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City of San Mateo Municipal Code.

27.19.050 Development Standards

An ADU, as defined in Section 27.04.165, shall comply with the following development standards:

(a) Permitted Zoning Districts. ADUs shall only be constructed on a lot zoned to allow residential uses.

(b) Number of Units. No more than the number of ADUs allowed by state law may be constructed on any lot.

(c) State Exempted "By Right" ADUs. An attached or detached ADU of at least 800 square-feet in size and of at least 16 feet in building height with four-foot side and rear setbacks may be constructed on a lot regardless of any size limitation based on the size of the proposed or existing primary dwelling, lot coverage, floor area ratio, daylight plane, open space, or minimum lot size that would otherwise preclude or limit its construction. The floor area of any new ADU in excess of 800 square feet shall be applied to the maximum floor area allowance of the subject lot as prescribed by the underlying zoning district.

(d) Maximum Unit Size. Floor area size maximums for both attached and detached ADU development in excess of 800 square feet are governed by the floor area maximum established by the underlying zoning district and remaining floor area allowance available on the lot. In instances when the existing floor area allowance of a lot has been fully utilized, only the state exempted "by right" ADU of up to 800 square feet is permitted.

(1) Accessory Dwelling Unit Conversions. An ADU conversion within the walls of an existing primary residence or accessory structure is not subject to size requirements. ADU conversions may be expanded up to 150 square feet beyond the existing footprint provided the expansion is used to accommodate ingress and egress to the ADU.

(2) Detached Accessory Dwelling Unit and Junior Accessory Dwelling Unit. Subject to the floor area requirements, one new detached ADU may be constructed on a lot with a junior accessory dwelling unit (JADU).

(e) Height. The specific height maximums are applicable to all zoning districts which permit ADUs.

(1) Attached Accessory Dwelling Units. Maximum building height for an attached ADU is 24 feet to top plateline and 32 feet to roof peak as measured from existing grade.

(2) Detached Accessory Dwelling Units. Maximum building height for a detached ADU is 16 feet to top plateline and 24 feet to roof peak as measured from existing grade.

(3) Alternative Discretionary Review for Detached Accessory Dwelling Units. For a detached ADU proposed to exceed the height limit, an application for discretionary review, pursuant to Section 27.19.045, shall be submitted.

(f) Setbacks. An ADU shall have setbacks as follows:

(1) No setback shall be required for an existing legally permitted garage or accessory structure that is converted to an ADU which is constructed in the same location and to the same dimensions as the existing structure.

(2) A minimum setback of four feet shall be required from the side and rear lot lines for all new ADU construction not meeting the criteria set forth in subsection (1) above, including ADUs constructed above an existing legally permitted garage.

(3) A minimum setback from the front lot line, as required in the lot's underlying zone district, shall be required for all new ADU construction not meeting the criteria set forth in subsection (1) above, including ADUs constructed above an existing legally permitted garage.

(g) The ADU, whether attached or detached, shall provide a separate exterior entrance, and permanent provisions for living, sleeping, eating, cooking, and sanitation.

(h) Location of Accessory Dwelling Unit. The ADU may be within, attached to, or detached from the primary dwelling unit. ADUs shall be accessory to the primary residence and are permitted in the same locations on the parcel as the primary residence as specified in the underlying zoning district.

(i) Number and Type of Required Parking Spaces.

(1) General Requirements. There shall be a minimum of one standard size off-street parking space for each ADU. All required parking spaces shall be a minimum of 10 feet wide by 18 feet long without any obstructions. Parking associated with ADU or JADU development, whether required or voluntary, must be located entirely on the same lot, and may be provided in a garage, carport, uncovered, or in tandem orientation. Additionally, maximum driveway width for single-car garages may be up to 20 feet to accommodate additional off-street parking. The required parking for the primary residential dwelling unit must comply with current standards, as specified by the underlying zoning district.

(2) Location of Parking for Accessory Dwelling Units. Parking for the accessory dwelling unit may be located in the required front, side, and rear yard setback areas.

(3) Replacement off-street parking is not required when a garage, carport, or covered parking structure is demolished in conjunction with the construction of an ADU or converted to an ADU.

(4) Parking Exemptions. No additional off-street parking shall be required for an ADU in the following instances:

(A) The ADU is located within one-half (1/2) mile walking distance of public transit; or

(B) The ADU is located within an architecturally and historically significant historic district; or

(C) The ADU is in part of the proposed or existing legally permitted primary residence or an existing legally permitted accessory structure; or

(D) In an area requiring on-street parking, permits are required but not offered to the occupant of the ADU; or

(E) When the ADU is located within one block of a car share location.

(5) Voluntary Parking. If no parking space is required, a maximum of one off-street parking space per ADU or JADU may be voluntarily provided.

(j) Architectural Standards. New ADUs and conversions of an existing legally permitted structure shall be designed to comply with the following standards:

(1) Attached Accessory Dwelling Units. ADUs that are attached to the primary dwelling unit shall:

(A) Be of the same architectural style as the primary dwelling unit;

(B) Be constructed of similar exterior materials, finishes, and family of colors as the primary dwelling unit;

(C) Offset windows from neighbor's windows to maximize privacy; and

(D) Be designed to meet the daylight plane requirements of the R-1 zoning district as applied to the nearest adjacent side and rear lot lines.

(i) Application of the daylight plane requirement shall not preclude a State Exempted "By Right" ADU as defined in this Chapter.

(2) Detached Accessory Dwelling Unit. An ADU that is detached from the primary dwelling unit shall:

(A) In instances when an ADU is attached to an accessory structure, provide a uniform and integrated design with that accessory structure;

(B) Offset windows from neighbor's windows to maximize privacy;

(C) Provide and maintain obscured glazing on second-story windows up to five feet from the finished floor when the windows are located within five feet of a lot line with an abutting residential lot;

(D) When an exterior staircase is proposed, it shall have a setback of at least five feet from the nearest lot line and the size of the second story landing shall be limited to the minimum area required to allow ingress and egress as specified by the California Building Code;

(i) For an ADU that seeks to have a larger second story deck, an application for discretionary review, pursuant to Section 27.19.045, shall be submitted.

(E) Be designed to meet the daylight plane requirements of the R-1 zoning district as applied to the nearest adjacent side and rear lot lines.

(k) Development Impact Fees. Development impact fees for ADUs shall be established in an amount set forth by resolution of the City Council.

(l) Utility Service. If an ADU is constructed within existing space, a separate water connection, a sewer service connection, or power connection is not required for an ADU. If an ADU is not constructed within existing space, the City may require new or separate utility connections.

(m) Utility Fees. For an ADU located within an existing structure, payment of a connection fee or capacity charge is not required. For an ADU that is separate from an existing structure, the City will require payment of a connection fee or capacity charge that is proportionate to the ADU's burden.

(n) Address Assignment. An application for a building permit for an ADU must include application for a separate address assignment.

(o) Other Requirements. All other zoning requirements shall be complied with unless an authorized variance is approved.

(p) For ADUs proposed within an existing multi-family structure or on a lot with an existing multi-family dwelling, the provisions of Government Code Section 65852.2(e)(1) apply.