Designing Your ADU in California: Who May Design It, Room Sizes, Light, Privacy, Accessibility, Sound, Fire, Roof and Drainage Rules

Designing an ADU is mostly a hunt for rules that do not exist. California law sets a short list of real design floors — room sizes, light, egress, fire separation — and then, unusually, it also tells cities what they may not demand: no architect mandate for a standard unit, no privacy review, no required patio, no matching-roof test that rests on an official’s taste. This guide separates the two lists, because mixing them up is how ADU budgets die in redesign.

Start with the finding our research kept returning to: state ADU law and the HCD ADU Handbook contain almost no layout rules at all. Grepping the March 2026 Handbook: “designer” 0 hits, “floor plan” 0, “orientation” 0, “universal” 0, “wheelchair” 0. What ADU law regulates is who reviews the design and by what kind of standard — objective versus subjective — while the room-by-room numbers live in Title 24: the California Residential Code (CRC), the California Plumbing Code (CPC), and the California Energy Code. One edition warning applies to the whole page: the 2025 CRC reorganized Chapter 3 and renumbered its habitability sections (the figures did not change). Citations below give the 2025 CRC number first, with the 2022 number in parentheses, because permits filed on or after January 1, 2026 are reviewed under the 2025 codes. This page is the Planning and Design chapter of our California Accessory Dwelling Unit Handbook, and it builds on our ADU floor plans gallery.

Who May Design Your ADU

The concept plan (Topic 152)

No statute or code section defines a “conceptual floor plan,” prescribes its contents, or sets a drawing scale — the concept plan is a design-process step, not a regulated document. What is regulated is the permit submittal: CRC section R106.1 requires construction documents drawn to scale and sufficient to show the work conforms to the code, and each city’s own plan-check checklist decides what a complete first submittal contains. Two programs are easy to confuse with a concept plan and are neither: a city’s preapproved plans (the AB 1332 program described in the HCD Handbook) are full permit sets a city has already reviewed — a detached ADU using a currently preapproved plan gets a 30-day approve-or-deny decision instead of 60 — and a city’s standard plans are local products on local terms. Pitfall: paying for a full permit set before feasibility is checked. The concept stage is exactly where setbacks, height, sewer, and panel questions belong — see our property eligibility guide and feasibility report.

Do you need an architect? (Topic 153)

No — not for a standard ADU. The Architects Practice Act exempts single-family dwellings of woodframe construction not more than two stories and a basement in height, multiple dwellings of up to four units on the same terms, and garages and other appurtenant structures (Business and Professions Code section 5537(a)). A typical one- or two-story wood-frame ADU sits in the center of that exemption. Hiring an architect is a scope and complexity decision, not a state mandate. What the Act does police is titles and paperwork:

• Title protection (section 5536(a)): it is a misdemeanor for an unlicensed person to call themselves an “architect” or an “architectural designer.” If someone markets with either title, check the license — that is a bright-line screening test.
• Everyone signs (section 5536.1(a)): all persons who prepare plans for others must sign them; only licensees affix a stamp.
• Written contract (section 5536.22): a licensed architect must use a written contract executed before work starts, stating the services, the basis of compensation, change and termination procedures, who owns the plans, and the architect’s license number. Plan ownership matters: under section 5536.4 your right to reuse the plans (for a second unit, or with another builder) is a contract term to settle up front, not an automatic right.
• Verify the license with the California Architects Board before you sign, and confirm the person in responsible control is the person whose stamp will appear.

There is no state fee schedule or published fee range for architects — section 5536.22 requires the basis of compensation in the contract and sets no amount, so any source quoting you a standard architect percentage as a rule is inventing it.

Designers, drafters, and contractors (Topic 154)

There is no California license titled “designer,” “building designer,” or “residential designer.” An unlicensed designer, the homeowner, or a contractor may lawfully prepare ADU plans inside the section 5537 exemption — provided they sign the plans (section 5536.1(a)), never use a protected title, and the design stays within conventional wood framing. The exemption ends in four places: more than two stories plus basement, non-woodframe construction, more than four units on a lot, or — the one that matters most in practice — any portion that deviates from substantial compliance with conventional framing requirements. For that portion, the building official shall require plans and calculations by or under a licensed architect or registered engineer, stamped and signed (section 5537(b)). A licensed contractor may design within section 5537 and their CSLB classification, including systems necessary to work they have contracted to perform (section 5537.2) — that is the legal footing of design-build. Certified Interior Designers hold a title certification (administered by the California Council for Interior Design Certification, not a state license) covering nonstructural, nonseismic interior work — kitchen and bath layouts, cabinets, finishes (sections 5800–5812, 5538). Note for evergreen planning: that chapter is scheduled for repeal on January 1, 2027 under section 5810 as amended, so verify its status before relying on CID status in 2027. Pitfall: paying an unlicensed designer for a design that plan check then routes into section 5537(b) — the licensee’s redraw is a second design fee. Run the exemption analysis in the first conversation.

