum — it fails on back-to-back outlet boxes, recessed lights, and medicine cabinets punched through it.
• One wall is enough. The double-wall and 2-hour common-wall regimes in the code are townhouse rules (R302.2). A house plus attached ADU is separated by a single rated assembly under R302.3 — there is no CRC rule requiring the shared wall to be double-stud, staggered-stud, or masonry. Those are sound and structure choices, not fire-code mandates.

Fire Separation (Topic 382)

Between the two homes: CRC R302.3 (2025 structure)

Dwelling units in a two-family dwelling must be separated by fire-resistance-rated assemblies “regardless of whether a lot line exists between two dwelling units” (R302.3). The 2025 code organizes the rule into subsections:

• The rating (R302.3.2): 1 hour, or ½ hour where the building is equipped throughout with an automatic sprinkler system installed per CRC section R309. Ratings are per ASTM E119 / UL 263 or the CBC section 703.2.2 analytical method.
• Stacked units (R302.3.5, new in 2025): where one unit sits above the other and sprinklers per R309 are not provided in both units, the separation assemblies — including an interior stairway serving as the upper unit’s means of egress — must limit the transfer of smoke, and each unit must have a notification appliance connected to the smoke alarms in the other unit. Note the test is sprinklers in both units, a different question from R302.3.2’s “equipped throughout.”
• Shared accessory rooms (R302.3.6, new in 2025): a garage or storage room both units use must be separated from each unit per Table R302.3.6 — at least ½-inch gypsum board on the room side of walls, ⅞-inch Type X where habitable rooms sit above or below, no opening directly into a sleeping room, and other doors 1⅜-inch solid or 20-minute, self-closing.

Whether a given house + attached ADU is enforced as a two-family dwelling under R302.3 is a plan-check determination — confirm the code path (CRC vs CBC) at intake; California jurisdictions do apply the 1-hour separation to attached ADUs (Placer County’s official ADU FAQ states it in those terms). One warning: national content circulates an “exception” under which an ADU inside an existing house needs no separation if smoke alarms are interconnected. That exception is a Washington State amendment. It is not in the California code. Do not design to it here.

Across the yard: exterior walls by distance (R302.1)

For a detached ADU, fire separation distance is measured from the building face to the closest interior lot line, to a street/alley/public-way centerline, or to an imaginary line between two buildings on the same lot — placed so the existing dwelling also complies. The gap between house and detached ADU is therefore split by that line, and each facing wall is judged at half the gap. The 2025 tables (R302.1(1) unsprinklered / R302.1(2) sprinklered throughout per R309):

At the state-law 4-foot setback (section 66314(d)(7)), that puts an unsprinklered ADU’s lot-line wall in the 1-hour + 25%-openings band, and a sprinklered one in the unrated + unlimited-openings band — the single most useful fire-design fact in ADU planning. Two cautions: the tables’ exception for “accessory buildings” of a dwelling does not exempt a detached ADU, which is itself a dwelling unit, not an accessory building; and a subdivision-wide footnote allowing unrated walls at 0 ft in fully sprinklered subdivisions is not a single-lot tool.

The garage that stays a garage (R302.5 / R302.6)

Where a garage remains next to either unit: no opening from the garage directly into a sleeping room; other garage-to-dwelling doors must be 1⅜-inch solid wood, solid- or honeycomb-core steel, or 20-minute fire doors, self-latching and equipped with a self-closing or automatic-closing device (R302.5.1, 2025 text); ducts through the separation at least No. 26 gage sheet steel with no openings into the garage (R302.5.2); and Table R302.6 separation — ½-inch gypsum on the garage side of walls, ⅞-inch Type X under dwelling space above, including the row for a garage sitting less than 3 feet from a dwelling unit on the same lot.

Sprinklers themselves

The ADU never triggers them on its own: “Accessory dwelling units shall not be required to provide fire sprinklers if they are not required for the primary residence,” and building an ADU never triggers sprinklers in the existing home (Gov. Code section 66314(d)(12); the same rule for section 66323 units at section 66323(d)). Our fire sprinkler guide covers the trigger analysis. For conversions, section 66323(a)(1)(C) separately requires the converted unit’s side and rear setbacks to be “sufficient for fire and safety” — the phrase that lets fire review reach a zero-setback conversion.

Sound Isolation (Topic 381)

California’s residential code sets no sound rating for the wall or floor between a house and its attached ADU. A full-text search of the 2025 CRC finds STC/IIC only in its sound appendix — Appendix BG (renumbered from Appendix AK) — whose own header states it is “not mandatory unless specifically adopted by a state agency or referenced in the adopting ordinance.” Where adopted, BG asks for STC 45 (field NNIC 42) and IIC 45 (field NISR 42). The STC 50 / IIC 50 rule people quote (45 on field tests) is CBC section 1206, and it binds buildings permitted under the Building Code — for example, an ADU that is a unit inside a Group R multifamily building. Some cities also enforce sound standards locally (Los Angeles’s LAMC section 91.1206 reaches dwellings on its face), so confirm at intake. The practical answer for a typical house + attached ADU: the code-required assembly between the units is the fire separation above; choose a tested assembly whose published STC you can point to in the Gypsum Association manual or a lab report. Fire and sound are different tests — a 1-hour listing is not automatically an STC 50 assembly, and no honest builder quotes an STC figure for a wall build-up without a tested assembly behind it. Our design guide covers the same ground from the design side.

