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California ADU Rules

Last updated November 3, 2026

California Government Code section 66323 requires every city and county to allow four kinds of ADUs: a converted unit with a junior ADU, a new detached unit of up to 800 square feet, converted units in a multifamily building, and detached units on a multifamily lot. The state housing department (HCD) explains the rules in its ADU Handbook (March 2026).

The Four Units Every City Must Allow

Government Code section 66323 lists units a local agency must permit no matter what its own ordinance says. HCD calls them 66323 Units.

• Converted unit in a single-family home or accessory building. One ADU and one junior ADU per lot, inside existing or proposed space, with exterior access.
• New detached unit on a single-family lot. One detached ADU of up to 800 square feet, with side and rear setbacks of no more than four feet.
• Converted units in a multifamily building. At least one, and up to 25 percent of the existing units, in space not used as living space such as storage rooms, attics, basements, or garages.
• Detached units on a multifamily lot. Up to two on a lot with a proposed multifamily building, or up to eight on a lot with an existing one, not more than the number of existing units.

On a single-family lot these can be combined, so a converted unit, a junior ADU, and a new detached ADU can all be allowed together (HCD ADU Handbook (March 2026)).

Size, Height, And Setbacks

• A city cannot set limits that prevent an 800 square foot ADU with four-foot side and rear setbacks (Government Code section 66321).
• Detached ADUs have a base height limit of 16 feet. It rises to 18 feet near major transit, plus up to two feet for roof pitch to match the main home. Attached ADUs can be limited to 25 feet or the main home's limit, whichever is lower.
• Where a city has no compliant ordinance, the default maximum for a new detached ADU is 1,200 square feet of interior living space.

Fees, Parking, And Occupancy

• Impact fees. None for an ADU of 750 square feet or less or a JADU of 500 or less. Larger ADUs pay in proportion to the main home (Government Code section 66311.5).
• Parking. A city cannot require more than one space per unit or per bedroom, whichever is less (HCD ADU Handbook (March 2026)). It cannot require parking at all in several cases, including ADUs within a half mile walking distance of public transit, ADUs that are part of the primary residence or an accessory structure, and 66323 Units (Government Code section 66322).
• Owner occupancy. A city cannot require the owner to live on the property for an ADU (Government Code section 66315). JADUs with shared bathrooms are different. See Junior ADUs.
• Rental terms. A city may require rentals longer than 30 days.

Approval Is Supposed To Be Ministerial

Ministerial means a city checks your plans against objective standards and does not hold a discretionary hearing. Timing rules are covered in Permits And Timeline. If you think a city is breaking state ADU law, the HCD ADU Handbook (March 2026) says to raise it first with the permitting agency and then use the technical assistance portal on the HCD ADU page.

Local Rules Still Matter

Your city or county can add rules within the limits above, and special areas such as the coastal zone, fire hazard zones, and historic districts may add review steps. Read your local ordinance, or ask the planning counter which rules apply to your lot.

Related guides: ADU Cost In California, Garage Conversion To ADU, Detached ADU.