When engineering is actually required (Topic 155)

The trigger list is short, and story count is not on it. “Two-story ADUs require an engineer” is false as stated — two stories in conventional wood framing is the center of the exemption (sections 5537(a), 6737.1(a)). A licensed engineer (or, for the deviating portion, an architect — the statute gives the choice) is required when:

1. A portion deviates from conventional framing (sections 5537(b) / 6737.1(b)) — long spans, wide openings, a second story over a wide garage opening are the common examples; there is no separate code section naming any one of them.
2. The design exits the CRC’s prescriptive path (CRC section R301.1.3) — expansive-soil foundations under CBC section 1808.6, hillside alternates under section 1808.7, deep foundations under section 1810, each standing on a geotechnical investigation (our construction and foundations guide maps these exits).
3. The building official requires it under CRC section R106.1 for the nature of the work.

Engineering documents must be prepared by or under the responsible charge of the appropriately registered engineer and signed and sealed when final (section 6735); verify the license and branch with the Board for Professional Engineers, Land Surveyors, and Geologists. A civil engineer may do a building’s structural design; “structural engineer” is an added authority title on top of civil licensure, and engaging one is a complexity choice (hillside, retaining, irregular), not a category requirement. No primary source states any price for engineering or soils work — this guide publishes none.

The Rooms: What the Code Actually Requires

Bedroom layout (Topic 156)

Every habitable room — bedrooms included — must clear the same floor (2025 CRC sections R312–R325; 2022: R304–R303):

Two verified negatives belong here. A bedroom does not need a closet to be a legal bedroom in California — the habitability sections impose no closet requirement (appraisal conventions are not building code). And no city may cap the number of bedrooms in an ADU: HCD’s Handbook states a bedroom limit could be construed as discrimination against protected classes such as familial status (Government Code section 65008) — and an ADU may lawfully have no bedrooms, because efficiency units are an allowable ADU type. Pitfall: designing to a window’s nominal size instead of its net clear opening — a nominal 20-by-24-inch window delivers far less than 5.7 sq ft of clear opening once frame and sash are counted, and it is the classic bedroom-egress failure at plan check.

Bathroom count (Topic 157)

The rule is exactly one full set: every dwelling unit shall have a water closet, a lavatory, and a bathtub or shower (2025 CRC section R326; 2022: R306.1), connected to an approved water supply and sanitary system, with hot and cold water to the kitchen sink, lavatory, tub, shower, laundry tubs, and washing-machine outlets. There is no second-bathroom trigger at any size or bedroom count, and no maximum — “two-bed ADUs need two baths” is market advice, not law. Additional half-baths are a free design choice governed by the same fixture clearances. The clearances themselves come from the California Plumbing Code, and one figure is the most-copied bathroom error in national content:

• Water closet: at least 15 in. from its centerline to any side wall or obstruction, and at least 30 in. center-to-center to an adjacent fixture (CPC section 402.5).
• Clear space in front of the water closet and in front of a lavatory: at least 24 in. in California (CPC section 402.5) — not 21 in., which is the IPC/model-code figure. (CPC section numbers verified in the 2022 CPC; confirm the 2025 CPC numbering at submittal — the figures are the long-stable UPC values.)
• Showers: at least 1,024 sq in. of finished interior area, able to encompass a 30-in. circle, with an opening at least 22 in. wide (CPC sections 408.5–408.6); tub and shower walls finished with a nonabsorbent surface to at least 6 ft above the floor (CRC R327 / R307.2); ceiling over a shower at least 6 ft 8 in. over a 30-by-30-in. area at the showerhead.
• Bathroom ventilation: 3 sq ft of glazing, half openable, or artificial light plus exhaust vented outdoors at 50 cfm intermittent / 20 cfm continuous (CRC R325.2 / R303.3).

