Fairfield uses the term “Second Dwelling Units” for ADUs in the Municipal Code section 188.8.131.52:
Second Dwelling Units and “Granny Flats” (collectively referred to as “second unit(s)”) are permitted on any lot where one single family detached dwelling is permitted or exists. Definitions for Second Dwelling Units and Granny Flats are provided in Section 25.50. Second units shall not be counted as an additional dwelling for the purposes of calculating permitted General Plan or residential zoning density.
B. Restrictions. All second units shall comply with the following regulations:
1. One second unit is allowed on a parcel 5,000 square feet or greater containing a detached single-family dwelling. Any second unit proposed on a lot with a detached garage or any lot with alley access shall be exempt from this minimum lot size requirement.
2. Second units are not allowed on lots with two or more dwellings.
3. Any applicant seeking approval for a second unit shall be both an owner and current resident of the property for which a second unit is proposed. An exception to the occupancy requirement is allowed where a second unit is to be constructed concurrent with the principal dwelling, and the owner will reside on the property upon completion of construction.
4. A second unit may only be rented, leased, and/or occupied for residential purposes. Property containing a second unit shall not be subdivided from that on which the principal dwelling exists.
5. Second units may be contained in an addition to the principal dwelling, within the principal dwelling, or in a detached building.
7. Setbacks. Second units shall comply with all setbacks established in Table 25-7, Accessory Structures and Room Addition Regulations, except as follows:
a. In any property in the RM Zoning District whose rear property line abuts a public alley, the rear yard setback applicable to a garage (i.e., three feet) shall also apply to any second unit constructed above the garage.
b. No additional setback shall be required for conversion of an existing garage or an existing accessory structure to a second unit as long as the structure and location meet fire and building code requirements.
c. A setback of no more than five feet from the side and rear lot lines shall be required for a second unit that is constructed above an existing garage.
8. Construction of a second unit must avoid creating any adverse impacts on any real property that is listed in the California Register of Historic Places.