Building an ADU in unincorporated Contra Costa County

Unincorporated areas only

The County's ADU rules, in Chapter 82-24 of the County Code, apply in unincorporated Contra Costa County. If your property is in a city, the city's ADU rules apply; check with that city's planning department. Statewide rules are on our California page.

Five cities use the County for building permits. DCD issues building permits for Clayton, Hercules, Lafayette, Orinda and Moraga, but you need planning approval from the city first. [1][2]

What you can build

The Board of Supervisors replaced Chapter 82-24 with Ordinance No. 2025-07 on May 13, 2025. ADUs that meet the location and development standards are approved ministerially, without discretionary review or a public hearing.

Where. ADUs are allowed in single-family residential districts (R-6 to R-100), the F-1 water recreation district, P-1 planned unit districts for residential use, multiple-family districts (M-6 to M-29) and agricultural districts (A-2 to A-80). On land under a Williamson Act contract, ADUs are allowed unless the contract prohibits them.

Streamlined units. Some ADUs skip the County's location and development standards entirely:

• On a single-family lot: one ADU and one JADU converted from the house or an existing accessory building (with up to 150 sq ft added for entry and exit), plus one new detached ADU of up to 800 sq ft with 4-foot side and rear setbacks that meets the state height limits.
• In an existing apartment building: ADUs converted from non-living space, up to 25% of the existing units.
• On a multifamily lot: detached ADUs with 4-foot side and rear setbacks, up to the number of existing units or 8, whichever is less (2 with a proposed building).

An ADU can't be split onto its own lot. [2]

Size, height and setbacks

Detached ADUs. Up to 1,000 sq ft, or 1,200 sq ft on a lot of 12,000 sq ft or more or in an agricultural district. In the Kensington (-K) combining district, 850 sq ft with one bedroom or 1,000 sq ft with more than one.

Attached ADUs. The same caps apply, and an attached ADU can't be more than 50% of the house's living area, except that it can be 850 sq ft with one bedroom or 1,000 sq ft with more than one. An attached ADU must be a conversion of an attached garage or other space in the house, or an addition.

Height and setbacks. ADUs follow the zone's usual front, side and rear yard and height rules, with exceptions. New ADUs need only 4-foot side and rear setbacks. Conversions, and ADUs rebuilt in the same place and size as an existing building, need no setback. Any part of an ADU in a yard area, or in Kensington, is held to the state ADU height limits: generally 16 feet for a detached ADU, or 18 feet on a lot within half a mile of a major transit stop or high-quality transit corridor, with allowance for matching the house's roof pitch.

Other standards. Every ADU needs a permanent foundation and its own exterior entrance on the side or rear of the building, or not visible from the street. A garage attached to a detached ADU is capped at 500 to 800 sq ft depending on lot size. [2][3]

Parking

A lot with an ADU must provide one extra off-street space, which can be in a setback area or in tandem. You don't have to replace parking when a garage, carport or parking space is converted or demolished for the ADU.

No extra space is needed if the ADU is within half a mile walking distance of public transit, in a historic district, a conversion, in a permit-parking area where the tenant can't get a permit, or within one block of a car-share pick-up spot. An ADU applied for together with a new house or apartment building is also exempt, but only if it meets one of those other conditions. The County's ADU page also says a new detached ADU of up to 800 sq ft and 16 feet needs no parking. [1][2]

Junior ADUs

A JADU can be up to 500 sq ft and must be inside the footprint of a single-family house, including an attached garage. JADUs are allowed only in single-family residential districts. Before getting a JADU permit, you must sign and record an agreement with the County that bars selling the JADU separately and limits its size. You must also include a disclosure statement in future sale documents.

County code doesn't set an owner occupancy rule for JADUs. Under state law, the owner must live on the property only if the JADU shares a bathroom with the house. See our junior ADU guide. [1][2][3]

Fees

ADU and JADU permit fees are set in DCD's fee schedule. The County says ADUs and JADUs pay all fees that apply to new development, to the extent state law allows. If your property has a septic system, Environmental Health must also review your plans.

State law limits what can be charged. No impact fees may be charged on an ADU of 750 sq ft or less of interior livable space or a JADU of 500 sq ft or less; larger ADUs pay in proportion to the size of the house. School districts can't charge impact fees on ADUs under 500 sq ft. Utilities can't treat an ADU or JADU as a new residential use for connection fees unless it is built along with a new house. See our ADU cost guide. [1][2][3]

Legalizing an existing ADU

Existing ADUs need both an ADU permit and a building permit; one built without them can be legalized through the same process as a new ADU. Under state law (AB 2533), the County won't deny a permit for an unpermitted ADU or JADU built before January 1, 2020, just because it doesn't meet building or ADU standards, unless the building is substandard under Health and Safety Code Section 17920.3.

You must show the unit was built and occupied before January 1, 2020. You can get an optional third-party inspection from a licensed contractor (B license or better), and you submit only a site plan, elevations, floor plan and structural plan. [1][2]

Pre-approved ADU plans

ADU vendors can get plans preapproved by the County for use in its service area. If you use one, you still need site-specific planning and building review: a signed authorization form, a site plan, foundation plans and details, a geotechnical report if required, and fire and safety compliance. The County lists approved plans and vendor contacts on its ADU page. Preapprovals last for the current three-year building code cycle. [1][4]

Renting it out or selling it

ADUs and JADUs can't be rented or offered for rent for less than 30 days, and the County says ADUs can't be permitted for short-term rental use. A JADU can't be sold separately from the house. An ADU can't be sold separately except under Government Code Section 66341, the state's narrow exception for certain nonprofit-built units.

State review. The state housing department (HCD) found that the County's earlier ADU ordinances didn't comply with state law, in April 2022 and December 2024, including on how many ADUs a single-family lot could have. The County adopted its current ordinance, 2025-07, in May 2025. Its streamlined rules list one converted ADU and one converted JADU, plus a new detached ADU that the code says may be combined with the JADU. HCD's handbook says state law requires allowing a converted ADU, a JADU and a new detached ADU together on a single-family lot; if you want all three, confirm with DCD. [1][2][3][5][6]

Permit steps in Contra Costa County

1. Check the code and your lot

   Read County Code Chapter 82-24 and look up your zoning. If you're on septic, plan for Environmental Health review. [1]

2. Apply for planning approval

   Register in the ePermits Center and start a Planning "General Application" with the ADU Planning Application Submittal Checklist. You get a temporary "ZZ" number until staff check completeness and you pay fees. [1]

3. Get your planning approval

   Planning staff review the application and email the approval documents. You must have planning approval before you apply for a building permit. [1][2]

4. Apply for the building permit

   Start a Building Permit in the ePermits Center with the ADU Building Permit Submittal Checklist. You get an interim "BISUB" number; once the application is complete, you pay plan check fees and it goes to review. [1]

5. Pay fees and download your permit

   After plan check, pay the building permit fees online and download the approved plans and permit card. [1]

Pre-approved and sample plans

• Contra Costa County preapproved ADU plans

  Vendor ADU plans preapproved by the County for its service area, listed with vendor contacts; site-specific planning and building review is still required. [1][4]

Who to call

• Contra Costa County DCD Application and Permit Center

  ADU planning approvals and building permits; email apcplanning@dcd.cccounty.us (planning) or permits@dcd.cccounty.us (building). Counter by appointment only.

  925-655-2700

  30 Muir Road, Martinez, CA 94553

  [1][7]

Questions people ask