Two questions come first for most owners: how big can the ADU be, and how tall? State law answers both with minimums that every city and county must respect. Cities may be more generous, never stricter.
LA ADU Guide is an information site. We are not a contractor, architect, lender or law firm. Rules change, so confirm everything with your building department before you spend money. This page is general information, not legal advice.
Two tracks: "state exempt" and "all other"
State law has two tracks. ABAG's July 2025 summary calls the first "State Exempt ADUs" or "66323 Units." These are the types listed in Gov. Code section 66323: conversions, one small detached unit on a single-family lot, and multifamily units. They must be approved ministerially, and local standards like lot coverage, front setbacks and design standards cannot be applied to them.
Everything else falls under the general standards in sections 66314 to 66322. Cities may set objective size and height standards for these, but only within the state limits below.
Size limits at a glance
| ADU type | What state law says |
|---|---|
| Detached, state exempt (66323) | Up to 800 square feet |
| Detached, other | City must allow at least 850 sq ft (studio or 1 bedroom) and 1,000 sq ft (2+ bedrooms) |
| Attached | Same 850 / 1,000 floor; a city may cap at a percentage of the main home, but not below those numbers |
| Conversion of existing space | Must fit in the existing or proposed home or accessory structure; an existing accessory structure may grow by up to 150 sq ft for entry and exit |
| JADU | Up to 500 sq ft, inside the single-family home (a JADU may use an attached garage) |
| Minimum ADU | As small as 150 square feet |
Source: ABAG summary, tables 2 and 4. For a city with no ADU ordinance at all, the detached limit rises to 1,200 square feet, and an attached ADU can be up to 50 percent of the main home. Los Angeles has an ordinance, so ask the city which cap it applies to your lot.
One more floor matters. Under section 66321, a city cannot use limits on lot coverage, floor area ratio, open space or minimum lot size to block an ADU of at least 800 square feet and 16 feet in height with four-foot side and rear setbacks. HCD added front setbacks to that list. See ADU setbacks and lot coverage.
Height limits at a glance
| Situation | Minimum height a city must allow |
|---|---|
| Detached ADU | 16 feet |
| Detached ADU within a half mile walk of a major transit stop or high-quality transit corridor | 18 feet, plus 2 feet to match the main home's roof pitch |
| Detached ADU on a lot with a multistory multifamily building | 18 feet |
| Attached ADU (for example over an attached garage) | 25 feet, or the zone's limit if lower |
Source: ABAG summary, tables 2 and 4. In HCD's words about the transit rule, the extra two feet allows "a height of up to 20 feet."
Where Los Angeles fell short
In November 2024, HCD reviewed the City of Los Angeles ordinance (No. 186481). Among its 17 findings:
- The ordinance limited some detached ADUs to 16 feet and did not reflect the 18-foot transit height or the extra two feet for roof pitch.
- A rule limiting ADUs to one story "prevents ADUs from being constructed above detached garages or accessory structures" that meet state height limits, HCD said, and the city should remove it.
- The ordinance's size language omitted updates to section 66321(b)(3).
Where a city ordinance conflicts with state law, state law controls. The City may have amended its ordinance since HCD's letter, and we did not verify that. Check LADBS or LA City Planning for the current text, or ask staff to cite the code section they are applying.
How to measure
Height and floor area definitions are local. Ask your building department how it measures:
- Height: to the roof peak, the midpoint of a pitched roof, or the top plate.
- Floor area: whether it counts interior walls, stairs, covered porches and attached garage space.
- Bedrooms: some caps change between one bedroom and two or more.
A few square feet of difference can decide whether your plan falls into the 800 square foot exempt category or the "other" category.
Checklist before you design
- [ ] Confirm which agency permits your lot: City of Los Angeles, unincorporated LA County, or another city.
- [ ] Decide the type: detached, attached, conversion or JADU.
- [ ] Check whether your address is within a half mile walk of a major transit stop. Ask the city for its map, since "major transit stop" has a legal definition.
- [ ] Ask how the agency measures height and floor area.
- [ ] Check fire, hillside, flood and historic overlays on your parcel, covered in ADUs in fire hazard zones.
- [ ] Ask the agency in writing which code sections it relies on.
Recent changes
The ADU law was renumbered into Gov. Code sections 66310 to 66342 effective March 25, 2024 (SB 477). Older articles cite section 65852.2. The substance of the size and height floors is largely the same, but section numbers differ. For the broader picture, read California ADU rules explained, or browse the ADU rules hub.