Where the New Foundation Meets the Old (Topic 378)

Here is a verified negative worth knowing before your plans are drawn: no code section prescribes how a new footing must meet an existing one. Dowel schedules, epoxy embedment, step-footing geometry, and pour-together-vs-separate decisions appear nowhere in the CRC or CBC as prescriptive figures — the interface is engineered, project by project. What the code does fix is everything around the junction:

• The new footing must be supported on undisturbed natural soils or engineered fill and transmit its loads to the soil (CRC R403.1) — a footing bearing half on undisturbed soil and half on loose trench backfill is the classic differential-settlement seam.
• Exterior footings sit not less than 12 inches below the undisturbed ground surface (R403.1.4). Many older Southern California homes have footings shallower than that; expose the existing footing’s depth before the new one is drawn.
• Where a geotechnical investigation is required, excavation adjacent to existing foundations is a required investigation topic, with underpinning and protection plans (CBC section 1803.5.7), and the soils report must address adjacent-structure loads and settlement (section 1803.6).
• Adjoining property — on a single lot, that includes your own house — must be protected from damage during construction, expressly including footings and foundations, with water runoff and erosion controlled (CBC section 3307.1).
• Worker-safety law states the hardest-edged underpinning trigger: where excavation endangers an adjoining structure, shoring, bracing, or underpinning shall be provided, and excavation below the base of an existing footing is not permitted except with a support system, in stable rock, or with a registered engineer’s approval that no hazard exists (Title 8, section 1541(i)). Underpinning is therefore not always required for an attached ADU — the trigger is depth relative to the existing footing, and endangerment — but digging below your own footing’s base without a support plan is precisely the prohibited case.
• Neighbor-facing trenches carry their own duties: Civil Code section 832 entitles each owner to lateral and subjacent support, requires reasonable notice stating the intended depth and start date, and gives the owner of an endangered building at least 30 days to protect their foundations where the excavation goes deeper than those foundations. The base building code’s excavation notice is 10 days (CBC section 3307.1); Los Angeles requires 30 days in the deeper-and-closer case and will not issue the permit until notice is evidenced (LAMC 91.3307.1; LADBS Information Bulletin P/BC 2026-060). Publish neither number without its jurisdiction.

Structural Load Transfer (Topic 380)

California’s rule is one sentence long and it governs every attached-ADU structural decision: construction must provide “a complete load path that meets the requirements for the transfer of loads from their point of origin through the load-resisting elements to the foundation” (CRC R301.1). Two applications matter at the seam:

• Shared loads are allowed; unexamined loads are not. Nothing in R302.3 or R301 requires an attached ADU to stand structurally independent of the house — structural independence is a townhouse rule (R302.2.6). An ADU may be framed to bear partly on the existing structure if the load path is complete and adequate. But no prescriptive table certifies an existing footing or wall’s spare capacity — the footing-size tables size new footings. “The house has stood for 60 years” is not a capacity calculation.
• The engineered exit. Where any portion deviates from substantial compliance with conventional woodframe framing, that portion must be designed to accepted engineering practice, and the building official shall require its documents to be approved and stamped by a California licensed architect or engineer (R301.1.3, R301.1.3.1, citing Business and Professions Code sections 5537 and 6737.1). Whatever supports a rated separation assembly must itself carry an equal or greater fire rating (R302.3.4) — load path and fire path meet in the same wall on a stacked attached ADU.

State the design posture on the plans: (a) ADU loads on new footings only, framed against but not bearing on the house, or (b) loads shared into the existing wall and footing — posture (b) is the one that pulls engineering in for the existing elements. Our construction and foundations guide owns the footing tables, shear, and seismic detail behind this section.

The Roof Tie-In (Topic 379)

Where the new roof meets the old one, the code’s rule is performance plus flashing at every joint: “Flashings shall be installed in a manner that prevents moisture from entering the wall and roof through joints” (CRC R903.2), expressly required at wall and roof intersections, wherever there is a change in roof slope or direction, and around roof openings, with a flashing to divert water where the eave of a sloped roof intersects a vertical sidewall; flashing metal must be corrosion-resistant and not less than 0.019 inch thick (R903.2.1). A cricket or saddle is required behind any chimney or penetration more than 30 inches wide (R903.2.2), and unless the roof drains over its edges, drains go at each low point (R903.4) — a tie-in must never create an undrained pocket against the existing wall. For asphalt shingles (the figures are covering-specific; tile, metal, and slate carry their own R905 subsections), the numbers are:

• Sidewall flashing (the classic tie-in leak): continuous or step flashing not less than 4 inches in height and 4 inches in width, directing water away from the wall and onto the roof or into the gutter, its vertical leg running continuous under the siding (R905.2.8.3). Surface-mounting the new flashing over the existing siding is the most common tie-in failure.
• Valleys (a tie-in often creates one where the new slope dies into the old): open valleys lined with metal not less than 24 inches wide; closed valleys lined with a ply of smooth roll roofing or self-adhering underlayment not less than 36 inches wide (R905.2.8.2).
• Base and cap flashing per the shingle manufacturer’s instructions (which R905.2.8 incorporates by reference) — metal at least 0.019 inch, or mineral-surfaced roll roofing at least 77 lb per 100 sq ft (R905.2.8.1). Drip edge at eaves and rakes, overlapped at least 2 inches (R905.2.8.5).

