ADU size, setback and height limits in California (2026)

Law verified as of 10 September 2026 against the text published by the California Legislature. Every figure below is quoted to its section.

The short answer

Your city sets ADU rules, but it does not set them freely. State law fixes floors it cannot go below. Three of them decide most projects:

Standard	The floor your city cannot go below	Statute
Maximum size	850 sq ft interior livable space; 1,000 sq ft if the ADU has more than one bedroom	§ 66321(b)(2)
Size protected from lot coverage / FAR / open space / front setback / minimum lot size	800 sq ft with 4 ft side and rear setbacks	§ 66321(b)(3)
Height, detached	16 ft	§ 66321(b)(4)(A)
Height, detached, within ½ mile walking distance of a major transit stop or high-quality transit corridor	18 ft, plus 2 ft to align the roof pitch with the house	§ 66321(b)(4)(B)
Height, detached, on a lot with an existing or proposed multifamily multistory dwelling	18 ft	§ 66321(b)(4)(C)
Height, attached to the primary dwelling	25 ft, or the local limit for the primary dwelling, whichever is lower	§ 66321(b)(4)(D)
Decision deadline	60 days from a complete application, then deemed approved	§ 66317(a)(3)
Completeness determination	15 business days, then deemed complete	§ 66317(a)(2)

Both sections were amended by SB 543, Stats. 2025, ch. 520, effective 1 January 2026.

The 800 versus 850 confusion, and why it matters

Almost every ADU guide states a single size number. The statute has two, doing different jobs, and conflating them is what produces bad advice.

850 square feet — and 1,000 square feet where the ADU has more than one bedroom — is the floor on a maximum-size ordinance. Section 66321(b)(2) says a local agency shall not establish a maximum square footage requirement below those figures. Your city can be more generous. It cannot be stingier.

800 square feet is a different protection. Section 66321(b)(3) bars a city from applying lot coverage, floor area ratio, open space, front setback or minimum lot size rules — or demanding a separate zoning clearance, or sizing your ADU as a percentage of the house — in any way 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 to be constructed in compliance with all other local development standards.”

The practical consequence: a small or awkward lot that fails the city’s lot-coverage test is not automatically disqualified. If the city’s standards would block an 800 sq ft ADU with four-foot side and rear setbacks, those standards do not get to block it.

Front setbacks are not the same as side and rear

This is the detail most often reported wrongly. The four-foot figure in § 66321(b)(3) is expressly side and rear. Front setbacks appear in the same subdivision, but as one of the standards that cannot be applied so as to prevent that 800 sq ft ADU — not as a dimension the state fixes for you.

In practice, read your local front setback and test it against the 800 sq ft protection rather than assuming either that it is waived or that it controls. On a deep lot it rarely binds. On a shallow one it is often the constraint that decides where the unit goes.

What the deadlines actually give you

Section 66317 is more useful to a homeowner than it looks, because it converts vague municipal slowness into dates.

1. 15 business days — the agency must determine whether your application is complete and tell you in writing. If it does not, § 66317(a)(2)(F) deems the application complete.
2. The incompleteness list is binding. If the agency says your application is incomplete, it must give you the list of missing items and how to cure them — and under § 66317(a)(2)(D) it cannot later demand an item that was not on that list.
3. 60 days — from a complete application, the agency must approve or deny. Under § 66317(a)(3), if it does neither, the application is deemed approved.
4. A denial has to be specific. Section 66317(b) requires a full set of written comments listing what is defective or deficient and describing how you can remedy it.
5. Appeals get 60 business days for a final written determination, under § 66317(d)(2). Being able to appeal to both the planning commission and the governing body does not extend that period.

The 60-day clock tolls if you request a delay, and the agency may hold an ADU decision if you filed it together with an application for a new primary dwelling. Those are the exceptions the statute names.

Why your guide probably cites the wrong law

California ADU law used to sit at Government Code § 65852.2. SB 477 (Stats. 2024, ch. 7) moved it into a new Chapter 13 of the Planning and Zoning Law, §§ 66310–66342. Query § 65852.2 on the Legislature’s site today and it returns no section text.

A very large share of ADU pages — including pages that rank well — still cite § 65852.2 and quote figures that predate SB 543. If a page cites the repealed section, it has not been reviewed since 2024, and its numbers should be checked rather than trusted.

The state floor is not the whole answer

Everything above is what your city cannot do. What your city does do sits on top of it, and in this region it varies more than most homeowners expect. Agoura Hills, Calabasas, Westlake Village, Hidden Hills, Malibu and Thousand Oaks each run their own building department with its own ordinance, submittal format and queue. Woodland Hills, Tarzana, Encino and Sherman Oaks are City of Los Angeles and go through LADBS. Unincorporated pockets go to Los Angeles County.

Fire hazard severity zones, hillside ordinances, Coastal Zone jurisdiction, septic capacity and utility connections all sit outside the state ADU floors and can be the actual constraint on a given lot. The state statute tells you the city cannot say no on size, setback or height grounds within these limits. It does not tell you your lot works.

That is a site-specific question, and it is worth answering before you pay anyone to draw anything.