The ADU/JADU fork decides more plumbing than any fixture choice: an ADU must have its own complete sanitation (Government Code section 66313(a) — complete independent living facilities), while a JADU may share sanitation with the house — but sharing triggers owner-occupancy (section 66333(b), as amended by AB 1154) and, where the JADU has no bathroom of its own, an interior entry to the main living area in addition to its separate exterior entrance (section 66333(e)).

Kitchen layout (Topic 158)

The entire CRC kitchen rule is two sentences: each dwelling unit gets a kitchen area, and every kitchen area gets a sink with hot and cold water (CRC R326 / R306.2, R306.4). Kitchens are expressly excepted from the 70-sq-ft and 7-ft room rules. So, as verified negatives: there is no minimum kitchen size in California residential law — no minimum counter length, cabinet count, or appliance size for a standard ADU — and there is no work-triangle code rule. The triangle (legs of 4–9 ft, 26 ft total) and the 36/42/48-in. walkway and work-aisle figures are NKBA Kitchen Planning Guidelines — good design guidance, attributed to the NKBA, never to the code. ADU law adds only that an ADU must include permanent provisions for cooking (section 66313(a)). For a JADU, the efficiency-kitchen definition (section 66333(f)) — a cooking facility with appliances, plus a food-preparation counter and storage cabinets of reasonable size in relation to the unit — is a ceiling on what a city may demand: HCD states a city may not add specified counter or cabinet sizes, appliance types, or electrical/gas connections, and 220V circuits are not required for an efficiency kitchen. Circuits, GFCI protection, and range exhaust are real requirements — they live in our electrical and HVAC guide, not in a layout rule.

Storage planning (Topic 159)

There is no storage requirement for a standard ADU or JADU — no closet count, no storage volume, nothing in the CRC habitability sections or in ADU law. The single closet mandate in California dwelling law belongs to a different unit type: an efficiency dwelling unit must have a separate closet, a kitchen sink, cooking appliance, and refrigerator each with at least 30 in. of clear working space in front, and a separate bathroom (CBC section 1208.4). (Published sources conflict on that unit type’s minimum living area, so this guide states no figure for it — verify the current CBC section 1208.4 text if you are permitting one.) Storage still has one real legal consequence: closets and laundry rooms count as interior livable space under section 66313(e) — the measure the state size caps use — so on a unit designed tight against a size line, a walk-in closet is not free area. Count it from the first sketch.

Laundry placement (Topic 160)

An ADU is not required to have laundry at all. Section 66313(a)’s list of required facilities — living, sleeping, eating, cooking, sanitation — omits laundry entirely; in-unit laundry, a shared laundry room, or no on-site laundry all satisfy state law. If laundry is provided: a laundry room’s ceiling may be as low as 6 ft 8 in. (CRC R313 / R305.1), washing-machine outlets get hot and cold water (R326 / R306.4), its area counts toward interior livable space, and its dryer exhaust, drainage, and circuits follow the plumbing and electrical rules in our plumbing guide and electrical guide. Placement itself — stacked on the wet wall with the bath and kitchen is cheapest to plumb — is design guidance, not a code rule; no code fixes where laundry may or may not sit. One clarification that saves confusion later: sharing laundry with the house does not trigger the JADU owner-occupancy rule — that trigger is shared sanitation (section 66333(b)).

Light, Air, Connection, and Privacy

Indoor-outdoor connection (Topic 161)

No state rule requires a patio, deck, porch, or private open space for an ADU. Private outdoor space is a selling point, not a mandate — and state law goes the other way on access: “No passageway shall be required in conjunction with the construction of an accessory dwelling unit” (section 66314(d)(6)). The code’s actual indoor-outdoor links are egress links: the means of egress is a continuous, unobstructed path from all portions of the dwelling to the required egress door without travel through a garage, and that door must open directly into a public way or to a yard or court that opens to a public way (CRC R318 / R311.1). The egress door itself: side-hinged, at least 32 in. clear width and 78 in. clear height (R318.2 / R311.2). On tight side-yard lots, that door-to-public-way geometry — not amenity space — is the binding constraint. Landscape and architectural standards may reach an ordinance-track ADU as objective standards (below), but they do not apply to section 66323 units or to conversions.