What does not exist: any code-mandated tie-in overlap length, shingle-course count, or “strip back X feet of existing roof” figure. Those are manufacturer-instruction and roofer-practice details, and any source quoting one as code is inventing it.

Entrances, Access Paths, and the Shared Yard (Topics 383–385)

The separate entrance (Topic 383)

There is no single ADU statute captioned “separate entrance” for a standard ADU — the requirement arrives through three verified doors. Conversions: a section 66323(a)(1) unit qualifies only if “[t]he space has exterior access from the proposed or existing single-family dwelling” (section 66323(a)(1)(B)), and any expansion beyond an existing accessory structure is capped at 150 sq ft, limited to accommodating ingress and egress (section 66323(a)(1)(A)) — spend that 150 on the entry, not on living area. JADUs: a separate entrance from the main entrance is required (section 66333(e)(1)), and a JADU with no separate bathroom must also have an interior entry to the main living area (section 66333(e)(2)). Every unit’s egress door: CRC R318 requires a continuous, unobstructed egress path from all portions of the unit to the required egress door without travel through a garage, the door opening directly into a public way or to a yard or court that opens to a public way (R318.1); at least 32 inches clear width and 78 inches clear height, openable from inside without a key (R318.2); a landing on each side at least the door’s width and 36 inches in the direction of travel, sloped no more than 2 percent (R318.3). The practical conclusion for an attached ADU follows from the definition: a unit with “complete independent living facilities” (section 66313(a)) whose only door opens into the other home’s living room has no egress path of its own. No state rule, however, fixes the entrance’s side of the building, screening, or distance from the main entrance — and no state rule assigns the ADU its own address or mailbox either; addressing is local agency practice (Batch 7 verification, linked from our comparison guide).

The backyard access path (Topic 385)

Your city cannot require a passageway for an ADU: “No passageway shall be required in conjunction with the construction of an accessory dwelling unit” (section 66314(d)(6)) — and the law defines the thing it forbids as “a pathway that is unobstructed clear to the sky and extends from a street to one entrance of the accessory dwelling unit” (section 66313(j)). What the no-passageway rule does not repeal is egress: under R318.1 the ADU’s door must still open to a public way, or to a yard or court that itself opens to one. A detached ADU behind a fully enclosed rear yard with no opening to a public way fails — not because a passageway is required, but because egress is. In practice: a gate and a walkable route out, designed for the people who live there. There is no state minimum path width, paving, lighting, or gate standard — width figures circulating from other codes are not California ADU law. Fire-apparatus access is a separate, local fire-code matter (see the alley section below), not a path spec.

Shared outdoor areas (Topic 384)

State ADU law never divides the yard. No statute or code section gives the ADU a private patio, assigns the side yard, or sets a minimum outdoor area for either home — the only outdoor-space power in ADU law is a city’s ability to set objective landscape standards in its ordinance (section 66314(b)(1)), and those do not reach conversions or section 66323 units at all. Who uses which part of the yard is the owner’s decision — and, once the ADU is rented, a matter of what the lease says (our landlord guide, Batch 16 in this program, will own the lease side). Design it deliberately anyway: a defined patio and path for the ADU, screening at the shared wall’s windows, and drainage that still falls away from both foundations (6 inches of fall in the first 10 feet, CRC R401.3). Do not fence the ADU’s yard in a way that landlocks its egress door — and do not believe any source quoting a minimum private-outdoor-area figure as California ADU law; none exists.

Living Through the Build (Topic 386)

No California rule promises you a quiet build: ADU law does not address living through construction at all — no occupancy-during-construction standard, no state construction-hours or dust rule (hours and noise are set by your city’s ordinance; check it before promising start times to neighbors). What the law does require is that the work protect the property it touches, including foundations, party walls, and roofs, with runoff and erosion controlled (CBC section 3307.1). The real difference between the configurations is structural, and it follows from the sections above: a detached ADU shares no wall, roof, or foundation with the occupied house, so the home’s envelope is opened zero times — the disruption is trenches, deliveries, staging, and utility tie-in days in the yard. An attached ADU necessarily opens the shared wall and roof lines, and each opening is a dust, noise, weather, and security event inside the occupied home. Get the living-through-it terms in writing in the contract — dust containment at shared-wall openings, same-day temporary dry-in at the roof tie-in, scheduled utility shutoffs with same-day restoration, a secured temporary separation between living space and work area, daily cleanup. Those are builder practices to specify, not code promises; when we describe our own process as low-disruption, that is a commitment about how we work, not a standard the code imposes. We publish no disruption durations or “you must vacate for X weeks” figures, because no source supports them.

