Contra Costa County

Unincorporated Contra Costa County ADU rules

Research summary of local ADU rules on file for Unincorporated Contra Costa County in Contra Costa County. Not a determination for a specific lot.

Last checked October 23, 2026.

City or county ADU page

Unincorporated Contra Costa County ADU page

Check any California address for free

Unincorporated Contra Costa County rules on file

Local rules recorded in this project for this jurisdiction. Where a city figure on file is stricter than the state baseline on file, both are shown.

Units allowed without local standards
In residential or mixed-use zones these are allowed ministerially without the County's location rules or development standards: one ADU and one junior ADU within an existing or proposed home or accessory structure (up to 150 sq ft expansion for an accessory structure); one new detached ADU of up to 800 sq ft with 4-foot side and rear setbacks and state height limits, which may be combined with a junior ADU; conversions of non-living space in multifamily buildings up to 25% of units; and detached ADUs on multifamily lots (up to the lesser of existing units or eight, or two with a proposed multifamily building).
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.006(b), accessed 2026-10-23.

Application
Apply to the Department of Conservation and Development before applying for a building permit. Include owner and applicant names, unit sizes, a scale drawing of the lot and structures, water and sewage details and the legal description. Applications that meet the chapter are allowed ministerially without a hearing.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.006(a); 82-24.008, accessed 2026-10-23.

Where allowed
ADUs are allowed on lots with a proposed or existing home in the County's single-family (R), planned unit (P-1), multiple-family (M), forestry (F-1) and agricultural (A) districts, with limits on Williamson Act lands. An ADU lot may not be subdivided.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.010, accessed 2026-10-23.

ADU size
Detached or attached: up to 1,000 sq ft, or 1,200 sq ft on lots of 12,000 sq ft or more and in agricultural districts; in the Kensington (-K) combining district, 850 sq ft (one bedroom) or 1,000 sq ft (more bedrooms). An attached ADU is also limited to 50% of the main home's living area, but may reach 850 sq ft (one bedroom) or 1,000 sq ft (more bedrooms).
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.012(a), accessed 2026-10-23.

Basic requirements
Complete independent living facilities, a permanent foundation, and independent exterior access on the side or rear of the building or not visible from the street. A private septic system or well must be signed off by the health officer before final inspection.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.012(b) to (e), accessed 2026-10-23.

Garage attached to a detached ADU
Up to 500 sq ft on lots of 20,000 sq ft or less, 600 sq ft on larger lots under five acres, and 800 sq ft on lots of five acres or more or in agricultural districts.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.012(g), accessed 2026-10-23.

Side and rear setbacks and height
No setback for an internal conversion or a rebuild in the same place and size; otherwise 4 feet from side and rear lot lines. ADUs in a front, side or rear yard area, and all ADUs in Kensington, follow the state height limits in Government Code 66321(b)(4); otherwise the zone height applies.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.012(h)(2) to (4), accessed 2026-10-23.

Parking
One added space (tandem or in a setback area allowed), not required for ADUs within a half mile walk of public transit, in a historic district, that are part of the home or an accessory structure, where on-street permits are not offered to the occupant, within one block of car share, or filed with a new home. Garage or carport spaces lost to an ADU do not have to be replaced.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.012(i), accessed 2026-10-23.

Rentals and sale
No rental for less than 30 days. A junior ADU may not be sold separately from the main home, and an ADU may be sold separately only as allowed by Government Code 66341.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.014, accessed 2026-10-23.

Junior ADU deed restriction
A deed restriction for a junior ADU (no separate sale; size and attributes) must be recorded, with a disclosure statement. The County code does not require owner occupancy.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.016, accessed 2026-10-23.

Nonconforming homes and unpermitted units
An ADU may be allowed on a lot with a nonconforming main home, and unpermitted ADUs may be legalized under Government Code 66332.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.018; 82-24.020, accessed 2026-10-23.

Fees
Fees are set by the Board of Supervisors' fee schedule.
Source: Contra Costa County Ordinance Code, Chapter 82-24 (Accessory Dwelling Units), Ord. 2025-07, 82-24.022, accessed 2026-10-23.