Privacy from the main house (Topic 162)

This topic has the clearest answer in California ADU law, because HCD chose the word itself as the example. The Handbook’s permitting Q&A: language such as “privacy,” “compatibility with neighboring properties,” “promoting design harmony and balance,” or requiring “high quality materials” states subjective standards, which may not be imposed on ADU development (section 66313(i) defines objective standards as those involving no personal or subjective judgment, uniformly verifiable against a benchmark knowable before submittal). So:

• There is no state privacy setback, window-placement distance, or screening mandate. The only state setback geometry is the familiar one: no setback for a converted existing structure, and no more than 4 feet from side and rear lot lines for a new ADU structure (section 66314(d)(7)). That 4 ft is a development standard, not a privacy guarantee — two lawful 4-ft setbacks can still put facing windows 8 ft apart.
• What a city may do on the ordinance track is adopt an objective standard — a numeric, pre-published rule (a specified screening height or material, a numeric window offset). A correction notice demanding “privacy screening” or “compatibility” changes on a ministerial ADU is applying a standard HCD names as impermissible; the remedy is the appeal and HCD framework in our permit process guide, not a redesign.
• Section 66323 units sit outside the local design lane entirely (section 66323(b)), and HOA rules are a separate, private system: an HOA may impose limited objective design restrictions within reason, but may not unreasonably increase cost or effectively prohibit the ADU (Civil Code section 714.3).
• Windows near a lot line are limited — by fire, not privacy (Topic 170 below).

Sight-line design — offsetting doors and windows, clerestories, planting screens — remains excellent practice for livability and rentability. Do it because tenants stay longer, not because a rule demands it.

Natural light (Topic 163)

The rule, per habitable room: glazed openings of at least 8% of that room’s floor area, facing directly onto a street, alley, or public way, or a yard or court on the same lot — including through a roofed porch whose longer side is at least 65% open with a ceiling at least 7 ft high (CRC R325.1.1 / R303.1). Skylights count as glazing. Ventilation is the twin rule: an openable area of at least 4% of the floor area (R325.1.2 / R303.1) — fixed picture windows count toward the 8% and not toward the 4%. Two qualifications keep the rule honest. First, glazing is not required where artificial light provides an average of 6 footcandles at 30 in. above the floor — a windowless habitable room is lawful when that exception is engineered and ventilation is separately satisfied mechanically. Second, bathrooms are not habitable rooms for this rule; they follow the 3-sq-ft-or-exhaust rule in Topic 157. Pitfall: counting glass area instead of glazed-opening area, or discovering after framing that an interior room’s borrowed-light arrangement does not meet the adjoining-room conditions — check the per-room math on the window schedule, room by room.

Window orientation (Topic 164)

California law does not tell you which way your ADU’s windows must face. No Energy Code, CRC, or ADU-law provision requires south-facing glass or prohibits windows on any elevation (Handbook grep: “orientation” 0). Orientation enters only indirectly, through the Energy Code’s fenestration budget for new construction (2025 Energy Code section 150.1(c)3, Table 150.1-A): a prescriptive maximum U-factor of 0.27 in Climate Zones 1–5, 11–14, and 16, and 0.30 in Zones 6–10 and 15 (tightened from 0.30-everywhere in the 2022 cycle); a maximum SHGC of 0.23 in Zones 2, 4, and 6–15, with no SHGC requirement in Zones 1, 3, 5, and 16; a mandatory U-factor ceiling of 0.40 (section 150.0(q)); and prescriptive caps on total and west-facing fenestration area as a share of conditioned floor area. Exceeding a cap does not prohibit the design — it moves the project to the performance path. Additions follow the banded allowances of section 150.2(a) instead (our electrical and HVAC guide covers the classification). Verify your parcel’s climate zone first: Southern California spans several, and no single pair of numbers serves the whole region.

Passive cooling (Topic 165)

No California code requires an ADU to be passively cooled — and no passive technique substitutes for mechanical ventilation in a new unit. The CEC’s ADU guidance is explicit: there are no exceptions to the section 150.0(o) indoor-air-quality ventilation requirements for newly constructed ADUs of any size; that section requires whole-dwelling mechanical ventilation per ASHRAE 62.2 as amended by section 150.0(o), plus local exhaust. Operable windows satisfy the CRC’s natural-ventilation provision; they do not discharge the Energy Code duty. The code-side passive measure that does exist is the whole-house fan, and only on the prescriptive path in Climate Zones 8–14 (section 150.1(c)12) — with new units of 500 sq ft or less exempt, and additions of 1,000 sq ft or less exempt. Shading, cross-ventilation layout, and night-flush operation are design practice and performance-path modeling inputs; no primary source supports an energy-savings percentage for any of them, so this guide states none. Cool roofs are the other regulated passive measure — see Topic 171.

