ADU types
ADU size: how big an ADU can be
By the HyreADU research desk. Last checked .
The short answer
ADU size is set by your city within state limits. In California, a city cannot cap an ADU below 850 square feet, or 1,000 with more than one bedroom, cannot block an 800 sq ft unit with four-foot setbacks, and cannot set a minimum that bars an efficiency unit.
A junior ADU is 500 sq ft at most. At 750 sq ft or less, no impact fees apply.
| Size line (California) | What it changes | Source |
|---|---|---|
| Efficiency unit | A city may not set a minimum that bars one | Gov. Code § 66321(b)(1) |
| 500 sq ft | JADU maximum; school-fee treatment under 500 | § 66313; § 66311.5(c)(3) |
| 750 sq ft | No impact fees at or under; proportional above | § 66311.5 |
| 800 sq ft | A unit this size with 4-ft setbacks may not be blocked | § 66321(b)(3) |
| 850 / 1,000 sq ft | Lowest maximum a city may set (1,000 for more than one bedroom) | § 66321(b)(2) |
How local caps work
Cities set their own maximum ADU size, often as a fixed number or as a share of the main house. Under California law, those caps cannot go below the floors above.
Other limits, such as lot coverage and floor area ratio, can also shrink what fits, but they cannot block the 800 sq ft unit with four-foot setbacks. Read lot coverage and floor area ratio.
How size is measured
The fee rules use interior livable space, not the outside footprint. Porches, garages and some mechanical spaces may not count. Ask your city how it measures, and ask your designer to state the interior area on the plans.
Find the size your lot allows
Your lot may allow less than the city cap once setbacks and coverage are applied. Run your numbers through the size envelope calculator and the ADU square footage calculator. Our size distribution research shows what sizes permits record.
Outside California
Other states set their own limits. Montana, for example, links ADU size to the main house, and Maine sets a 190 square foot minimum. Our ADU laws by state research covers the laws we have read.
A mistake to avoid
The mistake to avoid is designing to the city cap without checking your lot. A city may allow 1,200 square feet, but setbacks, an easement or coverage may leave room for far less. Measure first, design second.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
What is the minimum size for an ADU?
California bars minimums that prohibit an efficiency unit. Building codes set room minimums.
Can an ADU be bigger than the house?
Many cities cap ADUs as a share of the main house. Check your ordinance.
Why are many ADUs 749 square feet?
Because impact fees apply above 750 sq ft in California.
Does a porch count?
Usually not toward interior livable space, but it may count toward lot coverage.
Sources
- Gov. Code § 66321 (size and height standards a city may not impose). Checked September 5, 2026.
- Gov. Code § 66311.5 (ADU impact fees, connection fees and capacity charges). Checked September 5, 2026.
- Gov. Code § 66313 (ADU and junior ADU definitions). Checked October 7, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax, lending or engineering advice.
Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.