What is the maximum ADU size in California?

There is no single statewide maximum that applies to every lot — instead, California sets protected minimums that cities must allow and then permits local ordinances to grant more. In practice, most ADUs fall in the 800 to 1,200 SF range, and every city must allow at least an 800 SF unit with a 16-foot height no matter what its local code says. Statewide minimums apply, but local ordinances and lot conditions vary, so confirm your specific allowance with your local building department.

How the size limits actually work

State law layers several rules on top of one another. The governing number for your project is whichever one lets you build the larger unit:

• The protected 800 SF / 16-ft floor — a city can never reduce your ADU below 800 SF or below 16 feet in height, even if local lot-coverage, floor-area-ratio, or open-space rules would otherwise block it.
• One-bedroom and studio units — local ordinances commonly cap these around 850 SF.
• Two-or-more-bedroom units — local ordinances commonly cap these around 1,000 to 1,200 SF.
• Attached ADUs — often limited to a percentage of the primary dwelling's floor area, but still no less than the 800 SF protected minimum.
• JADUs (Junior ADUs) — capped at 500 SF and created within the existing home's footprint.

Conversions and detached units

If you convert existing legal space — an attached garage, a basement, or part of the house — that converted square footage is generally allowed regardless of size caps, because you aren't adding new floor area. New detached construction is where the 800 to 1,200 SF tiers and the protected minimum do most of their work.

A useful rule of thumb: if a local size limit would push you below 800 SF or below 16 feet, state law overrides it. Above those floors, the local cap controls.

Why size drives your MEP scope and cost

Square footage isn't just a zoning number — it determines the engineering and the price of your plans:

• Heat-load (Manual J) calculations and HVAC capacity scale with conditioned area.
• Electrical load calculations and panel sizing grow with the unit's size and appliance count.
• Plumbing fixture counts and drain/supply sizing increase with bedrooms and baths.

That's why our pricing is tiered by ADU square footage. A Full MEP Package starts at $1,495 for smaller units and scales up with size; single-discipline plans start at $995 and two disciplines at $1,195. Remember that code cycles and local ordinances change — verify the maximum that applies to your parcel before finalizing your design, and see our California ADU Requirements overview for the full picture.