Designing for Every Age and Ability

Universal design (Topic 166)

This topic is a stack of verified negatives, and the stack is the answer: no state or federal law requires a private single-family ADU to be universally designed, visitable, or wheelchair accessible.

• ADA: does not apply to individually owned or leased private housing not used as a public accommodation — including single-family homes. “Your ADU must be ADA-compliant” is the most common false claim in this area; do not let a plan checker, vendor, or blog sell it to you.
• CBC Chapter 11B: scopes to public buildings, public accommodations, commercial facilities, and public housing facilities (section 11B-233.3: at least 5% of units, no fewer than one, with mobility features). A private ADU is not a public housing facility.
• CBC Chapter 11A / CRC section R320: California’s private-housing accessibility chapter covers multifamily buildings of 3 or more apartment units or 4 or more condominium units. Single-family dwellings and duplexes are expressly outside it — a house plus one ADU is two units.
• Federal Fair Housing design rules: cover buildings of 4 or more units first occupied after March 13, 1991. A house-plus-ADU lot is not covered.
• HCD says almost nothing: the Handbook’s only accessibility treatment (one hit in the entire document) answers whether an agency may impose accessibility standards on an ADU converted from existing space with “Maybe” — and a referral to HCD Codes and Standards. That open question is all that exists; treat conversion accessibility as a confirm-at-intake item.
• The one universal-design statute touching single-family homes (Health and Safety Code section 17959.6) requires a developer of a new for-sale development to offer buyers a list of accessibility features, standard/optional/not-available. It is an offer duty on for-sale developers — not an install mandate, and not a design standard for an owner-built ADU.

None of that argues against designing accessibly — it argues for doing it deliberately, using CBC 11B / ICC A117.1 / the FHA’s seven design features as a voluntary reference standard, named as such in the plans. The dimensions in the next two topics carry that label every time.

Aging-friendly layout (Topic 167)

“Aging-friendly” and “aging in place” are design goals, not California code standards — no law defines or requires them for an ADU, and no state “aging-friendly certification” exists. The closest thing California law has to an official feature menu is the section 17959.6 offer list, and it is a sensible checklist to design from voluntarily: an accessible route of travel; accessible routes from parking and garage to the entries; an accessible primary door and threshold; routes from the primary entrance to at least one bedroom, bathroom, and the kitchen; reachable switches, outlets, and thermostats; rocker switches; adjustable closet rods; kitchen clear floor space; and grab-bar backing or grab bars. Single-story living — bedroom and full bath on the entry level — is the layout decision that does most of the work, and it is pure practice: no code provision cited here requires it, which is exactly why it must be decided at concept stage rather than discovered at framing. One boundary: once the unit is rented, a disabled tenant’s fair-housing right to make reasonable modifications at the tenant’s own expense is a landlord-obligation rule, not a build requirement — see our rentals coverage in the Handbook.

Wheelchair access (Topic 168)

California has no visitability law for private homes. Nothing in ADU law, the CRC for one- and two-family dwellings, CBC 11A or 11B, the ADA, or the FHA design rules requires an accessible route, a zero-step entry, or any wheelchair feature in a private ADU. If you choose full access — for a parent now, or for the unit’s next thirty years — design to the voluntary reference dimensions borrowed from CBC 11B / ICC A117.1, labeled here as practice, not law: a 32-in. clear door opening; a 36-in. minimum route; a 60-in. turning space; lever handles; a zero-step entry (threshold no more than ½ in., beveled, under the reference standards); and blocking in the bathroom walls so grab bars can be installed later without opening the wall — note that even in federally covered buildings the rule is reinforced walls for later installation, not bars on day one. Keep those voluntary numbers distinct from the bathroom clearances that are law for every ADU (Topic 157: CPC 15/30/24 in.). One scoping flag: unit counting for 11A and FHA is per building — ADUs created inside a larger multifamily building can change that building’s analysis. Detached ADUs on a single-family lot do not; multifamily projects should confirm scoping at intake.

