ADU law
What California ADU law lets you build
State law sets a floor your city can't go below. Here's what that floor is, what changed in January 2026, and which parts your city still controls.
The short version
- State law sets a minimum every California city has to allow. Your city can be more generous. It can't be stricter.
- The floor: 800 sq ft, four foot side and rear setbacks, 16 feet tall, a decision within 60 days.
- There's no hearing and no neighbor vote. Meet the written standards and it gets approved.
- Two changes took effect in January 2026: how square footage is measured, and the rules for junior ADUs.
A lot of what you'll be told about ADU rules is a city rule, and plenty of city rules are older than the state law that overrode them.
California's ADU rules live in the Government Code, sections 66310 to 66342. They set a minimum every city and county has to allow. So when someone tells you the city won't let you do that, the first question is whether the city has a choice.
The floor, in numbers
- Size
- At least 800 sq ft has to be allowed
- Side and rear setbacks
- 4 feet from the property line
- Height, detached
- 16 feet. 18 if you're within half a mile of a major transit stop.
- Height, attached
- Whatever your main house is zoned for, up to 25 feet
- Decision
- 60 days. 30 days if you use the city's pre-approved plans.
- Type of review
- Ministerial
That last row does the heavy lifting. Ministerial means a planner checks your drawings against the written standards and approves or denies. No design review board, no neighbor comment period, no vote at a Tuesday night meeting.
What changed in January 2026
- SB 543. Square footage is now measured as interior livable space, so exterior walls and stairs stop counting against you. It also gives the city 15 business days to say whether your application is complete. Miss that, and it's complete by default.
- AB 1154. A junior ADU now only requires you to live on the property if it shares a bathroom with the main house. One with its own bathroom doesn't. Junior ADUs also picked up a 30 day minimum rental term.
The completeness clock is the quiet win. The old version of this was your application sitting on a desk for six weeks before anyone mentioned a missing sheet.
Fees
- Under 750 sq ft: no impact fees. None.
- 750 sq ft and up: impact fees have to be proportional to the size of your main house, not charged as though you'd built a second one.
- Under 500 sq ft: school fees come off too.
Worth knowing before you round a floor plan up. Going from 740 to 780 sq ft buys you forty square feet and, in some cities, the whole impact fee schedule. Ask what yours charges before you settle the size.
Parking
- Within half a mile walking distance of transit, no parking can be required at all.
- Converting your garage doesn't obligate you to rebuild those spaces elsewhere on the lot.
That second one used to kill garage conversions on narrow lots, routinely.
Living there, and renting it out
You don't have to live on the property. AB 976 made that permanent, so the house and the ADU can both be rented.
Junior ADUs are the exception, and since January 2026 it's a narrower one: owner occupancy applies only if the JADU shares a bathroom with the main house.
Short stays are separate. Cities can require rental terms longer than 30 days, and most do. If your plan runs on short-term rental income, check that rule in your city first.
What your city still controls
The floor is a floor, not the whole rulebook. Still local:
- Objective design standards, meaning things like siding materials and roof pitch
- Fire setbacks, if you're in a high or very high fire hazard severity zone
- Utility connection requirements
- How fast the counter actually moves, which varies more than you'd expect
One that catches people: a Whittier mailing address doesn't mean Whittier's rules. If your lot sits outside the city limit you're under the county, and the setbacks are different.
If you want to know which rules apply at your address rather than in general, the lookup on our ADU page will tell you which jurisdiction you're actually in and hand you the planning department's phone number.
Sources
- California Government Code sections 66310 to 66342
- SB 543 (2025) and AB 1154 (2025), effective January 1, 2026
- AB 976 (2023), owner occupancy
- HCD Accessory Dwelling Unit Handbook
This is our read of the rules, not legal advice. We write these because they're the questions we get asked, and we keep them current. ADU law changes most Januaries, and every city adds its own rules on top of the state's, so before you spend money on anything here, confirm it with your city or county planning counter. Or call us and we'll confirm it for you.
More guides
Choosing
ADU, JADU, or garage conversion: which fits your lot
Four ways to add a unit, what each one costs you in constraints, and the questions that decide it before anyone draws anything.
Planning
Nine things to check before you spend a dollar
Site conditions decide more ADU budgets than design choices do. These are the ones that turn into change orders when nobody looks first.
Process
How an ADU permit actually moves
The six stages between an idea and a building permit, which ones have a legal clock on them, and where the time really goes.
Want this answered for your address?
State law is the floor. What your city allows on top of it is the part that decides your project. Tell us where you are and we'll tell you what fits.