The Detached Foundation (Topic 388)

A detached ADU stands entirely on its own — the full CRC Chapter 4 foundation system applies, with nothing borrowed from the house and no interface to engineer (contrast Topics 378 and 380). The verified frame, detailed in our construction and foundations guide:

• Footings (R403.1): all exterior walls on continuous solid or fully grouted masonry or concrete footings (or other approved systems), sized from the soil’s presumptive bearing value (Tables R403.1(1)–(3), keyed to Table R401.4.1(1)) and bearing on undisturbed natural soil or engineered fill; exterior footings at least 12 inches below undisturbed ground (R403.1.4).
• Slab-on-grade (R506, 2025 numbering): at least 3½ inches thick (R506.1); site prepared with all vegetation, topsoil, and foreign material removed (R506.3); a 4-inch base course where the slab is below grade, except on well-drained Group I soils (R506.3.2); and — note the edition change — a vapor retarder of at least 6-mil polyethylene or approved material, joints lapped at least 6 inches, between slab and base course, with a capillary break per CALGreen Chapter 4, Division 4.5 (R506.3.3, R506.3.3.1). The 2022 code’s 10-mil figure (former R506.2.3) is superseded for permits filed under the 2025 code; a heated ADU slab takes no vapor-retarder exception.
• Slopes (R403.1.7): where a slope is steeper than 1 vertical to 3 horizontal, the building must clear an ascending slope by the smaller of half the slope’s height or 15 feet, and a footing must sit back from a descending slope by the smaller of one-third the slope’s height or 40 feet. Siting a detached ADU tight to a descending edge for the view is the classic way to fail this section.
• Expansive soil exits the prescriptive tables entirely: foundations and slabs on expansive soils are designed under CBC section 1808.6 (R403.1.8).
• Drainage (R401.3): grade falls at least 6 inches in the first 10 feet from the foundation, impervious surfaces within 10 feet slope at least 2 percent away, and drainage is diverted to a storm sewer or approved collection point.

No code or HCD source ranks foundation types, prices them, or states what “most” detached ADUs use — slab vs raised vs pier-and-grade-beam is selected by your lot’s soil, slope, flood exposure, and plumbing access, and any cost figure for it is a project quote, not a code fact.

Detached Utility Routing (Topic 387)

A detached ADU’s utilities cross the yard in trenches, and the rules are about how you dig, not how far — there is no state maximum run length and no universal trench depth. The verified frame, owned in full by our plumbing and utilities guide:

• Before digging: an 811 ticket — DigAlert in Southern California — operator marks respected, and hand tools inside the 24-inch tolerance zone on each side of a mark before power equipment (Gov. Code sections 4216, 4216.4, 4216(u)). A standard ticket is valid 28 calendar days. Your own private lines — laterals, private electric, irrigation — are never marked; “no marks” does not mean “nothing there.”
• Water and sewer together: they may share a trench only on the Plumbing Code’s conditions — both pipes of material approved for use within a building — and where they cross, the water pipe sits above the sewer on a solid shelf (CPC section 609.2). The 5-foot separation figure in national content is the IPC, not California’s CPC; do not let it onto a California plan.
• Depth and distance: burial depth runs through CPC sections 609.1/312 and each utility’s standards for its own facilities — there is no single publishable number. Distance shows up in two real places: excavation cost, and the sewer’s slope budget — a gravity sewer must hold its code slope across the whole run, so a long flat backyard is where ejector solutions enter as a design decision. Connection requirements and capacity charges are regulated separately (Gov. Code section 66311.5). Recorded easements bound the route: the shortest line is not automatically a buildable one.

Height: Two Stories, and What “Neighborhood Context” May Not Do (Topics 391–393)

The height floors (Topic 391)

California law contains no ADU height cap and no story limit. What it sets are floors — the lowest height limit a city may impose (Gov. Code section 66321(b)(4), read in full as amended by SB 543, effective January 1, 2026). A city may allow more; it may not allow less:

Section 66323 detached units take the same (A)/(B)/(C) floors by incorporation (section 66323(a)(2)(B), (a)(4)(A)). How height is measured — grade reference, roof-form treatment — is not defined in the ADU chapter; it is a local-ordinance and building-code question.

Two-story detached feasibility is arithmetic, not permission. HCD states the protection precisely: a local agency must allow at least two stories if they can be accommodated under the applicable height limit while complying with all applicable building standards, and a two-story detached ADU that fits the state-required height allowances plus the building code may not be denied merely because the underlying zoning restricts a primary dwelling to one story (sections 66321(b)(4)(D), 66314(d)(8)). The condition is part of the rule: at the bare 16-foot floor, most two-story designs will not fit once floor structure, code ceiling heights, and a roof are counted — that is a tape-measure problem, not a permission problem. Measure feasibility against your city’s adopted limit, which may exceed the floor. Our second-story ADU page covers the configuration itself. (Any copy anywhere that reads “California allows two-story ADUs up to 16/18/25 feet” misstates the law in both directions: those numbers are minimums a city must allow, not maximums, and 25 feet is the attached floor.)

Second-story privacy (Topic 392)

No California rule tells you where a second-story ADU window may face. The HCD Handbook’s grep is total: “overlook” 0 hits — and “privacy” appears as the state’s own example of a subjective standard that may not be imposed on an ADU (section 66313(i); see Topic 393 below). There is no state privacy setback, window-placement setback, or privacy-glazing mandate at any height. The only lawful local lane is a pre-adopted, measurable objective standard on an ordinance-track unit — and section 66323 units are exempt from local design standards altogether (section 66323(b)). The window limit that does bind an upper wall at a 4-foot setback is fire, not privacy: openings are barred under 3 feet of fire separation distance and capped at 25 percent of the wall at 3 to under 5 feet, unsprinklered (Table R302.1(1), above). Design choices that prevent friction no rule will mediate later — offsetting upper windows from the neighbors’ and the main house’s bedroom windows, high-sill or obscured glazing by choice on the tight elevation, main glass on the yard-facing elevation — are good practice, labeled as choices, never requirements.