The Shared Walls: Sound and Fire

Sound insulation (Topic 169)

Whether a sound rating is law between your house and an attached ADU depends entirely on which code governs the building — and the precise answer corrects both common claims. CBC section 1206 requires wall and floor-ceiling assemblies separating dwelling units to provide STC 50 (laboratory; 45 field-tested) and floor-ceiling assemblies IIC 50 (45 field) — when the CBC governs the building, as with a unit in a Group R multifamily building. The CRC imposes no STC or IIC rating on a wall between a house and its attached ADU: the residential code’s body chapters contain no sound rating (the model code’s sound appendix, Appendix AK at STC/IIC 45, is not mandatory unless an adopting ordinance references it). So “California requires STC 50 between an ADU and the main house” is not a blanket rule — and “there is no sound rule at all” is equally wrong, because CBC-path projects carry section 1206 and some cities enforce sound standards locally (Los Angeles’ LAMC section 91.1206 expressly includes dwellings). Practical upshot: on a CRC attached ADU, the rating the shared assembly must carry is its fire rating (next topic), and sound performance should be designed deliberately on top of it — a 1-hour assembly is not automatically an STC 50 assembly; only a tested assembly (for example, from the Gypsum Association’s design manual) proves both. Never accept an STC figure quoted for a wall build-up without a tested-assembly citation.

Fire-resistance design (Topic 170)

Fire design for an ADU turns on one measurement: fire separation distance — 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. Under CRC Table R302.1(1) (unsprinklered) and (2) (sprinklered):

Now place the state-law ADU on that table: a new detached ADU at the standard 4-ft side or rear setback sits squarely in the 1-hour + limited-openings band unless the building is sprinklered. A full window wall on the setback elevation fails the table twice at once — this is the fire fact that must reach the floor plan before the glass is drawn. Between two dwelling units in a two-family dwelling, wall and floor assemblies must be at least 1-hour (one-half hour where sprinklered throughout) under CRC section R302.3 as verified in the 2022 CRC — the 2025 CRC restructures that section, so confirm its current subsection layout at plan check rather than relying on any secondary summary, this one included. How an attached ADU plus house is classified for that rule is a plan-check determination; design the shared assembly to the 1-hour line and confirm at intake. Garage separations (where a garage remains a garage beside the unit) follow CRC sections R302.5–R302.6 as covered in our garage conversion coverage. Sprinklers themselves follow the primary dwelling: an ADU needs sprinklers only when the primary residence is required to have them, and building an ADU never triggers sprinklers in the existing house (sections 66314(d)(12), 66323(d)) — full treatment in our fire sprinkler guide. In a Fire Hazard Severity Zone or designated WUI area, CRC section R337 (2022 CRC cite; the 2025 cycle created a standalone California WUI Code in Title 24 Part 7 — confirm the current home of these rules) adds Class A roofing and ignition-resistant exterior construction — zone-triggered only, never statewide.

Roof and Exterior

Roof architecture (Topic 171)

May a city make your ADU’s roof match the house? The precise answer has three lanes. Ordinance track: section 66314(b)(1) lets a city impose objective standards including architectural review, and HCD’s own examples of objective design standards include “specified colors or materials” — so an objective, pre-published roof standard (a stated pitch range, a stated material palette) may bind an ordinance-track ADU. Subjective track: a “match the main house / blend with the neighborhood” test that needs an official’s judgment fails the section 66313(i) objectivity definition and may not be imposed. State-exempt track: section 66323 units and conversions are outside local design standards altogether. State law itself contemplates pitch alignment without requiring it: near transit, a detached ADU’s height allowance grows by an additional 2 feet to accommodate a roof pitch aligned with the primary dwelling’s (sections 66321(b)(4)(B), 66323(a)(2)(B)). No state source sets a roof form — gable, hip, shed, and flat are all lawful forms inside the height limits. The regulated roof items are performance items: cool roofs on the prescriptive path (section 150.1(c)11) — steep-sloped roofs over 2:12 in Climate Zones 10–15 need aged solar reflectance at least 0.20, thermal emittance at least 0.75, or SRI at least 16; low-sloped roofs only in Zones 13 and 15 (0.63 / 0.75 / SRI 75) — so a steep ADU roof in Zones 6–9 carries no prescriptive cool-roof duty, and additions of 300 sq ft or less are excepted. Roof fire classification: at least Class A in a fire hazard severity zone, at least Class C elsewhere (CRC section R902). And two “ready” rules that are routinely merged: solar-ready does not apply to an ADU on an existing lot (CEC guidance on section 110.10(a)1 — it addresses new homes in subdivisions of ten or more), PV is required only for newly constructed detached ADUs (section 150.1(c)14), and battery-ready provisions (section 150.0(s)) are a separate rule that does reach new detached ADUs — see our electrical and HVAC guide.

