Below are some of the more significant standards property owners need to know to plan an ADU project. For a complete list of development standards, please review Zoning Code and the Permit Sonoma ADU Checklist. 

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. the Z Accessory Dwelling Unit Exclusion Combining District and on properties subject to a Williamson Act contract in accordance with the current Uniform Rules for Agricultural Preserves and Farmland Security Zones. 

ADU Exclusion District

The Z Accessory Dwelling Unit Exclusion Combining District prohibits ADUs where there are natural hazards, public safety issues, or water availability or water quality issues. In 2019, the County reevaluated the application of this district and ultimately rezoned over 1,900 parcels to remove the Z Combining District and allow ADUs as a permitted use. Parcels that were not rezoned and still contain this Z Combining District may apply for a zone change. A zone change is a discretionary permit, considered on a case-by-case basis, and approval is not guaranteed. The proposal must be reviewed by the Planning Commission and ultimately approved by the Board of Supervisors.

More information about applying for a zone change

Check your parcel's zoning

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 multi-family buildings, the maximum number of ADUs will vary. Please review the ADU Checklist and the Zoning Code. 

Maximum and Minimum Sizes

ADUs must meet minimum building code standards for living space. ADUs must have permanent provisions for for living, sleeping, eating, cooking, and sanitation. The maximum size of an ADU is generally 1,200 square feet. 

Setbacks

ADUs must meet of at least four feet from rear and side property lines, provided that applicable fire-resistive construction standards are met. Front yard setbacks are determined by the base zoning district (check your parcel's zoning). Check with the Fire Prevention division for information about applicable fire safe standards. 

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 to establish 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 ADU 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 ADU 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 ADU. Parcels in areas with low groundwater availability ("water-scarce areas") must meet additional requirements. Water-scarce areas are identified on the Sonoma County Groundwater Availability Map and the County's Zoning and Land Use GIS Viewer, and each parcel’s Groundwater Availability Area classification can be found in its parcel report.

Well Yield Test

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. 

Zero Net Use Requirements

If the well is located in a Class 4 Groundwater Availability Area or a Critical Habitat Area, the applicant must demonstrate compliance with Permit Sonoma Policy and Procedure 8-2-2 Guidelines for Net Zero Groundwater Use. 

Multiple permits are usually required to establish an ADU. The most common permit types are for sanitation, water, and building. Some considerations for each permit type are described 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 to analyze the capacity and quality of 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 if one is required. 

If sewer or septic permits are needed, these permits should be submitted and in “Issued” status prior to applying for a building permit for the ADU. 

Step 2: Water

• Before applying for a building permit, find out whether there are any additional requirements related to water supply. Water requirements vary based on the type of water service and location.
• Will the ADU be served by a well in a water-scarce area (see above)? 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 your parcel’s Groundwater Availability Area classification in the parcel report.
• If the well is located in a Class 4 Groundwater Availability Area or a Critical Habitat Area, the applicant must demonstrate compliance with Permit Sonoma Policy and Procedure 8-2-2 Guidelines for Net Zero Groundwater Use. 
• 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 ADU.

Well permits should be submitted and in “Issued” status prior to applying for a building permit for the ADU. Well yield studies should be completed and submitted to the Well & Septic Division prior to applying for a building permit. 

Step 3: Building Permit

At minimum, newly constructed ADUs require a building permit, but depending on the scope of construction, a site review, grading permit, and/or encroachment permit may also be required. 

Apply for a building permit online

Most permit applications are submitted and processed electronically. Visit our  online permitting website to start and submit a building permit or other development permit application, pay fees, schedule an inspection, and more. 

Please review and ensure you have the minimum  submittal requirements for residential building permits.

All property owners must sign and submit the   Accessory Dwelling Unit Rules and Performance Standards with the building permit application.

Fees

The size of your ADU is one of several factors that determines building permit fees for an ADU. In addition to building permit or other permit processing fees, the following fees may apply to ADUs. 

• Impact Fees. ADUs less than 750 square feet in size are exempt from park and traffic impact fees. For ADUs greater than 750 square feet, impact fees are charged proportionately in relation to the square footage of the primary dwelling unit.
• School Fees. ADUs greater than 500 square feet in size may be subject to school fees by the applicable school district.  <a>Contact your local school district for information. 
• Water/Sewer Connection Fees. New or separate utility connection, or related connection fee or capacity charge, will not be required for the following types of ADUs:
  • Junior Accessory Dwelling Units
  • Internal conversions of existing space within a  <a>primary residence or an accessory structure 

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

The Napa Sonoma ADU Center is an organization that assists homeowners with the ADU planning process within Napa and Sonoma County jurisdictions. The center offers educational webinars, personalized feasibility consultations, a home match program, and more.