Height and neighborhood context (Topic 393)

California replaced “does it fit the neighborhood?” with numbers and a definition. An objective standard “involve[s] no personal or subjective judgment by a public official” and is “uniformly verifiable by reference to an external and uniform benchmark or criterion” known before submittal (section 66313(i)). A city ordinance may impose objective standards including height, setback, landscape, and architectural review (section 66314(b)(1)), and specified colors or materials are HCD’s own example of the objective kind. What a city may not impose, in HCD’s words, are standards framed as “privacy,” “compatibility with neighboring properties,” “promoting design harmony and balance,” “must maintain similar architecture style,” or “high quality materials” — subjective, and barred. Context arguments also cannot re-litigate the height floors parcel by parcel; ministerial approval (section 66317) removes that discretion. Note the statute’s one genuine context concession, read narrowly: the extra 2 feet in section 66321(b)(4)(B) rewards a roof pitch aligned with the primary dwelling’s — it nowhere requires pitch, form, or style matching. The one named exception lane is historic: an ordinance may include standards preventing adverse impacts on property listed in the California Register of Historical Resources (section 66314(b)(1)) — a lane for listed property, not for “historic-feeling” streets. HOA architectural rules are a separate private system (Civil Code sections 4751, 714.3), not permit conditions.

Privacy Screening and Landscaping for a Detached ADU (Topics 389–390)

Screening (Topic 389)

California law does not require privacy screening for a detached ADU — no fence, wall, hedge, or screen-planting mandate exists in sections 66310–66342 or the Handbook (“screening”: 0 hits) — and a city cannot demand it in the name of “privacy,” the very standard HCD names as subjective and non-imposable. A screening rule survives only if a city adopted it in advance as a measurable objective standard (a specified height or material in the ordinance text), on an ordinance-track unit, without precluding the guaranteed 800 sq ft at 4-foot setbacks — and it can never apply to a section 66323 unit (section 66323(b)). No “6-foot privacy fence” figure is state law; fence heights are local zoning, and a dividing fence between neighbors is general property law — adjoining owners share its cost under the Good Neighbor Fence Act presumption, with 30 days’ prior written notice before incurring costs (Civil Code section 841). Screening between your own two units is an amenity you choose: orient the ADU’s door and main windows away from the house’s, stagger facing windows, plant or fence the gap — as design, not compliance.

Landscaping (Topic 390)

State ADU law doesn’t design your yard. It allocates landscaping to the local objective-standards lane (section 66314(b)(1)), then fences that lane: open-space limits may not preclude the 800 sq ft / 4-foot configuration (section 66321(b)(3)); section 66323 units “are not subject to local open space and landscaping requirements in an ADU ordinance” (section 66323(b)); and objective landscape standards do not apply to conversions at all. The landscaping code that does exist is MWELO, the state’s water-efficient landscape regulation — and it keys on landscape area, not on the ADU (23 CCR sections 491.2, 492): it applies to new construction projects with an aggregate landscape area of 500 sq ft or more (rehabilitated landscapes at 2,500 sq ft), with either prescriptive or performance compliance available from 500 to 2,500 sq ft and performance required at 2,500 and above. Under the prescriptive path, turfgrass is capped at 25 percent of the residential landscape area and compost is incorporated at 4 cubic yards per 1,000 sq ft. So a detached ADU project that installs under 500 sq ft of new landscape does not trigger MWELO by itself — and a project packaged with a full yard redo may. Two errors to avoid: “ADUs are exempt from MWELO” (the ADU-ordinance exemptions remove the city ADU ordinance’s standards; MWELO is a separate state regulation enforced locally) and “MWELO applies to every ADU yard” (equally false below 500 sq ft). The yard’s other binding rule is drainage (R401.3, above): keep the 4-foot setback strip serviceable and never plant or pave so water ponds against either foundation. Protected trees are local law, per parcel — our property eligibility guide covers that layer.

Alley-Facing Units (Topic 394)

There is no California alley rule for ADUs — “alley” appears zero times in the HCD Handbook, and no provision of sections 66310–66342 requires, forbids, or specially regulates an ADU that faces an alley. An alley-facing unit is simply an ADU whose entrance fronts the alley instead of the street, and state law is indifferent to which side the door is on: the same size, height, setback, parking, and fee rules apply (rear setbacks are measured from the rear lot line whether or not an alley abuts it). A conversion’s exterior-access requirement (section 66323(a)(1)(B)) can be satisfied from the alley side, and an alley entrance does not trigger any street-side path requirement — passageways may not be required at all (section 66314(d)(6)). Your address is assigned by the city’s addressing authority, not by which side the door faces; no state rule entitles any ADU to a separate address or mailbox (USPS recognizes a unit designator as an address element — it does not assign one). The questions that actually decide an alley project are these:

• Title: is the alley public or private, who maintains it, and is it legal access for the parcel? Maintenance of a private right-of-way falls on the easement owners, shared where ownership is shared (Civil Code section 845).
• Fire access: where the alley is the access route, the fire code framework applies — approved fire apparatus access roads extending to within 150 feet of all portions of the first story, with at least 20 feet of unobstructed width and 13 feet 6 inches of vertical clearance (California Fire Code section 503.1.1, 503.2.1) — as administered by your local fire district, which is how many residential alleys are in fact reviewed. Do not read that as “your alley must be 20 feet wide”; it is the framework the fire district applies, and its determination for your parcel is the verify step.
• Fire separation: an alley-facing wall’s separation distance is measured to the alley centerline (Table R302.1), which is often more permissive than a side-lot-line measurement — the facing wall’s rating and opening budget are computed from there.

