Building an ADU in unincorporated Alameda County

Unincorporated areas only

Alameda County's ADU rules, in Chapter 17.55 of the County's zoning code, apply only in the unincorporated parts of the county. If your home is inside a city, the city's rules and permit office apply. See our Oakland and Berkeley pages, and for other cities check with that city's planning department. Statewide rules are on our California page.

The County's ADU web page still lists the older ordinance and handout as archived items, so rely on the current code text on Municode and confirm with the Planning Department. [1][2]

What you can build

The County adopted its current ADU chapter with Ordinance No. 2026-1 in January 2026. ADUs are allowed in all zones that permit single-family, multifamily or mixed-use homes, on a lot with an existing or proposed home. JADUs are allowed only with a single-family home. The County approves allowable ADUs and JADUs ministerially, without discretionary review or a hearing, in agricultural, residential and mixed-use zones.

• Single-family lot: one JADU, plus one ADU by new construction and one by conversion, for each single-family home on the property. No more than one ADU may be attached. ADUs can be attached, detached, or conversions of existing floor area.
• Existing apartment building: at least one ADU, and up to 25% of the existing units, converted from non-living space such as storage rooms, attics, basements or garages; plus up to 8 detached ADUs, but no more than the number of existing units.
• Proposed apartment building: up to 2 detached ADUs.

ADUs and JADUs don't count toward the lot's maximum density. [1]

Size, height and setbacks

Size. A new detached ADU with no bedroom or one bedroom can be up to 850 sq ft of interior livable space; with two or more bedrooms, up to 1,200 sq ft. A new attached ADU can be up to 50% of the house's floor area, capped at 850 sq ft (up to one bedroom) or 1,000 sq ft (two or more bedrooms); if 50% is less than 850 sq ft, 850 sq ft is allowed. Converting an accessory building or part of the house has no size limit, unless you expand it by more than 150 sq ft. Size is measured inside the exterior walls.

Height. A detached ADU can be 16 feet tall; 18 feet within half a mile walking distance of a major transit stop or high-quality transit corridor, or on a lot with a multistory apartment building; and, near transit, 20 feet if needed to match the roof pitch of the house. An attached ADU can be 25 feet or the zone's height limit, whichever is lower. ADUs are limited to two stories.

Setbacks. All new attached and detached ADUs need 4-foot side, street side and rear setbacks; conversions of existing structures don't. ADUs over 800 sq ft must meet the house's front setback and stay 6 feet from other buildings. These limits don't apply to the 800 sq ft ADUs guaranteed by state law (Government Code Section 66323).

Design. ADUs over 800 sq ft must follow the County's Residential Design Standards and Guidelines for the Unincorporated Communities of West Alameda County, except where they conflict with state ADU law. [1]

Parking

JADUs need no parking. Other ADUs need one on-site space each, which can be in the front or street side setback. ADUs built under the state's guaranteed categories (Government Code Section 66323) need none.

No space is required if the site is within half a mile walking distance of public transit, in a historic property or district, part of the house or an accessory structure, in an area where on-street permits are required but not offered to the tenant, within one block of a car-share vehicle, or if the ADU has no bedrooms. 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. [1]

Junior ADUs

A JADU can be up to 500 sq ft of interior livable space inside the walls of an existing or proposed single-family house. It can share a bathroom with the house, but then needs both an interior connection and a separate entrance. It needs at least an efficiency kitchen and may add up to 150 sq ft for entry and exit. JADUs aren't allowed on multifamily lots.

Owner occupancy. The owner must live in the house or the JADU only if the JADU shares bathroom facilities. No owner occupancy is required if the JADU has its own bathroom, or the owner is a government agency, land trust or housing organization.

