Development Standards for ADUs

Given a parcel’s zoning designations allow the addition of an Accessory Dwelling Unit (ADU), objective standards outlined below apply.

• For information on how zoning and other parcel designations may impact ADU development, see ADU Allowability and Zoning or refer to the Preliminary ADU Checklist (PJR-149).
• The following information is a summary of standards applicable to inland ADUs NOT located within the Coastal Zone. Corresponding code text may be found in 26-88-060 of the Sonoma County Municipal Code.
• For development standards applicable to ADUs located within the Coastal Zone, refer to Sec. 26C-325.1 of the Sonoma County Municipal Code.

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).

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 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 search.

• 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 Codefor 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.

The following applies to Accessory Dwelling Units (ADUs) located inland, not in the coastal zone. For parcels with CC Coastal Zone designation, refer to Coastal Zone ADUs.

ADUs are accessory to an existing or proposed primary dwelling. Per County Code Sec. 26-88-060, a certificate of occupancy for an ADU cannot be issued before the primary residence.

Building an ADU Before a Primary Dwelling

Existing dwellings that meet ADU standards may be re-designated as an ADU when a new primary dwelling is proposed, with zoning permit approval.

Number of ADUs Allowed

On a lot with an existing or proposed single-family dwelling:

• One attached ADU within or attached to an existing single-family dwelling or accessory structure
• One detached, new construction ADU

On a lot with an existing multifamily dwelling:

• Two detached ADUs
• Up to 25% of existing units (minimum one), converted from non-livable space in a multifamily structure

On a lot with a proposed multifamily dwelling:

• Two detached ADUs

Lot Size and Coverage

There is no minimum lot size for an ADU inland, but the parcel must be large enough to meet zoning and building code requirements. Lot coverage is determined by the base zoning district, unless compliance would prevent an ADU of at least 800 square feet.

Size Requirements

• Minimum Size: ADUs must meet building code standards for living, sleeping, eating, cooking, and sanitation.
• Maximum Size: Generally 1,200 square feet. Zoning setbacks may limit size, but at least an 800-square-foot ADU is permitted if it meets height and setback standards.

Fees and Exemptions

• Impact Fees: ADUs under 750 square feet are exempt from park and traffic impact fees. Larger ADUs are charged proportionally.
• School Fees: ADUs over 500 square feet may be subject to school district fees.

Setbacks

• Front Yard: Determined by base zoning unless it would prevent an 800-square-foot ADU.
• Side and Rear Yard: Minimum 4 feet.
• Septic and Waterways: Setbacks apply for septic areas and riparian corridors. See Chapter 7B – Flood Damage Prevention.
• Fire Code: Additional setbacks may apply. See ADU Fire Prevention.

Height Limits

Per Sec. 26-88-060(h)(4), height limits depend on ADU type and zoning.

• Detached ADUs: 18 feet or the base zoning limit for accessory structures.
• Attached ADUs: Base zoning limit applies.
• ADUs above accessory structures: Base zoning height limits apply.

Parking

One parking space is required unless the ADU meets one of the following exemptions:

• Within ½ mile walking distance of a transit stop
• Located in a historic district
• Part of an existing or proposed primary residence or accessory structure
• On a lot where on-street parking permits are required but not available to the ADU
• Within one block of a car-share vehicle
• Studio units
• Submitted with a new single-family or multifamily dwelling

Solar (Photovoltaic) Requirements

New ADUs must comply with California Energy Code solar requirements unless exempt. See Section 150.1.14.

Universal Design (Optional)

Universal Design promotes accessibility. Features such as wide doorways, step-free entries, and lever-style handles are encouraged. Learn more at the Universal Design page.

Affordable ADU Standards

ADUs intended for rent under affordability requirements must comply with Article 89 and Sec. 26-88-060(k).

ADUs must be designed and built to meet all relevant construction standards such as those listed below.

• California Building Code
• California Energy Code
• CalGreen Building Standards
• California Fire Code
• Sonoma County Building Code - Ch 7 of the Sonoma County Municipal Code
• Sonoma County Fire Code – Ch 13 and 13A of the Sonoma County Municipal Code
• Fire Prevention for ADUs
• Photovoltaic Construction Standards

Consider using a pre-reviewed ADU plan to reduce work, time, and money spent designing and permitting your ADU. Pre-reviewed plans take the burden of certain design and construction standards off your plate by providing a base plan for the ADU itself. See our ADU Ready page for more information about this new program.

Manufactured homes and Factory built housing are two additional options for reducing time and fees building an ADU. Stock plans featured in our pre-reviewed ADU design gallery may include designs of each type. Using these housing options may reduce time, effort, and costs associated with designing, permitting, and constructing an ADU.

Factory Built Housing is a type of housing where the components are built in a factory then shipped to a project site for assembly. The components can vary from individual wall panels to 500+ square foot modules. Interior and exterior finishes can vary from completely finished at the factory to all finishes installed on site. Once assembled, there is no difference between Factory Built Housing and a site built residential structure. Both are under the jurisdiction of the County of Sonoma. Plans for Factory Built Housing must be stamped by a California Department of Housing and Community Development (HCD)-approved Design Approval Agency on each page. The same building codes and the same permitting process apply to both Factory Built Housing and site built residential structures with one exception: plan check fees are not charged for portions of the structure that are Factory Built Housing. 

Review Policy 4-0-9: Factory Built Housing Permits for procedures, fees, and additional information.

Manufactured Homes, previously known as mobile homes, are movable structures made of one or more sections, designed and set up as one or more home units and usable with or without a foundation. However, foundations are required for all accessory dwelling units including those that are manufactured homes.

The California Department of Housing and Community Development (HCD) oversees the manufacture of units that are sold, leased, and used within the State of California. Units manufactured outside the state of California must meet standards set by the Federal Department of Housing and Urban Development (HUD) and the California Health and Safety Code Section 18551. As a general rule, federal and state regulations preempt local building standards and establish a procedure for permit issuance. However, local zoning regulations are allowed to limit installation options. Permit Sonoma’s responsibility consists of verifying the submitted information complies with either HUD or HCD standards as limited by our zoning regulations. Project scope associated with building a manufactured ADU outside of the ADU itself must still be reviewed.

Review Policy 4-0-10: Placement of Manufactured Homes on Private Property for procedures, fees, and additional information.

• PJR-149 Preliminary ADU Checklist (PDF)
• ADU Allowability Map
• ADU Development Standards
• ADU Ready: Pre-reviewed ADU Plans
• ADU Ready Project Application Procedure and Checklist (PDF Coming Soon)
• CSS-026 Owner-Builder Information and Authorization (PDF)
• Schedule an Inspection
•  Frequently Asked Questions

Contact Permit Sonoma divisions directly with questions.

Find Division Contact Information