Building an ADU in unincorporated Sonoma County

Unincorporated areas only

Permit Sonoma's ADU rules apply in the unincorporated parts of Sonoma County. If your property is inside a city, that city's rules and permit office apply; see our Santa Rosa page, and for other cities check with that city's planning department. Statewide rules are on our California page.

Within the unincorporated county, the rules depend on whether your parcel is inland or in the coastal zone (parcels with "CC" zoning). Check your parcel in Permit Sonoma's Zoning and Land Use Map Viewer. Most of this page covers inland parcels; see the coastal section below. [1][4]

What you can build (inland)

The Board of Supervisors adopted new ADU and JADU rules on December 9, 2025 (Ordinance No. 6544), effective January 8, 2026. The ADU rules are in Section 26-24-020 of the County Code.

ADUs are allowed on lots whose base zone permits single-family homes, multifamily homes or residential mixed-use development, as long as a primary home exists or is proposed. There is no minimum lot size.

• Single-family lot: two ADUs of any type (detached, attached, converted, or a mix) plus one JADU. Existing ADUs count toward the two.
• Lot with an existing multifamily building: up to eight attached or detached ADUs, but no more than the number of existing units, plus at least one ADU (up to 25% of the units) converted from non-living space in the building.
• Lot with a proposed multifamily building: two attached or detached ADUs.

ADUs are approved ministerially, without a hearing, and are exempt from design review. ADUs are prohibited on land under a Williamson Act contract. [1][2][3]

Size, height and setbacks

Size. On a single-family lot an ADU can be up to 1,200 sq ft. Conversions of space in the house can be larger, as can conversions of a whole accessory building (plus up to 150 sq ft for getting in and out). An ADU attached to a new house can be larger if it fits within the house's allowed size. Permit Sonoma's standards page lists a 1,000 sq ft limit for a new ADU attached to an existing house or accessory structure, while the code sets 1,200 sq ft for ADUs on single-family lots; if you plan a large attached ADU, confirm with Permit Sonoma. On multifamily lots, new ADUs have no size limit. Floor area is measured inside the walls.

Height. 35 feet for new construction. A conversion can keep the height of the existing structure.

Setbacks. New construction needs 4-foot side and rear setbacks. On a single-family lot, the front setback applies only to ADU floor area beyond 800 sq ft within the front setback area, so an ADU can put up to 800 sq ft there. Converted space has no setback requirement. Fire codes may still require more space.

Lot coverage. On a single-family lot, the zone's lot coverage limit applies only to ADU floor area over 800 sq ft, and not at all to conversions. Lot coverage limits don't apply to ADUs on multifamily lots. [2][3]

Parking and access

No parking is required for an ADU or JADU. If you convert or demolish a garage, carport or other parking to build an ADU, you don't have to replace it. You may still need to improve access for fire code compliance, which can require grading or encroachment permits.

An ADU converted from an existing house, or attached to a proposed house, must have its own outside entrance. JADUs also need a separate outside entrance, and a JADU without its own bathroom must also have an interior door to the main living area. [2][3][5]

Junior ADUs

A JADU can be up to 500 sq ft and must be created inside an existing or proposed house, which can include an enclosed space such as an attached garage. It can't extend beyond the house's walls. A shared bathroom doesn't count toward the 500 sq ft. It needs an efficiency kitchen. One JADU is allowed per lot, in addition to the ADUs. JADUs are allowed only with a single-family home, not on multifamily lots. The JADU rules are in Section 26-24-150 of the County Code.

Owner occupancy. The owner must live in the JADU or the rest of the house unless the JADU has its own bathroom, or the owner is a government agency, land trust or housing organization.

Deed restriction. Every JADU owner must record a deed restriction that bars selling the JADU separately, limits its size and attributes, and lets the County enforce it. Permit Sonoma places a hold on the final inspection until you return a recorded copy. More in our junior ADU guide. [2][5][6]

Water, septic, fire and zoning overlays

Water. You must show an adequate water supply before applying for a building permit, such as a will-serve letter from your water provider. In water-scarce Class 3 or 4 groundwater areas, a well serving the ADU needs a dry-season yield test done between July 15 and October 1.

Septic and sewer. You must show sewer service or a septic system big enough for the new unit, and get any septic or sewer permits issued before applying for the building permit.

Fire. ADUs must meet fire standards for their fire hazard zone, which can mean extra setbacks, fire-resistant construction or sprinklers. Sprinklers are required in an ADU only if the main house must have them, and building an ADU won't trigger sprinklers in the existing house. Email Permit Sonoma's Fire Prevention staff at FirePrevention@sonomacounty.gov to ask.

