Accessory Dwelling Units

Accessory Dwelling Units (ADUs) are backyard cottages up to 1,200 square feet.

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Important Note

As of June 7, 2018, some aspects of Sonoma County’s Accessory Dwelling Unit ordinance have changed. The changes primarily relate to maximum unit size, minimum parcel size, and development impact fees. This webpage reflects those changes.

What?

An accessory dwelling unit is a small permanent home that may be established in addition to the main dwelling on a parcel zoned for residential use. Accessory dwelling units may be attached or detached from the main dwelling. Accessory units may involve new construction, conversion of an existing structure, or designation of an existing home as an accessory dwelling unit so that a new main residence can be built.

Who?

Anyone can live in an accessory dwelling unit. There are no requirements that accessory units be owner occupied or that the occupant be related to the property owner. Accessory dwelling units may not be rented on a transient basis (for periods less than 30 days) and may not be used as a vacation rental.

Where?

Allowed

Accessory units are allowed in residential zones (R1, R2, R3, RR, and AR) and in agricultural and resource zones (LIA, LEA, DA, and RRD).

Not Allowed

Accessory dwelling units are not allowed in the Planned Community (PC) zone, or in areas designated as an Accessory Dwelling Unit Exclusion Combining (Z) zone.

CC&Rs

Conditions, Covenants and Restrictions (CC&Rs) are binding recorded agreements between private parties that regulate the use of property. Commonly, the parties to CC&Rs are a homeowners’ association (HOA) and individual owners in a neighborhood. Some CC&Rs prohibit accessory dwelling units on lots where County zoning allows them. State and local law require that the County must issue a permit when an ADU application meets all zoning standards, but building and zoning permits do not override CC&Rs. Constructing an accessory dwelling unit that is prohibited by the applicable CC&Rs may result in legal action by the HOA against the property owner. The County does not have a role in that dispute because the CC&Rs are a private agreement between private parties. Accordingly, if there are CC&Rs that apply to your lot, you should review the CC&Rs and consult your HOA before you submit a permit application to the County. If your CC&Rs do not permit accessory dwelling units, you may wish to work with your HOA to amend the CC&Rs.

Permits Required

The type of permits and/or approvals required for accessory dwelling units vary according to a property’s features:

  • Location
  • Type of sewage disposal and water supply
  • Unit size

Primary Permits

  • Building
  • Zoning

Owners of properties that don’t meet the minimum parcel sizes shown below may still be able to apply for a minor use permit; contact planning staff at Planner@sonoma-county.org or (707) 565-1900 for more information.

Required Forms for Zoning Permit

All forms listed must be completed.

Supporting Permits

In addition to the zoning and building permits required for the accessory unit, supporting permits that may be required include one or more of the following:

  • Grading permit
  • Septic permit
  • Well permit
  • Sewer permit
  • Public water permit
  • Encroachment permit

Most accessory dwelling units can be approved with a zoning permit. Zoning and building permits for accessory dwelling units will be approved or denied within 120 days of a complete submittal if all required supporting permits are issued first.

Conversion of an Existing Structure in the R1 Zoning District

An accessory dwelling unit created from an existing structure in the R1 zoning district may qualify for a simplified permit process. A building permit will be issued if the proposed accessory unit meets each of the following requirements:

  • Is located in the R1 (Low Density Residential) zoning and not in the Z (Accessory Dwelling Unit Exclusion) combining zone; 
  • Is built within a portion of an existing single-family dwelling or a legal, permitted accessory structure existing as of January 1, 2017; 
  • Has an exterior entrance separate from the single-family dwelling; and
  • Has side and rear setbacks sufficient for fire safety.

If the unit qualifies for this simplified process, separate water and sewer connections are not required. The property owner will record a deed restriction before final occupancy of the unit. No other zoning standards, such as required parking and setbacks, apply. A planner will determine if the proposed accessory dwelling unit qualifies for this simplified process.

Unit Sizes

The maximum size of an accessory dwelling unit is 1,200 square feet on a parcel that meets the minimum parcel size.