Prefab Detached Options (Topic 395)

Detached changes nothing about build method — because ADU law never classifies by build method at all (“prefab”: 0 hits in the Handbook). A detached factory-built or manufactured unit takes the same height floors, 4-foot setbacks, ministerial permit, and — if it fits — the same section 66323(a)(2) category as a site-built one. What changes is who approved the box, and California law has exactly two factory buckets, detailed in our types of ADUs guide:

• Factory-built housing (FBH) — Health and Safety Code section 19971: manufactured offsite so that concealed parts cannot be inspected after installation without disassembly, assembled onsite to California Building Standards Code standards. FBH sold in California must bear HCD insignia of approval on each unit or component, with plan approval through HCD or a certified Design Approval Agency and in-plant inspection through an approved Quality Assurance Agency (section 19980). Insignia-bearing FBH is deemed to comply with local construction ordinances (section 19981(a)) — but locals keep zoning, setbacks, site development, and architectural review (section 19993), and local enforcement agencies inspect the installation on your lot (section 19992). The foundation is site work under local enforcement, per the manufacturer’s approved installation instructions.
• Manufactured home — section 18007: built on or after June 15, 1976 to the federal HUD standards on a permanent chassis. Installation on a foundation system as a fixture to real property requires a local building permit before installation, with title evidence and foundation plans, and the foundation document is recorded within five business days after the certificate of occupancy (section 18551(a)); chattel installation follows a different path (section 18551(b)).
• Energy split (CEC 2025 ADU FAQs, stated directly): “Factory-built homes must comply with Title 24, including the Energy Code” — so a newly constructed detached FBH ADU takes the section 150.1 package, solar PV included, like a site-built unit. Manufactured homes “must meet the Title 25 regulations and are not subject to the Energy Code.” Never call an FBH unit “HUD-code,” and never call a manufactured home “built to the California Building Code.”
• Not law: crane use, set-day procedure, transport routes, and street closures appear in no statute read for this guide — they are the manufacturer’s enforceable installation instructions plus local street-use permits and jobsite logistics. And no primary source supports any prefab speed or cost advantage; none is claimed here.

One detached-only sequencing note: in a qualifying post-disaster county, a detached ADU’s certificate of occupancy may issue before the damaged primary dwelling’s (section 66328(b)); attached units are expressly excluded (section 66328(c)). There is still no like-for-like disaster replacement entitlement — “prefab replacement” marketing does not create one.

The Attached Energy Envelope (Topic 397)

For energy purposes, an attached ADU is not new construction — it is an addition. The Energy Code defines an addition as any change that increases conditioned floor area and conditioned volume (section 100.1), and the CEC’s ADU guidance is explicit: attached ADUs — units that share a common wall, floor, or ceiling with another dwelling unit — are additions, as are conversions of existing unconditioned space (a garage, attached or detached). Both comply under section 150.2(a). Converting space that is already conditioned (a conditioned basement, a pool house) is an alteration under section 150.2(b) instead. Only a new detached structure is newly constructed (section 150.1). That single classification drives the whole envelope:

• No solar PV, no battery-ready, no electric-ready. The CEC answers it in one sentence: “Is solar PV required for attached ADUs? No, since attached ADUs are considered additions, solar PV is not required.” The battery-storage-ready (section 150.0(s)) and electric-ready duties are new-construction duties and do not reach additions either.
• Conversion walls: existing 2×6 walls of an addition-classified ADU may be insulated with R-21 cavity insulation per sections 150.2(a)1Ai and 150.2(a)1Bvi, and continuous insulation is not required for those walls (CEC FAQ). Extensions of existing wood-framed walls keep the existing dimensions with R-15 in 2×4 and R-21 in 2×6 framing. Existing walls in newly conditioned space are treated as “new” for the Energy Code.
• QII: the Quality Insulation Installation prescriptive requirement does not apply to ADU additions of 700 sq ft or less (section 150.2(a)1Bv) — unless QII was modeled for a performance credit, in which case verification applies.
• Cool roof reaches additions (steep-slope in Climate Zones 10–15; low-slope in 13 and 15), except additions of 300 sq ft or less. Lighting does too, at full strength: all dwelling units including additions must meet the mandatory lighting requirements of section 150.0(k) regardless of square footage (CEC FAQ). A newly installed vented range hood in an addition requires verification of its certification, airflow, and sound rating.
• Ventilation is the careful sentence: additions of 1,000 sq ft or less are excepted from the section 150.0(o) mechanical-ventilation airflow subsections and from whole-house-fan ventilation cooling — but ventilation in an addition may still be required by the California Mechanical Code, which may point back to section 150.0(o). Publish “excepted from the Energy Code airflow figures, CMC to confirm,” never “additions need no ventilation.”
• Its own HVAC: the existing furnace’s duct system may not simply be extended to serve the ADU — the Mechanical Code does not allow return air from an existing forced-air system to be discharged into another dwelling unit, so one ducted furnace may not serve both homes (CEC FAQ).
• The shared wall itself: no Energy Code provision anywhere in this program’s research assigns the house–ADU common wall an insulation R-value — the common wall’s role in the Energy Code is classificatory (it is what makes the unit “attached”). As the envelope is drawn on the project’s CF1R, the thermal boundary runs around conditioned space: a wall between two conditioned units is not an exterior envelope surface, while every new exterior wall, roof, and floor the addition adds is, and must meet the section 150.2(a) package above. Confirm that drawing on your project’s energy model — and expect the shared wall’s binding code life to be fire and sound, not energy.