California state rules on file

The statewide baseline recorded from state law and the HCD ADU Handbook.

Detached size cap described in state law
1,200 sq ft. Recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Side and rear setback cap for a new ADU
4 ft. Recorded in this rules file from Government Code section 66314, which describes a cap on side and rear setbacks for a new ADU.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Setback for a conversion of an existing structure
No additional setback, as recorded in this rules file from Government Code section 66314 for a conversion of an existing structure.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum lot size
A city may not impose a minimum lot size for an ADU, as recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum unit the handbook says cities must allow
At least an 800 sq ft ADU with 4 ft side and rear setbacks. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(3).
Source: HCD ADU Handbook, 66321(b)(3); handbook updated March 2026, accessed 2026-10-16.

Detached height described in the handbook
16 ft, or 18 ft or 20 ft in some cases. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(4).
Source: HCD ADU Handbook, 66321(b)(4); handbook updated March 2026, accessed 2026-10-16.

Permit decision deadline
60 days. Recorded in this rules file from Government Code section 66317(a)(3): the permitting agency must decide on (approve or deny) a completed ADU application within 60 days when an existing single-family or multifamily dwelling is on the lot. Section 66317(a)(1) says the application is considered ministerially, without discretionary review or a hearing.
Source: California Government Code section 66317, 66317(a)(1), (a)(3), accessed 2026-10-17.

Owner-occupancy requirement
None. Recorded in this rules file from Government Code section 66315: a local agency may not impose an owner-occupancy requirement on an ADU. It may require that the property be used for rentals of terms 30 days or longer. The HCD ADU Handbook (updated March 2026) describes the same rule.
Source: California Government Code section 66315, 66315; HCD ADU Handbook updated March 2026, accessed 2026-10-17.

Parking near public transit
No parking standards within one-half mile walking distance of public transit. Recorded in this rules file from Government Code section 66322(a)(1). Section 66322(a) lists several other cases where parking standards may not be imposed.
Source: California Government Code section 66322, 66322(a), accessed 2026-10-17.

Ministerial categories
The HCD ADU Handbook (updated March 2026) states that Government Code section 66323 sets four ministerial categories. This rules file does not restate those four categories.
Source: HCD ADU Handbook, 66323; handbook updated March 2026, accessed 2026-10-16.

Fire Hazard Severity Zones
If your lot is in a Fire Hazard Severity Zone, the CAL FIRE Office of the State Fire Marshal says in Information Bulletin 25-004 (issued March 3, 2025): “Fire Hazard Severity Zones (FHSZs) should not restrict the development of Accessory Dwelling Units (ADUs). However, additional building standards may be necessary, including but not limited to California Fire Code Chapter 49 and California Building Code Chapter 7A.” State ADU law does not itself bar an ADU there. A city or county may still apply its building code, including wildfire-resistant construction (Gov. Code 66314(d)(8)); require side and rear setbacks “sufficient for fire and safety” for a unit converted under 66323(a)(1); and decline parking in setbacks on specific fire and life safety findings (66314(d)(10)(B)). It may also consider public safety when it designates where ADUs may be permitted (66314(a)). Sprinklers can't be required in an ADU or JADU if the primary residence doesn't need them (66314(d)(12), 66323(d)). Some cities and counties still apply their own fire-zone limits. Where this summary has one, it is listed with the local rules. This summary does not look up your lot. Check the CAL FIRE Fire Hazard Severity Zone map viewer linked from the State Fire Marshal's Fire Hazard Severity Zones page: https://experience.arcgis.com/experience/5065c998b4b0462f9ec3c6c226c610a9
Source: CAL FIRE Office of the State Fire Marshal Information Bulletin 25-004, page 4, issued March 3, 2025; Gov. Code 66314, 66323, accessed 2026-10-20.

State baseline on the directory → · State ADU rules on the homepage →

Can I build an ADU in Unincorporated Contra Costa County?

State law sets a baseline for ADUs in Unincorporated Contra Costa County, and the local standards on file are listed on this page. Whether a specific lot can add an ADU depends on zoning, utilities, and other site facts this page does not measure. Confirm with the Contra Costa County Department of Conservation and Development, or run a free address summary.

Check any California address for free