However, an accessory unit of 640 square feet with a maximum of one bedroom is allowed on parcels of 1.0-1.99 acres that have potable water.

Parcel Requirements

Whether a particular lot can be developed with an accessory dwelling unit in addition to the main dwelling depends on the zoning of the parcel, the parcel size, the suitability for sewage disposal, and availability of adequate water supplies.

Minimum Parcel Size

Generally, a parcel must be at least 2.0 acres, but an accessory unit may be allowed on parcels between 1.0–1.99 acres if the accessory unit is less than 640 square feet, has only one bedroom, and the parcel has potable water. If served by public water and sewer and located within a designated urban service area, the minimum parcel size is 5,000 square feet

An applicant should confirm with Permit Sonoma the minimum parcel size requirement that applies to the applicant’s parcel. Accessory dwelling units must satisfy the same building code, septic system, water, and zoning requirements as any home constructed in the County.

Water and Sanitation ServiceParcel SizeMaximum Unit Size

Water: Well

Sanitation: Septic

2.0 acres1,200 sf

Water: Public or community water, or on-site well1

Sanitation: Septic

1.5-1.99 acres640 sf, one bedroom

Water: Public or community water

Sanitation: Septic

1.0-1.49 acres2640 sf, one bedroom

Water: Public water 

Sanitation: Sewer

Note: Must also be located in an urban service area

5,000 sf

1,200 sf

1On-site well must meet current potable water supply standards as defined in Sonoma County Code Chapter 25B-3

2Not in Waiver Prohibition Area

NOTE: Parcels that do not meet minimum parcel size or design/development standards can apply for an ADU with a minor use permit.

Septic System Suitability and Water Availability

Soil suitability for a septic system and water availability are significant factors that determine whether a permit for an accessory dwelling unit can be issued. Property owners are advised to investigate septic suitability and water availability 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 Scarce Areas

Accessory dwelling units served by a well in a Water Scarce Area have additional requirements. If the well is located in a Class 3 or Class 4 Water Scarce Area, the applicant must provide a well test conducted between July 15 and October 1 demonstrating the well meets minimum water yield requirements. If the well is located in a Class 4 Water Scarce Area, the applicant must also provide a report demonstrating the accessory unit will not generate additional groundwater use.

An applicant may use a variety of water-saving techniques to satisfy this requirement. Techniques include, but are not limited to, rainwater catchment, retrofitting the existing house with water-saving features, and replacing water-intensive landscaping with low-water-use-landscaping.

Water Scarce Areas are classified in the Sonoma County General Plan Water Resources Element Groundwater Basin and Subbasin Map.

Site Review

Accessory unit permits require a site review by a building inspector. Geotechnical or other reports may be required based upon this review.

Permit Fees

Permit application fees can vary depending upon the type of construction and the availability of public sewer and water. Prior to issuance of the building and zoning permits for any accessory dwelling unit, development impact fees (i.e. school, park, and traffic impact fees), sewer and water connection fees (if new connections are created), and/or well and septic plan check and permit fees are also required.

The cost of these fees varies depending upon the location of the property and the various districts that provide services to the parcel. For current estimated fees, see Permit Sonoma's Estimated Permit Fees handout.

Effective June 7, 2018, development impact fees for accessory dwelling units are reduced on a graduated scale: traffic and parks mitigation fees for units smaller than 750 square feet will be waived entirely, while units over 1,000 will pay the full cost of the fees.

 Size of UnitPercent of Development Impact Fees 
< 750 sf 0%
 751–1,000 sf 50%
 1,001–1,200 sf 100%

Contact Information

Contact Planning by Phone
Monday – Friday
10:00 AM – 12:00 PM
1:00 PM – 3:00 PM
Address
2550 Ventura Avenue
Santa Rosa, CA 95403
38.465074, -122.723705

State ADU Law

California Department of Housing and Community Development's web site describing state laws and regulations about Accessory Dwelling Units.

View State Web Site

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