Below are some of the most common standards property owners need to know to plan an ADU project. For a complete list of development standards, please review the checklist (see above). 

Note: 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. 

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. Front yard setbacks are determined by the base zoning district (check your parcel's zoning).

For properties with 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. 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.

Septic System Suitability

In areas where public sewer service is not available, the ability to dispose of wastewater on site is a significant factor in determining whether a permit for an accessory dwelling unit can be issued. Property owners are advised to investigate septic capacity on the property and/or in the existing septic system 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. The septic system must meet current code requirements and be adequately sized for the proposed sewage discharge (based upon total number of bedrooms in all the units to be served by that system).

Water Availability and Water-Scarce Areas

In areas where public water service is not available, the availability of sufficient well water is a significant factor in determining whether a parcel can can accommodate an accessory dwelling unit. Parcels in areas with low groundwater availability ("water-scarce areas") must meet additional requirements. 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. Review Section 7-12.2 of  the Sonoma County Building Code for additional requirements if the property is supplied by a well on another parcel with a water supply easement. 

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

Before applying for a building permit, find out whether there are any additional requirements related to wastewater disposal. 

• 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 there are any additional requirements related to water supply. 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

Contact Permit Sonoma divisions directly with questions. Find division contact information