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