Our electrical and HVAC guide covers the panel, solar, and systems side in full.

Site Coverage Tradeoffs (Topic 399)

Lot coverage cannot be used to stop your ADU — up to a point, and the point is exact. A local agency may not establish “limits on lot coverage, floor area ratio, open space, front setbacks, and minimum lot size… that does not permit an accessory dwelling unit with at least 800 square feet of interior livable space and with four-foot side and rear yard setbacks” (Gov. Code section 66321(b)(3), as amended by SB 543). HCD’s FAQ form: coverage and FAR limits “cannot preclude the creation of an ADU of at least 800 square feet,” and as applied beyond that they “must not unreasonably restrict the creation of ADUs” (sections 66321(b)(3), 66311). Keep the two halves distinct, because both errors circulate:

• At or below the guaranteed configuration, coverage is simply not a lawful denial ground — a preclusion ban, not a counting method. (No state rule says how coverage is counted — eaves, patios, and the primary dwelling’s treatment are local definitions.)
• Above the guarantee, coverage and FAR limits can apply to the extra area — a 1,200 sq ft detached design can be coverage-limited, subject to the section 66311 backstop and the size floors of section 66321(b)(2) (a city’s maximum-size ordinance must still allow 850 sq ft, or 1,000 sq ft for more than one bedroom). The state guarantee is an 800 sq ft unit, not an unlimited footprint.
• Section 66323 units and conversions sit outside local lot-coverage standards entirely (section 66323(b); HCD conversions FAQ). And density is a different rule: a compliant ADU is not counted as exceeding the lot’s allowable density (sections 66314(c), 66319) — do not merge it into the coverage analysis.

The tradeoff that remains is physical, not legal: every detached square foot consumes yard that drainage (R401.3), MWELO’s landscape-area counting, protected trees, fire separation distance, and your own outdoor use also claim. State law resolves the permit conflict in the ADU’s favor up to 800 sq ft. It does not repeal the geometry.

Renting It, Selling the Property (Topics 396 and 400)

Detached rental appeal — the legal frame, honestly labeled (Topic 396)

What state law contributes to a detached ADU’s rental appeal is a frame, not a figure. The unit may be rented separate from the primary residence (section 66314(d)(1)); no owner-occupancy requirement may be imposed on an ADU, so you need not live in either unit (section 66315); and rental terms are longer than 30 days — a local agency may require it generally (section 66315), and for section 66323 units the agency shall require it (section 66323(e)) — so no ADU copy here will ever pitch short-term rental use. The features themselves are definitional: complete independent living facilities (section 66313(a)) in a separate structure — HCD’s own framing is that ADUs let extended families be near one another “while maintaining privacy” and “can provide as much living space as many newly built apartments and condominiums.” On the financing side, HCD records that FHA policies allow ADU rental income in a borrower’s qualifying income (including 203(k)), and that Fannie Mae and Freddie Mac offer ADU financing options that recognize the rent. Beyond that frame, we publish nothing — because nothing exists to publish from: no statute, Handbook passage, or other primary source states that a detached ADU rents for more than an attached one, or states any rent premium, vacancy rate, or demand ranking by configuration. In our experience as builders, a detached unit gives a tenant a separate structure, no shared wall, its own entrance, and typically a defined outdoor area — the features landlords market, offered here as experience, not data. Project-level rent analysis for a specific property lives in our rental income guide; landlord obligations — leases, deposits, screening, habitability — are Batch 16 of this program.

Resale — what the law says, and what nobody can verify (Topic 400)

The Handbook contains no resale content at all (“resale”: 0 hits), and no primary source ranks ADU configurations by buyer preference or premium. Anyone who quotes you a resale premium for one ADU type over another is quoting an opinion. What is law, and what actually frames a resale, is the conveyance structure:

