Accessory Structures in San Mateo County, CA (2026)
6 county-level rules, plus city-specific rules for 2 cities in San Mateo County, California.
Verified from official government sources
ADU Rules
San Mateo County allows accessory dwelling units by right in the R-1, R-2, R-3, PC, CMU, NMU, R-E, RH, RM and TPZ districts and any other residential district under Zoning Regulations Section 8.392.030, and approves ADUs meeting Section 8.392.050's development standards ministerially, with no public hearing, under Section 8.392.040.
San Mateo County ADU Zoning Rules
Some RestrictionsSan Mateo County Zoning Regulations §§ 8.392.030, 8.392.040
Accessory dwelling units shall be allowed in the R-1, R-2, R-3, PC, CMU-1, CMU-2, CMU-3, NMU, NMU-ECR, R-E, RH, RM and TPZ districts, and all other districts in which residential uses are permitted, regardless of any regulations that might otherwise prohibit accessory dwelling units in those districts. Accessory dwelling units meeting all of the applicable requirements of Sections 8.392.050 thr...
ADU Impact Fees
San Mateo County waives development impact fees entirely for accessory dwelling units under 750 square feet, and for larger ADUs charges only a prorated share: Zoning Regulations Section 8.392.050(23) sets the fee at the standard impact fee multiplied by the ADU's square footage divided by the primary dwelling's square footage.
San Mateo County ADU Impact Fee Rule
Some RestrictionsSan Mateo County Zoning Regulations § 8.392.050(23)
23. Impact Fees. Accessory dwelling units of less than seven hundred fifty (750) square feet in size shall be exempt from all impact fees. Accessory dwelling units of greater than seven hundred fifty (750) square feet in size shall only be charged impact fees in an amount equal to the standard impact fee for such a unit, multiplied by the proportion of the square footage of the accessory dwelli...
ADU Owner Occupancy
Unincorporated San Mateo County requires the parcel owner to live in one unit whenever a junior accessory dwelling unit is built, secured by a recorded deed restriction.
San Mateo County Junior ADU Owner-Occupancy Rule
Significant RestrictionsSan Mateo County Code § 8.392.080(5)
Owner Occupancy. The owner(s) of the parcel on which a junior accessory dwelling unit is proposed shall be required to occupy one (1) of the units on the parcel. The owner(s) shall be required to record a deed restriction enforcing this requirement, which shall run with the land, and which shall be provided to the Planning and Building Department. The deed restrict...
ADU Rental Restrictions
San Mateo County bars accessory dwelling units built under its ADU chapter from short-term rental: Zoning Regulations Section 8.392.050(22) requires any ADU that is rented to be rented for a term longer than thirty (30) days, ruling out nightly or weekly stays regardless of the unit's eligibility for a short-term rental permit elsewhere in the code.
San Mateo County ADU 30-Day Rental Rule
Significant RestrictionsSan Mateo County Zoning Regulations § 8.392.050(22)
22. Short Term Rental. Accessory dwelling units created pursuant to the provisions of this Chapter, if rented, shall only be rented for a term longer than thirty (30) days. 23. Impact Fees. Accessory dwelling units of less than seven hundred fifty (750) square feet in size shall be exempt from all impact fees. Accessory dwelling units of greater than seven hundred fifty (...
Shed & Carport Rules
In unincorporated San Mateo County, a detached one-story accessory building in a residential district is capped at 30% of the rear yard, 1,000 square feet, and must sit at least 3 feet from the property line.
San Mateo County Accessory Building Size Rules
Some RestrictionsSan Mateo County Zoning Regulations § 8.348.020
In any residentially zoned district, a one-story detached accessory building may be constructed if the building coverage does not exceed thirty percent (30%) of the rear yard area, does not exceed 1,000 square feet in floor area, has a plate height that does not exceed then (10) feet, and is not within three (3) feet of a property line. The maximum building height shall be nineteen (19) feet ou...
Garage Conversions
Converting an attached or detached garage into an accessory dwelling unit in unincorporated San Mateo County does not require replacing the lost parking spot: Zoning Regulations Section 8.392.050(14)(d) waives replacement parking entirely, and Section 8.392.050(20) confirms garage-based ADUs need no separate use permit if they meet the chapter's standards.
San Mateo County Garage-to-ADU Conversion Rule
Few RestrictionsSan Mateo County Zoning Regulations §§ 8.392.050(14)(d), 8.392.050(20)
d. Garage Conversion. If an existing attached or detached garage is converted to an accessory dwelling unit, the parking previously provided by that garage is not required to be replaced, and if voluntarily replaced, may be replaced by uncovered parking of any type and configuration allowed by Section 8.392.050(14)(f), below, and no additional parking related to the accessory dwelling unit is r...
Accessory Structures by City in San Mateo County
2 cities in San Mateo County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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