Quick answer
ADU size is the floor area of an accessory dwelling unit, usually counted as interior livable square feet, and it is the number that state and local rules cap, protect and use to set fees.
In California the limits depend on the type of ADU and the bedroom count: a junior ADU tops out at 500 sq ft, a detached ADU’s state standard at 1,200 sq ft, and no city may block an 800 sq ft unit.
Quick facts
- Usually measured as
- Interior livable space, in square feet 3
- Smallest a city must allow (CA)
- An efficiency unit 3
- Lowest maximum a city may set (CA)
- 850 sq ft; 1,000 sq ft for more than one bedroom 3
- State cap, detached (CA)
- 1,200 sq ft 2
- State cap, attached (CA)
- 50% of the existing home’s floor area 2
- Junior ADU (CA)
- 500 sq ft max 1
- Impact fee threshold (CA)
- None at 750 sq ft or less (ADU), 500 or less (JADU) 5
Size is the number every other rule hangs on
Ask what an ADU may be and the first answer is usually a square-foot figure. Size decides how many bedrooms fit, which fees apply, which approval path you get and, roughly, what the project costs.
That makes it the most-quoted ADU number and the most misquoted, because the limit depends on which kind of unit you mean and which jurisdiction you are in.
Outside California, size limits come from each city’s zoning code, sometimes a state law. Many cities use a fixed cap, a percentage of the main house, or both. The patterns are compared on ADU laws by state. The rest of this page gives California’s rules, by section.
Key takeaways
- ADU size is the floor area of the unit, usually counted as interior livable square feet.
- The limit depends on the type of ADU and, in California, on the bedroom count.
- No California city may block an 800 sq ft ADU with 4 ft side and rear setbacks 3.
- A California city’s maximum may not be below 850 sq ft, or 1,000 sq ft for more than one bedroom 3.
- The state standard caps a detached ADU at 1,200 sq ft and a junior ADU at 500 sq ft 2 1.
- At 750 sq ft or less, a California ADU carries no impact fee 5.
Why size matters to you
Size touches almost every choice. It sets how many bedrooms fit. It sets which fees apply. It shapes the rent you can ask and the money you need to borrow.
It also decides which rules you meet. In California, a small new detached unit can go down the by-right path, where the city may cap it at 800 sq ft 4. A bigger unit follows the city’s own ordinance, within the state floors.
So pick a size on purpose. Do not let the first sketch pick it for you.
How ADU square footage is measured
California’s size rules count interior livable space. The statute defines livable space as space intended for human habitation: living, sleeping, eating, cooking or sanitation 1.
Wall thickness, exterior decks, unconditioned storage and a garage are generally outside it; see the ADU living space entry.
The 50% rule for attached units uses the existing home’s floor area, and "living area" is defined separately as the interior habitable area of a dwelling, including basements and attics but not a garage or accessory structure 1.
Cities may measure differently for their own rules (gross floor area for lot coverage or floor area ratio, for example). If a plan set shows two numbers, ask which one the ADU limit applies to.
California ADU size limits by type
| Type | Minimum | Maximum | Section |
|---|---|---|---|
| Detached ADU | Cannot prohibit an efficiency unit; 800 sq ft protected | State standard 1,200 sq ft; city cap not below 850 (1,000 for 2+ bedrooms) | §§ 66314(d)(5), 66321(b) 2 3 |
| Attached ADU | Same floors | 50% of the existing home, but never enforced below 800 sq ft; city cap not below 850 / 1,000 | §§ 66314(d)(4), 66321(b) 2 3 |
| Conversion of existing space | The existing space | The existing space plus up to 150 sq ft for ingress and egress (by-right path) | § 66323(a)(1) 4 |
| By-right new detached unit | — | The city may cap it at 800 sq ft | § 66323(a)(2) 4 |
| Junior ADU | — | 500 sq ft | § 66313(d) 1 |
How to find your size limit
California’s size rules are a ladder of six numbers. The chart puts them in order, so you can see which line your plan sits near before you work through the steps.
- Pick the type. Detached, attached, conversion or junior ADU each has its own rule.
- Count the bedrooms. In California, more than one bedroom raises the lowest cap a city may set to 1,000 sq ft 3.
- Read your city’s ordinance. It sets the actual maximum within the state bounds.
- For an attached unit, measure the house. The state standard is 50% of the existing home’s floor area 2.
- Check the lot. Setbacks, height and the space left over can make the real limit smaller than the legal one.
- Check the fee line. Decide if staying at 750 sq ft or less is worth it 5.
Worked example: one bedroom or two?
A hypothetical California city whose ordinance sets the lowest maximums the state allows. Inputs: a detached ADU on a single-family lot.
| Plan | City cap the state permits | Impact fee position (§ 66311.5(c)(1)) |
|---|---|---|
| Studio, 700 sq ft | 850 sq ft (cap may not be lower) 3 | No impact fee: 750 sq ft or less 5 |
| One bedroom, 850 sq ft | 850 sq ft 3 | Impact fees may apply, proportional to the main home’s size 5 |
| Two bedrooms, 1,000 sq ft | 1,000 sq ft for more than one bedroom 3 | Proportional impact fees may apply 5 |
The decision rule: staying at or under 750 sq ft removes impact fees, and adding a second bedroom raises the lowest cap a city may set from 850 to 1,000 sq ft. Connection fees are separate and can still apply. Estimate fees with the ADU fee estimator.
