Accessory Dwelling Units (ADUs) are known by many names like granny units, in-law units, second units, and backyard cottages. An ADU is a small dwelling unit that may be established in addition to single-family or multifamily dwellings in residential, mixed-use, or agricultural zones. An ADU may be detached, attached to the primary dwelling or other accessory structure, or located within a main dwelling and may involve new construction or conversion of an existing structure (such as a detached garage or other accessory structure, or a portion of a dwelling).

A Junior Accessory Dwelling Unit (JADU) is a specific type of ADU that is built within an existing residence, utilizes an existing bedroom, and has a small efficiency kitchen.

More Information on JADUs

The standards below apply to inland zones. For accessory dwelling unit regulations in the Coastal Zone, see Section 26C-325.1 of the the County's Coastal Zoning Ordinance.

Permitted Zones

ADUs are generally allowed in residential, mixed-use, and agricultural and resource zones but are prohibited in the ADU Exclusion (Z) Combining Zone.

Check Your Parcel's Zoning

State law does not allow private covenants such as CC&Rs to prohibit the construction or use of an ADU or JADU on a lot zoned for single-family residential use. The County does not enforce those private covenants.

Number of Units Permitted

A maximum of one ADU and one JADU may be permitted on parcels with a single-family dwelling. On parcels with a multifamily development, the number of ADUs permitted within the footprint of the existing structure may be up to 25 percent of the number of existing units, or one unit, whichever is greater. Up to two detached ADUs may be permitted on a lot with an existing, legal multifamily development.

Maximum and Minimum Sizes

The minimum size of an ADU is 225 square feet. The maximum size of an ADU is 1,200 square feet. ADUs must have permanent provisions for for living, sleeping, eating, cooking, and sanitation.

Setbacks

ADUs must meet at least four foot setbacks from rear and side property lines. For units on septic systems, setbacks will be required from septic areas and required septic reserve areas. Setbacks from waterways are also required where the Riparian Corridor Combining Zone applies. Front yard setbacks are determined by the base zoning district ( check your parcel's zoning). In addition, the Fire Code may require additional setbacks from property lines or between buildings.

Minimum Lot Size

There is no minimum lot size for an ADU, but adequate water and sewer or septic must be demonstrated.

Septic System Suitability

Water availability and soil suitability for a septic system are significant factors that determine whether a permit for an accessory dwelling unit can be issued. Property owners are advised to investigate water availability and septic suitability prior to proceeding with design plans. The construction of an accessory dwelling unit represents a new use on the property which must meet current codes with respect to the septic system for the unit. If one septic system is to serve both the main dwelling and accessory unit, the septic system must meet current code requirements and be adequately sized for the proposed sewage discharge (based upon total number of bedrooms in the two units).

Water Availability and Water-Scarce Areas

Accessory dwelling units served by a well in areas with low groundwater availability must meet additional requirement. If the well is located in a Class 3 or Class 4 Groundwater Availability Area, the applicant must provide a well test conducted between July 15 and October 1 demonstrating the well meets minimum water yield requirements according to the County’s Dry Weather Well Testing policies.

Water-scarce areas are identified on the Sonoma County Groundwater Availability Map and each parcel’s Groundwater Availability Area classification can be found in its parcel report.

Please be aware that multiple permits are typically required to establish an ADU. These permits are explained below.

Step 1: Sanitation

Property owners should investigate the property's septic suitability prior to proceeding with design plans.

• Do you have capacity in your current septic system or will you do a "bedroom swap" to create capacity for the new unit? Contact a consultant for professional help to research your current system, prepare a findings report, or prepare an application for a bedroom swap.
• Will you need to build a new septic system? Contact a consultant for professional help to design and apply for a permit for a new system.
• Do you have sewer service? Make sure your sewer provider will provide service to the new unit and contact a professional for help preparing an application for a sewer permit.

Step 2: Water

Before applying for a building permit, find out whether you have any additional requirements related to water. Water requirements may vary based on type of water service and location.

• Do you have a well in a water-scarce area? Contact a professional to conduct a dry weather well test (conducted between July 15 and October 1) demonstrating that the well meets minimum water yield requirements according to the County’s Dry Weather Well Testing policies. Find you parcel’s Groundwater Availability Area classification in the parcel report.
• Do you need a new well? Contact a professional for help preparing an application for a new well.
• Do you have public water service? Check with your provider to make sure they will provide service to the new unit.

Step 3: Building Permit

The type of permits required for ADUs can vary. At minimum, all units will typically require a building permit but may also require a site review, grading permit, and encroachment permit. All property owners must sign and submit the Accessory Dwelling Unit Rules and Performance Standards with their building permit.

See submittal requirements for residential building permits

Apply for a building permit online

More Information

If you have questions for multiple departments, the Permit Sonoma Permit Center is the best place for prospective applicants to meet with representatives from each permitting department to discuss potential permit requirements, submit applications, or being the process of creating an ADU or JADU. 

A separate planning permit is no longer required but building permits will be routed to a planner for a planning clearance. For zoning questions related to ADUs and JADUs, please contact planner@sonoma-county.org or (707) 565-2900 (option 5). 

Does my property qualify for an accessory dwelling unit or a junior accessory dwelling unit?

If your property is within a single-family or multifamily zone, has an existing or proposed dwelling, and can provide adequate water supply and sewer or septic capacity, it may qualify for an ADU.

Can accessory dwelling units be used for vacation or short term rentals?

ADUs may not be rented for periods of less than 30 days or used as vacation rentals.

Who can live in an accessory dwelling unit? 

Anyone can live in an ADU and they are not required to be occupied by an owner or relative of the owner.

How much are permitting fees?

Prior to issuance of the building permits for any ADU, the following fees are typically required:

• Development impact fees (e.g.: school, park, and traffic impact fees)
• Sewer and water connection fees (if new connections are created)
• Well and septic plan check and permit

Potential Fee Reductions

Permit fees are typically reduced for many ADUs and JADUs and can vary between roughly $15-35,000. Units built within the existing footprint of a single-family residence or accessory structure are not subject to sewer or water connection fees or capacity charges.

Traffic and parks mitigation fees in particular are reduced based on the size of the unit:

Find out more about estimated permitting fees