ADUs
How big can my ADU actually be?
Four numbers get quoted at homeowners as if they were the same rule. They are not, and knowing which one applies to you is worth an entire bedroom.
There is no single statewide maximum. State law sets floors on how small your city is allowed to cap you. A compliant local ordinance cannot limit you below 850 sq ft of interior livable space for a studio or one-bedroom, or 1,000 sq ft for a unit with more than one bedroom (Gov. Code §66321(b)(2)). Separately, an 800 sq ft unit with four-foot side and rear setbacks is protected against lot-coverage, floor-area-ratio, open-space and front-setback rules (§66321(b)(3)). Your city may allow more. It cannot allow less.
The four numbers, and what each one actually protects
| Number | What it means | Statute |
|---|---|---|
| 800 sq ft | The unit size that lot coverage, floor-area ratio, open-space rules, front setbacks and percentage-of-the-main-house limits cannot be written to prevent, given four-foot side and rear setbacks | §66321(b)(3) |
| 850 sq ft | The lowest maximum a compliant local ordinance may set for a studio or one-bedroom ADU | §66321(b)(2) |
| 1,000 sq ft | The lowest maximum a compliant local ordinance may set for an ADU with more than one bedroom | §66321(b)(2) |
| 1,200 sq ft | The maximum for a new detached ADU where the local agency has NO compliant ADU ordinance. Not a universal statewide ceiling | §66314(d)(4) |
| 500 sq ft | The cap on a junior ADU, which must sit within the walls of the existing house | §66313(d) |
850 and 1,000 are not minimum sizes you are entitled to build, and 1,200 is not a statewide maximum. They are limits on what your CITY may do. Read them as a floor under your rights, not as a description of your lot.
Size is measured as interior livable space now
As of 1 January 2026, the statute measures ADU size by interior livable space — space intended for human habitation, including bedrooms, bathrooms, kitchens, hallways and closets (Gov. Code §66313(e)). Garages, covered patios and exterior porches are outside the number.
Before this change the statute said “living area”, and cities differed on whether wall assemblies counted. Getting your exterior walls out of the calculation is worth real floor area on a small unit. If you are holding a plan set drawn before 2026, the sizing assumption inside it may be costing you space.
HCD has been enforcing this. It has issued findings against cities that both capped units below the statutory floor and measured by “floor area” instead of interior livable space — two separate ways the usable number comes out smaller than the law allows.
Two more allowances people miss
- Converting an existing accessory structure? It may be expanded by up to 150 sq ft beyond the existing footprint, but only where the expansion is needed for ingress and egress.
- Height. Local rules must generally allow at least 16 feet for a detached ADU, with greater allowances near qualifying transit and on some multifamily properties. Height is what decides whether a two-storey unit is on the table at all.
Why the legal maximum is rarely the number that decides your project
In our experience on lots in Santa Clara and San Mateo counties, the statutory cap is almost never the binding constraint. What decides the size of your unit is the usable rectangle left in the rear yard after setbacks, whether equipment and concrete can reach the back of the property, and how far the utility runs have to travel.
So we would rather establish those three things on your lot before anyone talks about square footage. A homeowner who arrives certain they are building 1,000 sq ft, on a lot that physically supports 600, has usually already paid someone to draw the wrong thing.
And the arithmetic that makes small units expensive per foot
A good-quality ADU here runs $300K–$360K for a good-quality build. We publish that as a project range and never as a rate per square foot, because roughly $13,500–$18,500 of soft costs are fixed whatever the unit size — plans around $10,000, survey and certification, and soils work. Shrinking the unit does not shrink those. The effective cost per foot goes up as the unit gets smaller, which is the opposite of what a per-square-foot quote implies.
There is one place where size does move money in a step rather than a slope: an ADU with 750 sq ft of interior livable space or less pays no impact fees at all. Cross that line and you are in a different fee regime, calculated on the whole unit. That threshold deserves to be in the conversation before the floor plan is settled, not after.
Common questions
What is the maximum ADU size in California?
What is the difference between the 800 and 850 square foot ADU rules?
How is ADU square footage measured in 2026?
How big can a junior ADU be?
Can I build a two-storey ADU?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works — not legal advice. For your specific contract, talk to an attorney.
Also Worth Reading
The rest of what we publish
Cost guides
What an ADU, an addition, a kitchen or a whole house actually costs here, and what moves each number.
Our own projects
Two jobs written up start to finish, including what we found once the walls were open.
Paying for it
Renovation lenders we work with. We take no referral fee, and we never quote you a rate.
Where we build
Sixteen cities across two counties, and what genuinely changes at each city line.
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