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How big can my ADU be?
Pick the ADU type and its size. We check it against California's state ADU law, section by section. It's free, with no sign-up.
Not legal advice; confirm with the city.
Check a size
It checks size only. Setbacks, height, your lot and your city's rules still apply. Your city may allow more.
The state's size limits
Each line cites its section on the official code, read on 31 Oct 2026.
| Section | What it says | Who sets the number |
|---|---|---|
| Gov. Code §66314(d)(5) | A detached ADU may be up to 1,200 sq ft. | The state |
| Gov. Code §66314(d)(4) | An attached ADU may be up to half the floor area of the existing home. A city can't use this to stop an 800 sq ft ADU. | The city, within the state's range |
| Gov. Code §66321(b)(3) | Lot coverage, floor area ratio, open space, front setback and minimum lot size rules can't stop an 800 sq ft ADU with 4 ft side and rear setbacks. | The state |
| Gov. Code §66313(d) | A junior ADU is up to 500 sq ft, inside the existing home (an attached garage counts as part of it, §66333(d)). | The state |
| Gov. Code §66321(b)(2)(A) | A city's size limit can't be under 850 sq ft of interior livable space. | The state |
| Gov. Code §66321(b)(2)(B) | A city's size limit can't be under 1,000 sq ft for an ADU with more than 1 bedroom. | The state |
| Gov. Code §66311.5(c)(1) | No impact fees for an ADU of 750 sq ft of interior livable space or less, or a junior ADU of 500 sq ft or less. Above that, in proportion to the home's floor area. | The state |
| Health and Safety Code §17958.1(a) | A city's minimum size can't ban an efficiency unit. A city may allow one from 150 sq ft. | The city, within the state's range |
Setbacks, height, parking and fees are in California ADU law, explained.
Check a real lot
A California site report runs the ADU size check on the lot itself, with the home already there.
Go further with a paid plan
A paid plan runs full site reports and area scans across California.
Starter: US$39 a month for 100 credits. Run full site reports, area scans and letter drops.
Or create a free account first. No card needed.
Not legal advice; confirm with the city.