Worked example: attached ADU on two houses
Two hypothetical California houses, using the 50% state standard in § 66314(d)(4) and the 800 sq ft floor in § 66321(b)(3).
| Existing house | 50% of it | What the state protects |
|---|---|---|
| 1,000 sq ft | 500 sq ft | 800 sq ft, because the floor wins 3 |
| 2,000 sq ft | 1,000 sq ft | Up to 1,000 sq ft under the state standard, within the city’s own cap 2 |
The lesson: on a small house, the 800 sq ft floor is the number to plan around. More worked cases sit on the page about ADUs joined to the house.
Size terms that get mixed up
| Term | What it measures | Used for |
|---|---|---|
| Interior livable space | Space for living, sleeping, eating, cooking or sanitation 1 | California ADU size limits and fee lines |
| Living area | Interior habitable area of a home, including basements and attics, not a garage 1 | Describing the main home |
| Floor area of the main home | The existing house | The 50% rule for attached ADUs 2 |
| Floor area ratio | Building floor area divided by lot area | A city’s bulk limit |
| Lot coverage | The share of the lot covered by buildings | A city’s footprint limit |
Costs and what drives them
Bigger usually costs more in total, but not in a straight line. Kitchens and bathrooms cost a lot for their size. So a small unit can cost more per square foot than a large one.
Fees also jump at set sizes. In California there is no impact fee at 750 sq ft or less, and above it impact fees must be in proportion to the main home 5. This page gives no price. See the cost per square foot entry and the ADU cost calculator.
Size rules outside California
Outside California, the size cap comes from each state or city. Common patterns are a fixed number of square feet, a percentage of the main house, or the smaller of the two. Some cities also tie size to lot size. Every number on this page is California’s. Check your own state and city.
Where the size number appears
- On the floor plan and site plan, often as both livable and gross area.
- On the fee schedule: the 750 sq ft line decides impact fees in California 5; see ADU fee statistics.
- On builder bids, as a cost per square foot. Divide the total by the same area each bidder used.
- On appraisals and rent estimates, where size drives value; see the rent estimator.
Small vs large: the trade-offs
Going smaller
Common mistakes
- Myth 800 sq ft is the most you can build.
- Reality No. 800 sq ft is the size California protects. Many cities allow more 3.
- Myth The size on the bid is the size the city counts.
- Reality Not always. Ask whether it is livable space or gross area.
- Myth A JADU can be as big as an ADU.
- Reality No. A California junior ADU is capped at 500 sq ft 1.
- Myth Size rules are the same everywhere.
- Reality No. They vary by state and city.
What to ask your city and designer
- What is the maximum size for my ADU type and bedroom count?
- Do you count livable space or gross floor area?
- For an attached unit, what floor area do you use for my house?
- Do lot coverage or floor area ratio limits apply here?
- What fees apply above 750 sq ft? Check with the fee estimator.
Checklist for choosing your ADU size
- Write down who will live there and how many bedrooms they need.
- Check the city maximum for that bedroom count.
- Check the lot fits it after setbacks.
- Decide whether staying at 750 sq ft or less is worth it.
- Ask the designer to show livable and gross area.
- Compare bids on the same area figure.
Size data and tools
- Size envelope calculatorWhat fits on your lot
- ADU size statisticsWhat owners actually build
- ADU size distribution research
- ADU design hub
- How size drives cost
Questions about adu size
What is the maximum size of an ADU in California?
The state standard caps a detached ADU at 1,200 square feet and an attached ADU at 50% of the existing home, and a junior ADU at 500 square feet. Cities set their own maximums, but not below 850 square feet, or 1,000 for more than one bedroom.
What is the minimum size of an ADU?
California does not set a numeric minimum; it bars cities from a minimum that prohibits an efficiency unit. Other building code rules on room size still apply.
Is 800 square feet the limit for an ADU?
No. 800 square feet is the size California protects: no local rule may prevent an 800 square foot ADU with four-foot side and rear setbacks. Many cities allow larger units.
Does a garage count toward ADU square footage?
California size rules count interior livable space, and the statutory definition of living area excludes a garage. If a garage is converted into the ADU, the converted space becomes livable space and counts.
Why does 750 square feet matter?
In California, no impact fee may be charged on an ADU of 750 square feet of interior livable space or less. Above that, impact fees must be proportional to the main home’s square footage.
How big is a typical ADU?
There is no single typical size, because it depends on the type and the lot. A California junior ADU is at most 500 square feet. Detached units under the state standard go up to 1,200. Real build data is on the ADU size statistics page.
How many bedrooms can an ADU have?
California sets no bedroom count. It sets size floors that rise with bedrooms: a city cannot cap an ADU with more than one bedroom below 1,000 square feet. The size limit and the building code decide how many rooms fit.
Can an ADU be bigger than the main house?
Usually not for an attached ADU in California, where the state standard is half the existing home, with an 800 square foot floor. A detached unit follows the city cap. Check your city’s ordinance.
What is the smallest an ADU can be?
California sets no number. It says a city may not set a minimum size that prohibits an efficiency unit. The building code still sets room sizes, so ask your building department what applies.
Sources
- California Legislature, Gov. Code § 66313 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66314, retrieved .
- California Legislature, Gov. Code § 66321 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66323 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66311.5 (formerly § 66324; renumbered by SB 543, eff. 1 Jan 2026), retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Every size and fee threshold was read in the current Chapter 13 sections on leginfo.legislature.ca.gov on 7 October 2026. Examples use hypothetical plans. Education, not legal advice.
Suggest a correction. We fix errors and say what changed.