The minimum ADU size in California is 150 square feet. That floor applies statewide, and no city or county can require anything larger. It comes from two statutes working together: Government Code Section 65852.2 bars local agencies from setting a minimum square footage that would prohibit an efficiency unit, and Health and Safety Code Section 17958.1 defines the minimum floor area of an efficiency unit as 150 square feet.1California Legislative Information. California Government Code 65852.2 – Accessory Dwelling Units2California Legislative Information. California Health and Safety Code 17958.1 – Efficiency Units
Where the 150-Square-Foot Floor Comes From
The number doesn’t appear in a single tidy sentence. State law gets there in two steps. First, the ADU statute tells local agencies they cannot establish a minimum square footage requirement for an attached or detached ADU that would prohibit an efficiency unit. Second, the Health and Safety Code sets the minimum floor area of an efficiency unit at 150 square feet.2California Legislative Information. California Health and Safety Code 17958.1 – Efficiency Units
The practical effect is preemption. If a jurisdiction tried to require a 400-square-foot minimum, that ordinance would be void because it would effectively ban efficiency units. The 150-square-foot floor is a right created by the state, not a privilege granted by your city.
One housekeeping note on statute numbers. In 2024, the legislature renumbered the ADU and JADU statutes as Government Code Sections 66310 through 66342. Many local ordinances and online resources still cite the older 65852.2 and 65852.22 sections. The substantive rules did not change.
What a 150-Square-Foot ADU Must Still Include
Hitting the minimum area is only part of qualifying as an ADU. The Government Code definition requires permanent provisions for living, sleeping, eating, cooking, and sanitation. All five functions have to fit inside the unit. It is tight at 150 square feet, but it is legal, and it is exactly the kind of compact housing the legislature meant to protect from local interference.
A Junior ADU works a little differently. A JADU must sit entirely within the walls of an existing or proposed single-family residence, including attached garages, and can be no larger than 500 square feet. The minimum follows the same 150-square-foot efficiency unit logic. A JADU must include an efficiency kitchen with a cooking appliance, a food preparation counter, and reasonably sized storage cabinets, but it does not need its own bathroom and can share sanitation facilities with the primary residence.3California Legislative Information. California Government Code 65852.22 – Junior Accessory Dwelling Units That shared-bathroom option matters for a reason beyond cost: starting January 1, 2026, owner-occupancy is required when the JADU shares sanitation with the main house, but not when it has its own separate bathroom.4California Department of Housing and Community Development. Accessory Dwelling Unit Handbook – 2026 Addendum
Interior Dimensions That Also Have to Work
Total square footage is the headline number, but the California Residential Code also governs how that space is shaped. This is where plans drawn to the minimum often fail plan check.
Every habitable room except the kitchen must have at least 70 square feet of floor area and measure at least 7 feet in every horizontal direction.5UpCodes. California Residential Code 2022 Chapter 3 – Building Planning A narrow 5-by-30-foot layout adds up to 150 square feet on paper, but it fails because no habitable room in it hits the 7-foot minimum dimension. That dimension applies to each habitable room independently.
Ceiling height is the other constraint. Habitable spaces, hallways, and basement areas used for habitable purposes must have ceilings at least 7 feet high. For rooms with sloped ceilings, at least half the required floor area needs the full 7-foot clearance, and any portion with a ceiling below 5 feet doesn’t count toward the minimum floor area. Kitchens have their own dimensional rules and are exempt from the 70-square-foot and 7-foot-width requirements, but counters and appliances still need adequate clearance for safe use.
Taken together, these rules explain why a functional 150-square-foot ADU is a design puzzle. A 10-by-15-foot rectangle satisfies the total area and the 7-foot minimum, and inside that box you still have to fit a kitchen, a bathroom, and a combined living and sleeping area, with each habitable room meeting its own requirements.
How This Fits With Maximum Size Limits
The 150-square-foot figure is the floor, not the target. State law also caps what local agencies can do on the upper end, so you have real flexibility above the minimum.
A local agency cannot set a maximum size below:
- 850 square feet for a studio or one-bedroom ADU
- 1,000 square feet for an ADU with two or more bedrooms
Above those thresholds, local agencies may allow more. For detached ADUs, the absolute state maximum is 1,200 square feet. For attached ADUs, total floor area cannot exceed 50 percent of the existing primary dwelling, though the local cap still cannot drop below the 850 and 1,000 numbers.1California Legislative Information. California Government Code 65852.2 – Accessory Dwelling Units
There is one additional backstop worth knowing. Regardless of other size limits, lot coverage rules, or floor area ratios a city adopts, those restrictions cannot prevent at least an 800-square-foot ADU that is 16 feet tall with four-foot side and rear setbacks from being built, so long as it complies with other local development standards.1California Legislative Information. California Government Code 65852.2 – Accessory Dwelling Units This is why many ADUs get built close to 800 square feet rather than near the 150-square-foot floor: it is the size the state guarantees you can build on almost any eligible lot.
Check Your Local Rules Before Designing to the Minimum
The 150-square-foot floor is a statewide right, but the rest of the design process still runs through your local planning and building departments. Permits must be issued ministerially, without discretionary hearings, and a complete application that is not approved or denied within 60 days is deemed approved by operation of law.6Association of Bay Area Governments. ADU State Laws Summary and Checklist Local agencies still apply design standards, setbacks within state limits, and building code review. If you plan to build near the minimum size, confirm with your local building department how they interpret the kitchen, bathroom, and ceiling-height rules for small units, because those details are where small ADU plans most often need revision.