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Permit Sonoma › Regulations & Long Range Plans › Regulations and Initiatives › Housing Types › Accessory Units and Junior Units › Allowability & Zoning
Accessory Dwelling Units

Allowability & Zoning

backyard accessory dwelling unit

Zoning determines how land can be used and what types of buildings can be constructed on a given property. 

ADU Zoning Map Viewer (Coming Soon)

Understanding your parcel’s zoning designation is essential for determining what can be built, including whether an Accessory Dwelling Unit (ADU) is allowed. Use our parcel search tool and read on to understand how these designations come into play. 

Potential Impacts of Zoning & Other Parcel Designations

In Sonoma County, we have various zoning types and unique land features that may require special consideration when building an ADU. Below is an overview of various parcel designations. 

Base Zoning

      • A parcel’s base zone refers to the land use designation assigned to the parcel. Full parcel zoning includes a base zone and any overlaying combining districts. Each has its own standards outlined in the Sonoma County Municipal Code. ADUs and JADUs are established secondary to an existing or proposed single-family or multi-family primary dwelling. There are areas where zoning prohibits these primary dwellings, therefore, by extension, ADUs and JADUs may not be built.

Inland Parcels

Coastal Parcels

      • Chapter 26C lists zoning information including base zones for parcels designated CC Coastal Zone.
      • See Coastal ADUs for more information.

Z ADU Exclusion Combining 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: Applying for a Zone Change

Williamson Act

F1 Floodway

      • Construction is prohibited in the area zoned F1 (Floodway) Combining District. This applies to the portion of a parcel the F1 zone overlaps. Construction of an ADU may still be possible on a parcel designated with F1 zoning if it is large enough to properly distance construction from the actual zone. See Article 56 of the Sonoma County Municipal Code for more information.

F2 Floodplain

      • Construction in an area overlapping with the F2 (Floodplain) Combining District must meet additional construction and design standards. See Chapter 7B of the Sonoma County Municipal Code for more information on construction standards and Chapter 58 for information on the F2 Combining District.

Geologic Hazard Area

      • No structures intended for human occupancy, including ADUs, are permitted to be placed across the trace of an active fault or within 50 feet of the surface trace of any fault. See Article 70 of the Sonoma County Municipal Code for more information.

Water-Scarce Areas

      • Critical Watershed: ADUs are prohibited in Critical Watershed areas unless it can be demonstrated that the ADU will have a net zero increase in water usage on the lot in accordance with Permit Sonoma Policy & Procedure 8-2-2.
      • Critical Habitat Area: ADUs are prohibited in Critical Habitat areas unless it can be demonstrated that the ADU will have a net zero increase in water usage on the lot in accordance with Permit Sonoma Policy & Procedure 8-2-2.
      • Class 3 Groundwater Availability Class: If the ADU would be served by a well in a Class 3 Groundwater Availability Area, the applicant must provide a well test conducted between July 15 and October 1 in accordance with 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 lot with a water supply easement.
      • Class 4 Groundwater Availability Class: ADUs are prohibited in Class 4 Groundwater Availability Areas unless it can be demonstrated that the ADU will have a net zero increase in water usage on the lot in accordance with Permit Sonoma Policy & Procedure 8-2-2. If the ADU would be served by a well in a Class 4 Groundwater Availability Area, the applicant must also provide a well test conducted between July 15 and October 1 in accordance with 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 lot with a water supply easement.

RC Riparian Corridor

      • Development on lots in the RC (Riparian Corridor) district must comply with the requirements in Article 65 of the Sonoma County Municipal Code. RC setbacks may not prohibit an ADU of no more than 800 square feet that meets applicable height standards with four-foot side and rear yard setbacks and that complies with requirements for development within the RC setback.

LG Local Guidelines

      • ADUs located in a Local Guidelines Combining District need to meet applicable objective standards, such as setbacks. Discretionary design review is not required for ADUs. Further information and a list of established LG Local Guidelines Combining Districts can be found in Article 63 of the Sonoma County Municipal Code.

SR Scenic Resources

      • ADUs located in a Scenic Resources Combining District need to meet applicable objective standards, such as setbacks. Discretionary design review is not required for ADUs. Further information for each type of Scenic Resource Combining District can be found in Article 64 of the Sonoma County Municipal Code.

HD Historic District

      • ADUs located in a Historic Combining District need to meet applicable objective standards, such as setbacks. Discretionary design review is not required for ADUs. Further information can be found in Article 68 of the Sonoma County Municipal Code.

State Responsibility Area (SRA)

      • ADUs in SRA must be built to Fire Resistive Construction standards (CA Residential Code Section R337). Standards include Responsibility Area fire-resistive siding, tempered windows, and protections for eaves and underfloor venting. ADUs must have fire Safe Standards (FSS) emergency vehicle access. Additional information can be found on Permit Sonoma’s ADU Fire Prevention Webpage. To see if your property address is located in the State Responsibility Area, please use the State Board of Forestry: SRA Viewer.

CC Coastal Zone 

      • ADUs in the Coastal Zone are referred to as “second dwelling units” or “second units”. Parcels in the coastal zone are held to different, usually more restrictive, standards and permitting requirements than inland parcels. 26C-325.1 of the Sonoma County Municipal Code provides guidelines for the development of second dwelling units in the Coastal Zone.
      • See Coastal Zone ADUs for more information.