• Default: the ADU “shall not be sold or otherwise conveyed separate from the primary residence” (section 66314(d)(1)). Resale is the sale of one property with two units. A JADU can never be sold separately — a recorded deed restriction prohibits it (section 66333(c)).
• The condo path (section 66342, AB 1033): a city may adopt an ordinance allowing the primary dwelling and the ADU to be conveyed separately as condominiums. Where one exists, it must conform to the Davis-Stirling Act and the Subdivision Map Act, require a safety inspection before the condo plan records, require each lienholder’s written consent (which may be refused), and carry the statutory notices. No ordinance in your city means no condo path there — we name no city’s status here, because it is verified city by city.
• The nonprofit path (section 66341): separate conveyance to a qualified low- or moderate-income buyer is mandatory-allowed only where the property was built or developed by a qualified nonprofit corporation, under a recorded tenancy-in-common agreement with a nonprofit repurchase option and 45-year affordability restrictions. It is an affordable-housing mechanism, not a general market path.
• Appraisal, hedged as lender practice: how an appraiser values a home with an ADU — comparable sales of homes with ADUs where they exist, the unit’s income where the loan program allows an income approach — is set by the lender’s program and appraisal standards for the specific loan, not by ADU law, and it does not come in a detached-vs-attached table. Our ADU value guide treats valuation as its own subject.

Future Household Needs (Topic 398)

No California code designs your ADU for the family you will have in ten years — there is no future-needs standard, no protected later expansion, and no reserved envelope. That part is entirely your choice, and the law’s contribution is that it mostly stays out of the way: parents now, a tenant later (no owner-occupancy rule; rentals over 30 days); accessibility built in by choice — nothing requires a private ADU to be universally designed or wheelchair accessible, and nothing forbids the zero-step entry, wider doors, and grab-bar blocking that make it work if mobility needs change (voluntary reference dimensions are in our design guide). What the law will not do is save your place: a future expansion is a future permit under the rules then in force, so the flexibility has to be built into the unit you permit today — a single-story or entry-level bed-and-bath layout, a room that can swing between office, nursery, and guest use, and rough-in decisions made at build time, because retrofitting them is the expensive version. Every item in that sentence is a design choice, labeled as one.

Choosing: The Decision Sequence

1. Pathway first: conversion (section 66323(a)(1)), the guaranteed detached category (section 66323(a)(2)), or the ordinance track — the pathway decides which local standards can even apply (section 66323(b)).
2. Attached? The binding constraints are the rated shared wall (R302.3), the engineered foundation interface, the flashed roof tie-in, the addition energy package (section 150.2(a)) — and the 50-percent-of-the-house cap (section 66314(d)(4)).
3. Detached? The binding constraints are the height floor that fits your design (section 66321(b)(4)), fire separation across the imaginary line (R302.1), full new foundations and trenches, and newly-constructed energy compliance (section 150.1).
4. Ignore invented rules: no privacy-screening mandate, no passageway, no path width, no coverage denial at 800 sq ft, no rent or resale premium by configuration. If a demand or a claim cannot be traced to a section, ask for the section.

Run in that order and the attached-vs-detached decision stops being a taste argument. When you want the lot-specific answers first, our ADU Feasibility Report works through eligibility, envelope, and utilities up front — or start with a free consultation and bring us the address.

Sources (verified November 2026): Government Code sections 66311, 66313 (including (a), (i), (j)), 66314(b)(1), (c), (d)(1), (d)(3), (d)(4), (d)(6), (d)(7), (d)(8), (d)(12), 66315, 66317, 66319, 66321(b)(2), (b)(3), (b)(4) (read in full on leginfo as amended by SB 543, Stats. 2025, Ch. 520), 66323 (as amended by SB 543), 66328(b)–(c), 66333(b), (c), (e), 66341, 66342 (leginfo / HCD Handbook pinpoint treatment); Civil Code sections 832, 841, 845, 4751, 714.3; Health and Safety Code sections 17959.6, 18007, 18550–18551, 19971, 19980, 19981, 19992, 19993 and HCD Fact Sheet FBH 314; California Residential Code (Title 24, Part 2.5, 2025) sections R301.1, R301.1.3–R301.1.3.1, R302.1 + Tables R302.1(1)/(2), R302.3.1–R302.3.6 + Table R302.3.6, R302.4, R302.5, R302.6 + Table R302.6, R302.11, R309, R318.1–R318.3, R401.3, R403.1, R403.1.4, R403.1.7, R403.1.8, R506.1–R506.3.4, R903.2–R903.4, R905.2.8–R905.2.8.5, Appendix BG; California Building Code sections 3307.1, 703.2.2, 1206, 1803.5.7, 1803.6, 1808.6; California Fire Code section 503; California Energy Code (Title 24, Part 6, 2025) sections 100.1, 150.0(k), (s), 150.1, 150.2(a)–(b) with the CEC’s 2025 Energy Code ADU FAQs; 23 CCR sections 491.2, 492 (MWELO); Title 8, section 1541(i); LAMC sections 91.1206, 91.3307.1 and LADBS Information Bulletin P/BC 2026-060 (Los Angeles examples, labeled local); HCD Accessory Dwelling Unit Handbook, March 2026 edition (attached definition, height, coverage, subjective-standards, conversions, sales, and funding treatments). Where this guide describes construction or design practice, or our own experience, it is labeled as such in the text; edition-specific citations are labeled by edition, and local standards are confirmed with the project’s city. This page is part of the California Accessory Dwelling Unit Handbook on this site (Attached and Detached ADUs, Questions 751–800). Related: Detached ADUs · Attached ADUs · Second-Story ADUs · Designing Your ADU · Construction & Foundations · Types of ADUs · ADU FAQs