From the full articles at:
Accessory Dwelling Units (ADUs) are backyard cottages of between 250 and 1,000 square feet.
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.
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 (periods less than 30 days) and may not be used as a vacation rental.
Accessory units are allowed in residential zones (R1, R2, R3, RR, and AR) and in agricultural and resource zones (LIA, LEA, DA, and RRD).
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. Certain private property restrictions may also prohibit accessory units. These restrictions are generally found in private Conditions, Covenants and Restrictions (CC&Rs) that affect specific developments or subdivisions. A property title report should provide references to these types of private restrictions.
The type of permits and/or approvals required for accessory dwelling units vary according to a property’s features:
- Type of sewage disposal and water supply
- Unit size
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. Use permits are no longer required.
Full article at http://sonomacounty.ca.gov/PRMD/Regulations/Accessory-Unit-and-Junior-Units/Accessory-Dwelling-Units/