California ADU Requirements: Size Limits and Floor Plans

California's ADU requirements do not set one ADU size. They set limits on the limits: a city can write its own accessory dwelling unit ordinance, but state law says how small its maximum size may be, how far back it may push an ADU, how low it may cap the height and when it may charge impact fees. This post sets out those state figures, then walks through an 800 ft² two-bedroom plan that sits inside them. It is a planning guide, not legal advice: your city's ordinance and the state housing department have the last word. Law current as of October 2026.

Where the rules live

ADU law is in the Government Code, sections 66310 to 66342. Those numbers are new: SB 477 moved the ADU provisions there in 2024 and SB 543 moved the fee rules to section 66311.5 in 2025, so older articles, city ordinances and permit handouts often still cite the former section 65852.2. The substance largely carried over; when a document quotes 65852.2, look for the matching section in the 66310 series.

The state's Department of Housing and Community Development (HCD) reviews local ADU ordinances and publishes findings letters when a city's rules go further than state law allows. Those letters are a useful check on any local figure that looks too strict. For how other states compare, see ADU laws by state; Hawaii's Act 39, for example, requires counties to allow two ADUs per lot.

Size: the floors under a city's cap

A city may set a maximum ADU size, but under Gov. Code 66321(b)(2), as amended by SB 543, that maximum cannot be lower than:

• 850 ft² of interior livable space for a studio or one-bedroom ADU.
• 1,000 ft² of interior livable space for an ADU with more than one bedroom.

Separately, a city cannot use lot coverage, floor area ratio, open-space requirements or front setbacks to stop an ADU of up to 800 ft² of interior livable space with 4 ft side and rear setbacks (Gov. Code 66321(b)(3)). In practice this is the guaranteed size: on most single-family lots, an 800 ft² ADU that keeps 4 ft from the side and rear lot lines cannot be refused on those grounds, even where the main house already uses up the lot's coverage.

Height and setbacks

Gov. Code 66321(b)(4) sets the lowest height limits a city may impose:

Side and rear setbacks cannot be more than 4 ft. Above 800 ft² a city may still apply its front setback, and fire, building and utility separations apply at any size.

Impact fees and the 750 ft² line

Under Gov. Code 66311.5(c) (section 66324 until SB 543 renumbered it), a city, special district or water corporation cannot charge impact fees on an ADU with 750 ft² of interior livable space or less, or on a junior ADU (JADU) of 500 ft² or less. Above 750 ft², fees must be proportional to the size of the main house. That makes 750 ft² a real design threshold: a plan just over it pays fees that a plan just under it does not.

Junior ADUs are the small option: up to 500 ft², made within an existing single-family home, typically by converting a bedroom. They have their own section of the law and their own rules. If you want a second full-size house rather than an ADU, SB 9 lot splits and duplexes are the other route.

SB 543 and how size is measured

SB 543 (chapter 520, Statutes of 2025, in force since January 1, 2026) writes the state's size figures, the 850, 1,000 and 800 ft² floors and the 750 ft² fee line, in interior livable space. That is a narrower figure than the outside dimensions of the building: it leaves out the thickness of the exterior walls, and depending on how a city reads it, possibly other areas too. Los Angeles County's planning department has published a memo on applying it.

This matters at the edges. The outside of a small building can measure 60 ft² or more than its inside, as the plan below shows. Whether your city's form wants the outside figure, the inside figure, or both, ask before you settle the size, especially if you are near 750 ft².

An 800 ft² two-bedroom plan

The FloorGen 800 sq ft 2-bed ADU template is a 40 × 20 ft rectangle measured to the outside faces of the walls, so it is exactly 800 ft² measured outside; for the state figures, which SB 543 measures as interior livable space, it is about 700 to 736 ft². It is drawn with 2×6 framed exterior walls (about 6½ in finished) and 2×4 interior walls (4½ in). The bedrooms are at the west end. A central hall leads to a 9 × 6 ft bath and an open laundry alcove. The kitchen, dining area and living room take the east end, and the entry door opens into the living room. Our 800 sq ft ADU floor plan walkthrough goes through the layout room by room.

The same building gives three different square footages. To the outside of the 6½ in exterior walls it is 40 × 20 ft, 800 ft². Inside the exterior walls, partitions included, it is about 38 ft 11 in × 18 ft 11 in, roughly 736 ft². The rooms alone, measured to the inner faces of every wall, add up to about 700 ft². So the plan is 800 ft² measured outside, but under the state's 750 ft² fee line, which is measured in interior livable space. Cities measure differently, so confirm the method before you settle the size.

Both bedrooms clear the International Residential Code's habitable-room minimum of 70 ft² and 7 ft in any direction (2021 IRC R304; 2024 IRC R312), and the template opens with FloorGen's IRC profile switched on and no room-size warnings. What FloorGen does not check is everything specific to a site: setbacks from your lot lines, height, fire separation from the main house, or your city's design standards.

Smaller ADUs

If 800 ft² is more than the lot or the budget allows, the next common size is a garage conversion. A two-car garage is commonly about 20 × 20 ft, around 400 ft², which fits a studio with a kitchen run and a shower bath. See 2-car garage conversion floor plans for that layout and the code items conversions run into.

Before you draw the final plan

• Read your city's current ADU ordinance, and check it against HCD's guidance; HCD has published findings letters against cities whose rules went further than state law.
• Ask how the city measures ADU size after SB 543, and whether it wants the figure to the outside or the inside of the walls.
• Check utility, fire and building separations on your actual lot. A 4 ft setback is the most a city can demand for zoning, not a guarantee that a building there meets fire code.
• Use the state figures in this post as the floor of what your city must allow. The ordinance and the plan checker decide what you can actually build.

For the rules that make a room a legal bedroom, including egress windows, see what makes a room a bedroom.