Rio Vista Municipal Code 17.44.030 Second Dwelling Units refers to ADUs.


Applicability. Second dwelling units are permitted on any lot where one single-family detached dwelling is permitted or exists. Definitions for second units are provided in Section 17.02.040.

Second units shall not be counted as an additional dwelling for the purposes of calculating maximum permitted residential zoning density.

General Regulations.

  1. One second unit is allowed on any parcel or lot with the minimum required site or building area of the zoning district in which the parcel or lot is located, on which exists a detached single-family dwelling;
  2. Second units are not allowed on lots with two or more existing dwellings, or on any lot with a guest house;
  3. Any applicant seeking approval for a second unit shall be an owner of the property for which a second unit is proposed;
  4. A second unit may only be rented, leased, and/or occupied for residential purposes;
  5. Property containing a second unit shall not be subdivided from that on which the principal dwelling exists;
  6. Second units may be contained within the principal dwelling structure or in a detached building;
  7. Any illegal building additions or accessory structures shall be brought into compliance with all applicable city codes and ordinances prior to approval of a second unit building permit;
  8. The placement of a second unit on any property shall comply with all building setbacks and other zoning regulations set forth in this code.

Floor Area, Lot Coverage and Minimum Yard Area(s)/Private Open Space.

  1. Floor Area. No second unit shall exceed forty (40) percent of the floor area of the principal dwelling or one thousand two hundred (1,200) square feet, whichever is less. Notwithstanding, the floor area of a second unit shall not be required to less than five hundred (500) square feet. The maximum floor area shall not exceed more than thirty (30) percent of the total buildable lot area.
  2. Lot Coverage. Total lot coverage of all structures located on the lot shall not exceed the maximum coverage allowed by the zoning district in which the lot is located.
  3. Yard Area/Private Open Space. The minimum required yard area(s) or private open space requirements of the zoning district in which the parcel is located shall apply.


  1. Number of Spaces Required. Exclusive of the parking required for the principal dwelling, the following parking shall be provided for a second unit:
  2. One space shall be required for an efficiency or one-bedroom unit.
  3. Two spaces shall be required for units having two or more bedrooms.
  4. Location and Size. Parking spaces for the second unit shall be located where they will not interfere with or obstruct required parking for the principal dwelling unit. Parking shall not encroach into a front or street side yard except in a driveway apron (e.g.: between a garage door and the curb cut). When the parking area is located in a required side yard, a two-foot landscaped buffer is required. The minimum dimensions of a required parking space shall be eight feet by eighteen (18) feet.

Design Standards and Site Location.

  1. The design shall be architecturally consistent with the principal dwelling, including form, exterior siding and/or trim, roof materials, and window placement/type. It shall also be compatible with the building height, mass and general design of other buildings in the vicinity of the site;
  2. If attached, the entrance to the second unit shall not be located on the same building elevation as the entrance to the principal dwelling. The second unit entry shall be clearly subordinate to that of the principal dwelling;
  3. A detached second dwelling unit shall not exceed a maximum height requirement of twenty-five (25) feet;
  4. No detached second dwelling unit shall be located closer to any structure than permitted by the Uniform Building Code; however, in no event shall a detached dwelling unit be located less than five feet from any dwelling unit
  5. At minimum, the second dwelling unit shall observe the accessory structure side and rear yard setbacks as indicated in Section 17.44.090;
  6. A second dwelling unit created by the internal conversion of an existing single-family dwelling shall not occupy more than forty-five (45) percent of the habitable floor area of the building after conversion, excluding garage area;
  7. All second dwelling units must be constructed with a permanent foundation;
  8. Second units located within ten (10) feet of a residence located on an adjoining property shall require landscape screening and/or fencing. The applicant must submit a landscaping plan for staff review and approval prior to occupancy.


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