Exterior materials (Topic 172)

State law does not pick your ADU’s siding. No source read requires or bans stucco, fiber-cement, wood, metal, or vinyl as such. Materials acquire legal force through exactly three lanes: (1) fire distance — Table R302.1 rates the wall assembly, and the rating belongs to the tested assembly, not the cladding brand; (2) wildfire zone — in a WUI/FHSZ area, exterior walls must be noncombustible, ignition-resistant, heavy timber, log wall, a tested SFM 12-7A-1 assembly, backed by 5/8-in. Type X sheathing, or the exterior portion of a 1-hour assembly (CRC R337.7, 2022 cite); and (3) local objective standards or HOA rules — a specified-materials standard on the ordinance track, or a reasonable HOA architectural restriction that does not unreasonably increase cost or effectively prohibit the ADU (Civil Code section 714.3 family; section 4751). Weather performance is its own verified lane — water-resistive barrier, flashing, stucco’s two layers and weep screed, and the 6-in./2-in. decay clearances are in our construction and foundations guide. “The ADU must use the same siding as the house by law” is false as a state claim; at most it is a local objective standard or an HOA rule, each with the limits above.

Water and Land

Drainage design (Topic 173)

There is no ADU-specific drainage code — an ADU lot is graded under the same rule as any California dwelling (CRC section R401.3; CBC section 1804.4, the same grade stated two ways): lots are graded to drain surface water away from foundation walls, falling at least 6 inches within the first 10 feet (5%), with surface drainage diverted to a storm sewer conveyance or other approved point of collection that does not create a hazard. Where lot lines, walls, slopes, or other barriers prohibit that fall, drains or swales must be built to ensure drainage away from the structure; impervious surfaces within 10 ft of the foundation slope at least 2% away, and CBC-path swales within 10 ft slope at least 2%. The plans must show how grading or the drainage system keeps water from entering the building (CRC section R300.2; CALGreen section 4.106.3). Foundation drains are a separate, conditional rule for below-grade space (CRC R405.1) — not a general slab-on-grade requirement. Pitfall: grading the ADU pad flat and planning to “fix it with a French drain” — the code order is grade first; drains and swales are the exception where grade is physically prohibited. On the small lots where ADUs live, whether 10 ft of fall even exists is a concept-stage survey question, not a framing-stage discovery.

Stormwater routing (Topic 174)

State ADU law is silent on stormwater — no retention, low-impact-development, or runoff-volume standard for ADUs exists in sections 66310–66342, and the Handbook’s stormwater count is zero. Routing is set by two layers, in this order: the building code’s drainage rule above (get water away from the building, to an approved point of collection), and then your city’s LID ordinance, adopted under the regional stormwater (MS4) permit, whose thresholds and design volumes are local law that varies by city. Los Angeles illustrates the shape (LAMC section 64.72): full LID standards attach to defined Priority Development projects — thresholds a typical single-lot ADU project falls below — with the city’s design volume defined from a 0.75-in. / 85th-percentile storm. Publishable conclusion, carefully scoped: “ADUs must retain the 85th-percentile storm on site” is not a California rule, and “ADUs are exempt from stormwater rules” is equally false — R401.3 always applies, and a large or multifamily project can cross a local Priority threshold. Downspouts, swales, dry wells, and rain gardens are design choices made against the collection rule and local guidance; infiltration tight against a foundation or a neighbor’s wall fails the hazard check regardless of the device chosen. Confirm your project’s category under your own city’s ordinance at intake.

Landscaping (Topic 175)

State ADU law sets no planting, tree, turf, or irrigation standard for an ADU. “Landscape” appears in section 66314(b)(1) only as a permissible category of local objective standard — and landscape objective standards do not apply to conversions, and may not be imposed on section 66323 units beyond that section itself. The landscaping code that can actually reach an ADU project is MWELO, the Model Water Efficient Landscape Ordinance (23 CCR sections 490–495, under Government Code sections 65591–65599), and its trigger is landscape area, not the ADU: it applies to new construction projects with an aggregate landscape area of 500 sq ft or more, and rehabilitated landscape projects of 2,500 sq ft or more (23 CCR sections 491.2, 492). Between 500 and 2,500 sq ft the applicant may choose the prescriptive or performance path; at 2,500 or more, performance is required. Prescriptive examples worth knowing: compost incorporated at 4 cubic yards per 1,000 sq ft, and turfgrass limited to 25% of the landscape area in residential landscapes — a prescriptive-option rule, not a universal cap. So building an ADU alone triggers nothing; an ADU project packaged with 500 sq ft or more of new landscape does — and “ADUs are exempt from MWELO” is not a verified claim anywhere. MWELO is enforced through your city’s adopted ordinance, which sets the submittal package; protected-tree rules are likewise local (see our eligibility guide). One exemption does not swallow the other: a conversion’s freedom from city design landscape standards says nothing about MWELO’s application to the landscape work itself.

