ADU Size and Setback Rules in California

How big can your ADU be, how close to the property line can it sit, and how tall can it go? California law sets floors your city cannot go below — and one pathway, the 800-square-foot guarantee, that overrides most local size traps.

Every ADU design starts with the same three questions: size, setbacks, and height. Get them wrong on paper and plan check sends you back; get them right and most lots in Southern California can carry a legal second home. This guide walks through the state rules exactly as the California Department of Housing and Community Development (HCD) states them in its ADU Handbook (March 2026 edition), with the Government Code section for each rule — plus the places where there is no statewide number and your city’s ordinance or the building code decides instead. For the law behind these numbers, see our California ADU law hub; for how they get checked, see the ADU permit process.

The Numbers at a Glance

A city may always be more generous than these floors — many allow the full 1,200 sq ft detached maximum by ordinance (Gov. Code §66325(b)). The floors are what a city may not go below.

The 800-Square-Foot Guarantee: The Most Important Number in ADU Law

If you remember one rule, make it this one. Government Code Section 66321(b)(3) prohibits a local agency from imposing any minimum or maximum size rule, percentage-of-primary-dwelling rule, or limit on lot coverage, floor area ratio, open space, front setbacks, or minimum lot size “that does not permit at least an 800 square foot accessory dwelling unit with four-foot side and rear yard setbacks to be constructed.” In plain English: whatever your lot’s coverage math, FAR math, or front-yard rules say, the city must still let you build an 800-square-foot ADU at four-foot side and rear setbacks, up to the height limit that applies to your lot.

What to verify: which pathway your design uses. There are two. A Section 66314 ordinance unit is designed under your city’s ADU ordinance, where objective local standards may apply (subject to the floors in this guide). A Section 66323 unit — HCD calls these “66323 Units” — is one of the state-guaranteed categories in Section 66323, and a city may not impose any objective development or design standard on it beyond what Section 66323 itself lists (Gov. Code §66323(b)). The detached 66323 category is one new-construction ADU of up to 800 square feet of livable space at four-foot side and rear setbacks (§66323(a)(2)). When a local rule blocks a bigger ordinance-unit design, the 800 pathway is often the fallback that still pencils.

Pitfall: the guarantee protects an 800-square-foot unit — not the 1,000- or 1,200-square-foot design in your first sketch. Homeowners who design to the maximum first, then discover a coverage or FAR conflict, lose weeks redrawing. We run the 800 test and the maximum test side by side during feasibility, before design money is spent.

How Big Can an ADU Be? Maximum Sizes, Explained

Detached ADUs

A city may set a maximum size by ordinance, but the cap cannot be lower than 850 square feet of interior livable space for a studio or one-bedroom ADU, or 1,000 square feet for an ADU with more than one bedroom (Gov. Code §66321(b)(2)). If your city has no compliant ADU ordinance, the state fallback maximum for a new detached ADU is 1,200 square feet (Gov. Code §66314(d)(5)). A city may also choose to allow more than 1,200 square feet — state law does not limit a city from being less restrictive (§66325(b)).

Attached ADUs

A city may cap an attached ADU at a percentage of the existing primary dwelling — commonly 50 percent — but only if the cap does not push the allowed size below the same 850 / 1,000-square-foot floors (§§66314(d), 66321(b)(2)). Under the no-ordinance fallback, an attached ADU may be up to 50 percent of the primary dwelling’s floor area (§66314(d)(4)), and at least 800 square feet must be allowed (§66321(b)(3)).

800 versus 1,200 square feet — which one applies to you?

Both numbers are real; they answer different questions. 800 is the size the state guarantees on essentially every eligible lot, overriding coverage, FAR, open-space, and front-setback conflicts. 1,200 is the largest detached size in the state fallback rules and the cap many city ordinances adopt. Between them sit the 850 / 1,000 floors on whatever maximum your city sets. The practical sequence: check your city’s ordinance maximum first, then test the design against coverage, FAR, setbacks, and height — and if a local standard blocks the larger design, drop to the guaranteed 800 pathway rather than abandoning the project.