Deed restriction. You must record a deed restriction with the County Recorder, and file a copy with the Planning Department, that bars selling the JADU separately and limits its size and features. The County's undated ADU process handout still says the owner must live on the property for any JADU; the 2026 code above is the current rule. More in our junior ADU guide. [1][3]

Fees

Park fees. The code says no park dedication fee is charged for ADUs under 750 sq ft of interior livable space or for JADUs. The County's ADU process handout lists a $5,775 flat park dedication fee for ADUs of 750 sq ft or more. State law says impact fees on an ADU over 750 sq ft must be proportional to the size of the main house, so confirm the current amount with the Planning Department.

Other fees. The handout says permit fees range from $1,000 to $10,000 and can be estimated with the County's fee worksheet. It lists a $140 address assignment fee, due with the application, and says other agencies (Public Works, Environmental Health, the fire department and your school district) may charge fees. Ask your utilities about connection fees. State law bars school impact fees on ADUs under 500 sq ft and limits utility connection fees. See our ADU cost guide. [1][3][4]

Septic, fire and hillside sites

If your property is on a septic system, the County Department of Environmental Health must approve the ADU. The County's handout says septic upgrades can cost $30,000 to $70,000, so talk to Environmental Health early.

In a Wildland-Urban Interface fire zone there may be extra permits and requirements; ADUs need fire sprinklers only if the main house has them. Hillside ADUs may need more detailed drawings, and extensive grading may need a grading permit. New electric or gas service from PG&E can add time and cost. [1][3]

Legalizing an existing ADU

The County must ministerially approve an application to legalize an unpermitted ADU or JADU built before the state's cut-off date (January 1, 2020, under Government Code Section 66311.7), unless correcting a violation is needed to meet the Health and Safety Code's substandard building standards. An inspector may check the unit and recommend fixes, and the County must approve permits to correct them without penalizing you. You can get a confidential third-party inspection from a licensed contractor first. Legalized units don't pay impact fees or connection charges unless new utility infrastructure is needed for health and safety. [1][4]

Renting it out or selling it

You don't have to live on the property to have an ADU. ADUs and JADUs can be rented to a separate household, but rental terms must be longer than 30 days, so short-term rentals aren't allowed. ADUs and JADUs can't be sold or owned separately from the house, except an ADU owned by certain nonprofit corporations under Government Code Section 66341.

State review. In April 2025, the state housing department (HCD) found that the County's 2024 ADU ordinance (O-2024-32) didn't comply with state law in many respects, including multifamily ADU limits and JADU rules. The County replaced its ADU chapter in January 2026. If a County rule seems stricter than state law, ask the Planning Department how it applies. [1][5]

Permit steps in Alameda County

1. Learn the rules for your property

   Use the County's online zoning map and read Chapter 17.55. Under the current code, a new detached ADU is generally limited to 16 feet and 1,200 sq ft, and an ADU under 750 sq ft pays no park dedication fee. [1][3]

2. Make a sketch and get a planner's advice

   Sketch your lot, house, other structures and the proposed ADU, then make an appointment with the Planning Department at 510-670-5400 for advice on the rules and the Zoning Determination process. [3]

3. Design the ADU and prepare the application

   Have your designer prepare plans, documents and calculations that meet the ADU submittal requirements. Sites on septic need Environmental Health approval. [1][3]

4. Submit online

   Submit the building permit application through the County's online portal and email the Address Assignment Form. Staff tell you within 60 days, in the portal, whether revisions are needed. [3]

5. Get the permit issued

   Once the application is approved and fees are paid, download your permit from the portal. It is valid for one year. [3]

6. Build and pass final inspection

   Your builder schedules inspections. After the final inspection you receive a Certificate of Occupancy; the ADU's address is assigned automatically. [3]

Who to call

• Alameda County Planning Department

  ADU rules for unincorporated Alameda County

  (510) 670-5400

  224 West Winton Avenue, Suite 111, Hayward, CA 94544

  [2]

• Alameda County Permit Center / Zoning Counter

  General ADU information and permits (in person by appointment only)

  510.670.5400

  399 Elmhurst Street, Suite 141, Hayward, CA 94544

  [2][3]

Questions people ask