Accessory Structures in Contra Costa County, CA (2026)

4 county-level rules, plus city-specific rules for 9 cities in Contra Costa County, California.

Verified from official government sources

Note: These county rules apply to unincorporated areas of Contra Costa County. If you live within an incorporated city, that city's own rules apply — see the city list below.

ADU Rules

Unincorporated Contra Costa County allows one accessory dwelling unit (ADU) on lots of at least 6,000 sq ft in single-family, multi-family, P-1, and agricultural districts under Chapter 82-24 of the County Ordinance Code.

ADUs allowed countywide in residential and ag zones

Few Restrictions

Contra Costa County Ordinance Code § 82-24.008

One accessory dwelling unit may be located on any lot in a single-family residential district (R-6, R-7, R-10, R-12, R-15, R-20, R-40, R-65, and R-100), planned unit district (P-1) for residential uses, or a multiple-family residential district (M-6, M-9, M-12, M-17, and M-29).

Full ordinance details →Official source ↗

Shed Rules

In unincorporated Contra Costa County, one-story detached accessory structures (sheds) with a floor area not exceeding 120 sq ft do not require a building permit, but they must still comply with zoning setbacks and may need planning, sanitary, or environmental health approval.

Sheds under 120 sq ft exempt from building permit

Few Restrictions

Contra Costa County Ordinance Code § 72-6.202

Per Contra Costa County Ordinance 72-6.202: No person, firm, or corporation shall erect, construct, enlarge, alter, repair, move, improve, remove, convert or demolish any building or structure regulated by this title... without first obtaining the necessary permits from the county building official. According to the 2022 California Residential Code (CRC), permits shall not be required for the f...

Full ordinance details →Official source ↗

Garage Conversions

Contra Costa County allows an attached ADU to be created through an internal conversion of an existing garage; no setback is required for an existing garage being converted to an ADU under Chapter 82-24.

Garage conversion to ADU streamlined; no setback required

Few Restrictions

Contra Costa County Ordinance Code Chapter 82-24

If an accessory dwelling unit is attached to a primary residence, the accessory dwelling unit must be an internal conversion of a garage or other area within the existing primary residence, or an addition to the existing primary residence. A setback is not required for an existing garage that is converted to an accessory dwelling unit.

Full ordinance details →Official source ↗

Carport Rules

Carports in unincorporated Contra Costa County are regulated as accessory structures under Title 8 (Zoning) of the County Ordinance Code and as covered structures under the California Building Code as adopted by the county. A carport must satisfy the yard-setback rules of the underlying residential zoning district (e.g., R-6 single-family per Chapter 84-4) and requires a building permit. The county's STR ordinance separately requires off-street parking and may restrict use of carports for STR guest parking.