San Mateo County Accessory Dwelling Units – ADUs

New second units shall be subject to the same requirements as any dwelling unit located on the same parcel in the same district, including but not limited to the requirements of Chapters 20 and 22 of the Zoning Regulations, with the following exceptions:

1. Minimum Lot Area. Second units shall be exempt from the minimum lot area per dwelling unit provisions in the applicable district.
2. Maximum Density of Development. Second units shall be exempt from any and all provisions limiting the maximum density of development in the applicable district.
3. Setbacks. Notwithstanding the required setbacks in the applicable district, minimum setbacks for second units shall be:

1. Detached Second Units of Sixteen (16) Feet or Less in Height. Side Setback: Five (5) Feet
   Rear Setback: Five (5) Feet
2. Detached Second Units Greater Than Sixteen (16) Feet in Height. Side Setback: Five (5) Feet
   Rear Setback: Ten (10) Feet
3. Setbacks Between Attached Second Units and Property Lines. Attached second units shall be subject to the same setback requirements as a primary residence in the same district, except as described in 6429.14, below. Second units constructed entirely within an existing garage shall not be subject to setback requirements. Second units constructed above an existing garage, regardless of height, will be subject to the setbacks in 6429.3(a).
4. Setbacks Between Detached Second Units and Property Lines. The setbacks required between a detached second unit and any property lines shall be as specified in this Chapter. If different setbacks to property lines are required by any other section of the Zoning Regulations, those requirements shall be disregarded, and the standards of this Chapter shall govern.
5. Detached Second Units in Front Of Primary Residence. If any portion of a second unit is located in front of the primary residence, then the front and side setbacks applicable to the second unit shall be those required of a primary residence in the same zoning district.
6. Distance Between Detached Second Units and Other Buildings. The distance required between a detached second unit and any other building on the same parcel must be a minimum of five (5) feet, measured from foundation to foundation. If a separation distance greater than five (5) feet is required by any other section of the Zoning Regulations, it shall be disregarded, and the standards of this Chapter shall govern.

4. Floor Area. The floor area of a detached second unit shall not exceed seven hundred fifty (750) square feet or thirty-five percent (35%) of the floor area of the existing or proposed primary residence, whichever is larger, up to a maximum of one thousand two hundred (1,200) square feet. The floor area of an attached second unit shall not exceed seven hundred fifty (750) square feet or fifty percent (50%) of the floor area of the existing or proposed primary residence, whichever is larger, up to a maximum of one thousand two hundred (1,200) square feet. The floor area of a second unit shall count against the total floor area allowed on a parcel, such that the total floor area of the second unit and the primary residence shall not exceed the maximum floor area allowed within the zoning district.

5. Height. The maximum height of the second unit shall be twenty-six (26) feet. Building height shall be measured as the vertical distance from any point on the lower of (a) finished grade, or (b) natural grade, to the topmost point of the building immediately above. Chimneys, pipes, mechanical equipment, antennae, and other similar structures may extend up to eight (8) feet beyond the building height, as required for safety or efficient operation. Second units built entirely within an existing building shall be subject to the height limit applicable to that building in the relevant district.
6. Balconies and Decks. Second units that do not meet the setback requirements that would apply to a primary residence in the same district shall have no rooftop decks, and no portion of any balcony or deck shall be located above ten (10) feet in height, measured in the same manner as height in Section 6429.5 except on the side of the second unit facing the primary residence. Second units that meet the setback requirements that would apply to a primary residence in the same district may have rooftop decks and balconies to the extent otherwise allowed in the relevant district.
7. Windows. Second units that do not meet the setback requirements that would apply to a primary residence in the same district shall have no windows located above or extending above ten (10) feet on the second unit except on (1) the side(s) of the second unit facing the primary residence, and (2) the side(s) of the second unit that comply with the normal setback requirements of the district. On the sides of the second unit that do not meet the normal setback requirements of the district, clerestory windows located above ten (10) feet on the second unit shall be allowed, if they have a lower sill height of no less than seven (7) feet from the nearest interior floor of the second unit, and a total window height no greater than twenty-four (24) inches. Skylights shall be allowed.

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