Pitfall: quoting “1,200 square feet allowed” from a headline without checking the ordinance pathway is the most common sizing mistake we see. The other is forgetting that a two-bedroom design raises the floor a city must allow from 850 to 1,000 square feet — bedroom count changes the legal math, not just the floor plan.

What Is the Smallest ADU California Allows?

There is no statewide minimum ADU size in square feet beyond this: a local agency “must not establish by ordinance a minimum square footage requirement that prohibits the development of an efficiency unit” as defined in Health and Safety Code Section 17958.1, and an efficiency unit may be as small as 150 square feet of floor area (HCD ADU Handbook, March 2026). An efficiency unit still has to work as a home — the state definition contemplates living space with a separate closet, a kitchen sink, cooking appliance and refrigerator, and a separate bathroom.

What to verify: building-code room minimums still apply inside the unit (habitable-room sizes, ceiling heights, light and ventilation under the California Residential Code). A 150-square-foot unit is legal in principle; most homeowners land between 350 and 750 square feet because kitchen, bath, and circulation space have to fit somewhere.

Pitfall: a city handout stating a 400- or 500-square-foot “minimum ADU size” is stating a rule state law preempts if it would prohibit an efficiency unit. Ask the planner to cite the ordinance section, and compare it against Section 66321 and the HCD Handbook before redesigning.

How ADU Square Footage Is Measured: Interior Livable Space

Since the SB 543 changes took effect January 1, 2026, the size numbers in state ADU law are measured in interior livable space (ILS). Government Code Section 66313(e) defines livable space as “a space in a dwelling intended for human habitation, including living, sleeping, eating, cooking, or sanitation.” HCD spells out what that includes: bedrooms, bathrooms, kitchens, laundry rooms, closets, and interior stairs count toward ILS.

Does a garage count toward ADU size?

A garage is not living space. The related statutory term “living area” is defined as the interior habitable area of a dwelling unit, including basements and attics, but not including a garage or any accessory structure (Gov. Code §66313(f)). So an attached garage that stays a garage does not inflate your ADU’s measured size — and a garage you convert becomes livable space once it is living space. Note the flip side for fees: school and impact-fee calculations have their own area definitions (see our law hub’s school-fee section), so the number on your plan set can be described differently by different agencies.

Do covered patios count?

State ADU law measures the dwelling unit’s interior livable space. An open or covered patio outside the unit’s exterior walls is outdoor space, not interior livable space — but whether a patio counts toward lot coverage or a city’s floor-area definition is a local-ordinance question, and cities answer it differently. Enclosing a patio later changes the answer: enclosed, conditioned living space is livable space.

Pitfall: mixing measurement systems is how projects get mis-sized. A plan drawn to a city’s old “gross floor area” habit — measuring to outside wall faces, or counting the garage — can show a different number than the ILS figure state law uses. Confirm, in writing in the plan notes, that the size shown is interior livable space as Section 66313(e) defines it.

Setbacks: The Four-Foot Rule and Its Exceptions

Side and rear setbacks

For a new attached or detached ADU, a local agency may require a setback of no more than four feet from the side and rear lot lines (Gov. Code §66314(d)(7)). A city may require less, or none — it may not require more. HCD adds one geographic caveat: additional setbacks may be required in the Coastal Zone if a certified Local Coastal Program requires them (Gov. Code §66329).

Conversions: no setback at all

No setback shall be required for an ADU or JADU created within an existing living area or accessory structure, or for an ADU created in a new structure in the same location and to the same dimensions as an existing structure (Gov. Code §66314(d)(7)). That is why a garage sitting two feet from the property line can usually convert in place: the conversion inherits the existing footprint. The 66323 conversion category states the same idea differently — side and rear setbacks need only be sufficient for fire and safety (§66323(a)(1)(C)) — and a conversion of an existing accessory structure may expand by up to 150 square feet beyond the structure’s dimensions, limited to accommodating ingress and egress (§66323(a)(1)(A)). Conversions are also not subject to unit-size requirements (§66323(a)(1)) — which is why an existing structure larger than the usual caps can still convert. Full detail lives in our garage conversion guide.