Z (ADU Exclusion) district. Permit Sonoma says ADUs are prohibited in the Z Combining District, though JADUs are allowed, and that it is reevaluating where Z applies. In January 2025, the state housing department (HCD) told the County it must remove the Z prohibition unless it adopts detailed findings supporting it. The ADU code that took effect in January 2026 says ADUs aren't permitted by right where a combining district doesn't allow homes by right. If your parcel has Z zoning, confirm with Permit Sonoma.

Other overlays (riparian corridors, floodways, scenic resources, historic districts) can limit where an ADU goes on your lot. [1][2][3][7]

Fees

Permit Sonoma collects fees in three stages: initial review fees when you apply; issuance fees (addressing, water and sewer connections, inspections) before the permit is issued; and any impact fees after the project passes its inspections, before you get your certificate of occupancy. Pre-reviewed, factory-built and manufactured ADUs get reduced plan check fees. See the Permit Sonoma fee schedules.

Impact fees. JADUs pay none. ADUs with more than 750 sq ft of interior livable space pay park and traffic impact fees, and fire impact fees in the Sonoma County Fire District and Sonoma Valley Fire District. School districts may charge fees on ADUs over 500 sq ft; you pay the school district and give Permit Sonoma proof.

Connection fees. ADUs and JADUs converted from existing space on a single-family lot don't need a new utility connection or pay a connection fee. Others may need a separate connection and fee. See our ADU cost guide. [8]

Rebuilding after a wildfire

Permit Sonoma's fire rebuilding page encourages owners rebuilding after a wildfire to include an ADU or JADU in their plans. It says the County "is allowing residents to build detached accessory dwelling units to live in while they are rebuilding their main residence." The same page lists a park and traffic impact fee reduction for new ADUs, meant in part to house fire victims while they rebuild: fees waived up to 750 sq ft and cut 50% for 751 to 1,000 sq ft.

Check the current rules. The ADU code that took effect in January 2026 says a certificate of occupancy for an ADU can't be issued before the one for the main home, and Permit Sonoma's current ADU fee page lists park and traffic fees for all ADUs over 750 sq ft. If you are rebuilding after a fire and want to live in an ADU first, or are counting on the fee reduction, confirm with Permit Sonoma's rebuild team at (707) 565-1788. [3][8][9]

Coastal zone ADUs

In the coastal zone (CC-zoned parcels), ADUs are called "second dwelling units" and follow Section 26C-325.1 of the Coastal Zoning Code, summarized on Permit Sonoma's Coastal Zone ADUs page. JADUs aren't yet allowed in the coastal zone.

A coastal ADU needs at least a building permit plus either a use permit (on LIA or LEA agricultural land) or a zoning permit (all other zones), and may need a coastal development permit. Limits on the number of units, lot size, size and setbacks depend on your zoning. Email Planner@sonomacounty.gov with project details to ask which permits apply. [4]

Permit steps in Sonoma County

1. Learn the rules for your lot

   Look up your parcel's zoning in the Zoning and Land Use Map Viewer and review Permit Sonoma's Standards for ADUs and JADUs. [2][6]

2. Budget and design

   Estimate fees and construction costs, then design the unit or use an ADU Ready pre-reviewed, factory-built or manufactured design. If you design or build it yourself, file the owner-builder form CSS-026. [6][10]

3. Get water, septic and other preliminary permits

   Before you apply for the building permit, get any planning permits, evidence of water and sewer service or septic capacity, and any well, septic or sewer permits issued. Consult Fire Prevention about emergency access. [2][6]

4. Apply online and pay initial fees

   Submit a building permit application through Permits Online, following BPC-021 and BPC-002, and pay initial review fees. [6]

5. Plan review and permit issuance

   Permit Sonoma may do a site evaluation, then reviews the plans: generally within 60 days of a complete application, or 30 days with pre-reviewed, factory-built or manufactured plans. Address any corrections, then pay issuance fees to get the permit. [6]

6. Build, record any deed restriction and get a certificate of occupancy

   Start work within a year of issuance and schedule inspections. Record the JADU deed restriction if you have one, pay any impact fees, and receive your certificate of occupancy. [6][8]

Pre-approved and sample plans

• ADU Ready pre-reviewed ADU plans

  Plans for detached ADUs pre-reviewed by Permit Sonoma and listed in its design gallery; using an unmodified plan cuts building permit review to 30 days, but site-specific requirements still apply. [10]

Who to call

• Permit Sonoma Planning

  ADU and JADU rules for unincorporated Sonoma County (phone hours Monday-Friday 10 a.m.-12 p.m. and 1-3 p.m.); email Planner@sonomacounty.gov

  (707) 565-1900, option 5

  2550 Ventura Avenue, Santa Rosa, CA 95403

  [1]

• Permit Sonoma Resiliency (Rebuild) team

  Rebuilding after wildfires, including ADUs; email Rebuild@sonomacounty.gov

  (707) 565-1788

  [9]

Questions people ask