The Design Sequence That Avoids Redesign

1. Track first: is the unit a section 66323 unit, a conversion, or an ordinance-track ADU? That answer decides whether local design standards exist for this project at all.
2. Exemption check: woodframe, two stories or fewer, conventional framing throughout? If any portion deviates, the licensee for that portion is budgeted from day one (sections 5537(b) / 6737.1(b)).
3. Hard geometry on the concept: 4-ft setbacks, the fire table at that distance (1-hour wall, 25% openings), the egress door’s path to a public way, and an escape window wall in every bedroom — before the pretty version is drawn.
4. Room math on the plan: 70 sq ft / 7 ft / 7-ft ceilings, 8% glazing and 4% openable per habitable room, net-clear egress — not nominal sizes.
5. Wet rooms to California numbers: CPC 15/30/24-in. clearances, the 1,024-sq-in. shower, 50/20-cfm exhaust — and the ADU-vs-JADU sanitation decision made explicitly.
6. Energy classification: new construction, addition, or alteration decides the fenestration budget, whole-house fan, cool roof, and PV questions — by climate zone, on the CF1R.
7. Access by choice, on paper: if aging-in-place matters, name the voluntary reference standard (11B/A117.1 dimensions, wall blocking, single-story living) in the plans — no law will add it later.
8. Water off the lot: 6 in. of fall in 10 ft or the drains/swales exception, every new impervious surface at 2%, an approved collection point, and the city’s LID category confirmed.

Run in that order and design stops being the phase where ADU projects lose months. When you want the lot-specific answers before a designer is engaged, 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): Business and Professions Code sections 5500.1, 5536, 5536.1, 5536.22, 5536.4, 5537, 5537.1, 5537.2, 5538, 5800–5812, 6731, 6735, 6737, 6737.1, 6745 (leginfo / California Architects Board / BPELSG texts); Government Code sections 65008, 66313, 66314, 66321, 66323, 66333, 65591–65599; Civil Code sections 714.3, 4751; Health and Safety Code section 17959.6; 42 U.S.C. section 3604(f)(3)(C) and the HUD/DOJ Joint Statement on FHA design and construction; ADA National Network scope guidance; California Residential Code (Title 24, Part 2.5) sections R106.1, R300.2, R302.1, R302.3 (2022 formulation; 2025 restructure flagged in text), R313/R305.1, R312/R304, R318–R319/R311–R310, R325/R303, R326–R327/R306–R307, R337 and R902 (2022 cites; WUI material’s 2025 home in Title 24 Part 7 to be confirmed), R401.3, R405.1; California Building Code sections 1206, 1208.3, 1208.4, 1804.4, and Chapter 11A / 11B-233.3 scoping; California Plumbing Code sections 402.5, 408.5–408.7 (2022 CPC numbering; figures as stated); California Energy Code (Title 24, Part 6, 2025) sections 110.10, 150.0(o), 150.0(q), 150.0(s), 150.1(c)3, 150.1(c)11, 150.1(c)12, 150.1(c)14, 150.2(a), Table 150.1-A, with the CEC’s 2025 Energy Code ADU FAQs; CALGreen section 4.106.3; 23 CCR sections 491.2, 492 (MWELO); LAMC sections 64.72, 91.1206 (Los Angeles examples, labeled local); NKBA Kitchen Planning Guidelines (design guidance, attributed); HCD Accessory Dwelling Unit Handbook, March 2026 edition (objective/subjective standards, bedrooms, efficiency kitchen, conversions, accessibility “Maybe,” HOA, preapproved plans). Where this guide describes design practice or voluntary accessibility references, it is labeled as such in the text; edition-specific citations are labeled by edition, and local ordinance standards are confirmed with the project’s city. This page is part of the California Accessory Dwelling Unit Handbook on this site (Planning and Design, Questions 301–350). Related: ADU Floor Plans · Construction & Foundations · Electrical & HVAC · Plumbing & Utilities · Property Eligibility · ADU FAQs