Can an ADU go in the front yard?

Sometimes — and state law decides when a city must allow it. A city may apply its front-yard setback to ADUs, but a front setback cannot preclude an ADU of at least 800 square feet, even if that unit would sit partially or wholly within the front setback (Gov. Code §66321(b)(3)). The pathway matters:

• Section 66314 ordinance unit: the city may require you to build in the backyard instead — if an 800-square-foot unit can actually be built there. If it cannot, the city must allow the unit within the front setback as necessary (§66314(b)(1), per HCD).
• Section 66323 unit: the city cannot impose local standards, including front setbacks, at all, and may not require the unit to be built in an alternate location (§66323(b)). HCD’s own summary table for the detached 66323 unit states it plainly: “May be in front setback.”

Pitfall: front-yard placement is the placement most likely to draw neighbor attention and design-standard questions even when it is legal. Verify the pathway (66314 vs. 66323) with the planning counter before drawing a front-yard design — the two pathways give opposite answers on whether the city can redirect you to the backyard.

Height: 16, 18 (+2), and 25 Feet

State ADU law contains no height limit of its own; it limits how low a local height limit may be. A local agency may not impose a height limit less than the following (Gov. Code §66321(b)(4)):

• Detached, base case: 16 feet on a lot with a proposed or existing single-family or multifamily dwelling (§66321(b)(4)(A)).
• Detached, near transit: 18 feet when the lot is within one-half mile walking distance of a major transit stop or a high-quality transit corridor — plus an additional 2 feet to accommodate a roof pitch that aligns with the primary dwelling’s roof pitch (§66321(b)(4)(B)). This is the source of the “18 or 20 feet” figures for the state-guaranteed detached unit.
• Detached, multistory multifamily lot: 18 feet on a lot with an existing or proposed multifamily, multistory dwelling (§66321(b)(4)(C)).
• Attached: 25 feet, or the height limit that applies to the primary dwelling in the local zoning ordinance, whichever is lower (§66321(b)(4)(D)).

Two-story ADUs

If a detached two-story ADU can be built within the height allowances above while complying with the building code, a city cannot deny it merely because the underlying zoning restricts the primary dwelling to one story (Gov. Code §§66321(b)(4)(D), 66314(d)(8)). A local agency must allow at least two stories where the height limit and building standards can accommodate them; it is not required to allow an ADU to exceed two stories. On a standard lot, two stories usually means designing to the 18- or 20-foot transit allowance, an attached design at up to 25 feet, or a city ordinance more generous than the state floor — a true two-story detached unit inside 16 feet is rarely workable once floor structure and roof pitch are counted. See our second-story ADU guide for how this plays out in design.

What to verify: how your city measures height (to the midpoint of a sloped roof, to the ridge, or to the top of the structure). State law sets the number; the measurement method is in the local ordinance and can change whether a design complies. Also verify transit proximity against the state’s own data — HCD points applicants to the California State Geoportal’s high-quality transit corridor mapping rather than a guess from a bus map.

Lot Coverage, Floor Area Ratio, and Open Space

These three standards are where legal ADUs most often get wrongly denied, so HCD answers each one directly:

• Lot coverage and FAR: “Limits on lot coverage or any floor area ratio requirements cannot preclude the creation of an ADU of at least 800 square feet of interior livable space that maintains four-foot rear and side setbacks.” Coverage and FAR can be applied to other (larger, ordinance-pathway) ADUs, but they must not unreasonably restrict ADU creation (Gov. Code §§66321(b)(3), 66311).
• Open space: a city’s open-space requirements may not be used to deny an ADU of at least 800 square feet of ILS with four-foot setbacks (§66321(b)(3)), and 66323 Units are not subject to local open-space and landscaping requirements at all (§66323(b)).
• Minimum lot size: prohibited outright for ADUs (Gov. Code §66314(b)(1)) — there is no lot too small, by size rule alone, to consider an ADU.

Pitfall: coverage and FAR are usually calculated against all structures on the lot — the main house, the ADU, and sometimes covered patios, decks, and accessory structures, per the local definition. A lot that “looks” half empty can already be near its coverage cap on paper. The calculation belongs in the feasibility study, with the city’s own definitions, before a footprint is promised.

Fire Separation: The Rule That Is Not in ADU Law

Homeowners often ask ADU law a question ADU law does not answer. HCD is explicit: “State ADU Law does not address the distance between an ADU and other structures on a lot.” A city may apply objective standards to some ADUs, but a minimum-distance rule may not be applied if it would unreasonably restrict ADU creation — unless it is a requirement of a Building or Fire Code (Gov. Code §66314(d)(8)). In practice, the separation rules that bind your design come from the California Residential Code (CRC), Section R302.1, measured by fire separation distance — the distance from the building face to the lot line, to a street/alley centerline, or to an imaginary line between two buildings on the same lot.

Under CRC Table R302.1(1) (dwellings without an automatic sprinkler system):

• Walls: an exterior wall with a fire separation distance under 5 feet must be 1-hour fire-resistance rated; at 5 feet or more, no rating is required by that table.
• Openings (windows and doors): not allowed at under 3 feet; limited to 25 percent of the wall area at 3 feet to under 5 feet; unlimited at 5 feet or more.
• Projections (eaves and overhangs): not allowed under 2 feet; at 2 feet to under 5 feet, projections must be 1-hour rated on the underside (or heavy timber / fire-retardant-treated wood); unrated at 5 feet or more.

A sprinklered dwelling is measured under the companion Table R302.1(2), which relaxes several of these thresholds — one more reason the sprinkler question (see our fire sprinkler guide) belongs in early design, not at plan check. Local amendments exist, so treat the table as the state baseline and confirm your city’s adopted version.

What this means at a four-foot setback: an ADU wall four feet from the property line is perfectly legal under ADU law, and under the CRC it will typically be a 1-hour rated wall with window area limited to 25 percent of that wall. Design the window plan for that wall accordingly — large picture windows belong on the yard-facing walls, not the setback wall.

Verify Your Property Line Before You Trust a Setback

Every number in this guide is measured from the property line — which makes the line itself the most important “standard” on the project. A four-foot setback measured from the wrong line is not a four-foot setback. Fences are the usual trap: fences are placed for convenience, replaced over decades, and routinely sit inches — sometimes feet — off the legal line, on one side or the other.

What to verify: start with your parcel map and any prior survey; check whether fences, walls, and the neighbor’s structures match the recorded dimensions; and if the line is disputed, unmarked, or the design sits close to the minimum setback, hire a licensed land surveyor to establish and mark the line before plans are drawn. The cost of a survey is small next to the cost of moving a foundation — or a neighbor dispute after the slab is poured. Our feasibility report flags line-verification risk on every lot; corner and hillside lots (below) get flagged most often.

Pitfall: building to a fence line that turns out to be inside your neighbor’s parcel can mean redesign, removal, or an encroachment fight — and “the fence was already there” is not a permit defense. When the ADU wall will sit at or near the four-foot minimum, verify first.

Corner Lots, Eaves, and Windows at the Setback Line

Corner lots: which yard is “front”?

A corner lot has two street frontages, and that changes the setback geometry. State law caps side and rear setbacks at four feet and separately protects the 800 pathway against front-setback rules — but which yard your city treats as the “front” yard on a corner lot is decided by your local zoning definitions, not by state ADU law, and cities define it differently (commonly by the lot’s narrower street frontage, the primary home’s orientation, or the address street). Get the city’s corner-lot determination for your parcel in writing during feasibility: it decides which lot lines get the four-foot treatment and which get the front-yard treatment, and it can change the buildable envelope more than any other single ruling on a corner lot.

Eaves and roof overhangs

ADU law’s four-foot setback is measured to the building; how far a roof eave may project into that setback is a separate question answered by your local ordinance and the building code — there is no single statewide eave number in ADU law. The fire code does set a floor: under CRC Table R302.1(1), projections are not allowed within 2 feet of the fire separation line and must be fire-protected on the underside between 2 and 5 feet. Many local ordinances separately cap how far an eave may extend into a required setback. Verify both before the roof is designed — an eave drawn to the property line is a plan-check correction waiting to happen.

Windows on a wall at the setback

State ADU law does not prohibit windows on a setback wall, and it does not impose a privacy-window rule. What limits windows there is the fire code (above): no openings under 3 feet of fire separation distance, and a 25-percent-of-wall-area cap from 3 to under 5 feet, under the non-sprinklered table. Some cities add objective privacy standards (sill heights, obscured glazing) for ordinance-pathway ADUs; those are local rules, they must be objective, and they cannot be applied to 66323 Units beyond what Section 66323 allows. Check the local standard, design the rated wall and window budget together, and put the big glass on walls with five feet or more of separation, where openings are unlimited.

Hillside Lots and Trees in the Building Envelope

Hillside grading

State ADU law sets no statewide grading limit, cubic-yard cap, or slope threshold for ADUs. Hillside projects are governed by local hillside and grading ordinances, geotechnical (soils) reporting, and retaining-wall rules — layered on top of the state size, setback, and height floors, which still apply. What changes on a hillside is cost and engineering, not eligibility: stepped foundations, retaining walls, drainage, and export/import of soil drive the budget, and a soils report is commonly required before the city will accept the design as buildable. Treat any grading number quoted without a soils report and the city’s hillside overlay in hand as a guess.

Trees inside the footprint

There is likewise no statewide ADU rule that either protects a tree in your building envelope or lets you remove one. Protected-tree rules are local — many Southern California cities protect specified native species (coast live oak is the common example) or trees above a trunk-diameter threshold, and some require permits, replacement planting, or root-protection measures during construction. A tree can also matter even if it is not removed: root zones influence foundation placement and trenching routes. Inventory the trees on and near the envelope during feasibility, check the city’s tree ordinance before the footprint is fixed, and never assume a tree can come out because the ADU itself is by-right — the ADU approval and the tree approval are separate questions.

Pitfall (both): hillside and tree constraints surface late and expensively when they are discovered at plan check instead of feasibility. Both are site facts, not design preferences — verify them first, then design inside them.

Before You Design: A Size-and-Setback Checklist

1. Pull your city’s ADU ordinance and note its maximum size, height, and any corner-lot and eave rules — then compare each against the state floors in this guide.
2. Run both tests: the city-ordinance maximum and the guaranteed 800-square-foot pathway.
3. Confirm the size on your sketch is interior livable space (Section 66313(e)) — not gross area, and not counting a garage that stays a garage.
4. Verify the property lines the setbacks will be measured from; survey if the line is unclear or the design hugs the minimum.
5. Check the fire separation for every wall near a lot line or the main house, and budget rated walls and window limits into the design.
6. Check transit proximity (for the 18/20-foot height allowance), coastal and hillside overlays, and protected trees.
7. If a local rule appears to block the guaranteed pathway, ask for the ordinance citation and compare it with Sections 66321 and 66323 — and see our law hub for how HCD enforcement works when a city rule conflicts with state law.

Sources (verified October 2026): California Department of Housing and Community Development, Accessory Dwelling Unit Handbook, March 2026 edition including the December 2025 addendum — Size Requirements, Setbacks, and Height Requirements sections; Government Code Sections 66313, 66314, 66321, 66323; Health and Safety Code Section 17958.1; California Residential Code Section R302.1, Table R302.1(1). This page is part of the California Accessory Dwelling Unit Handbook on this site (Size and Setback Rules, Questions 201–250). Related: ADU FAQs · Unincorporated LA County ADUs · ADU Cost Guide