5 county-level rules, plus city-specific rules for 4 cities in San Mateo County, California.
Verified from official government sources
In unincorporated San Mateo County, ADUs are governed by Development Code Chapter 8.392 (non-coastal) and Chapter 8.396 (coastal). Ministerial approval, no minimum lot size, 4-foot side/rear setbacks, and no short-term rental. Junior ADUs under 500 sq ft require owner-occupancy.
In unincorporated San Mateo County, a detached tool/storage shed needs no building permit if it is 120 sq ft or less and 12 feet or less in height. Detached accessory buildings are governed by Development Code Chapter 8.348, which sets a 3-foot minimum from property lines.
Unincorporated San Mateo County permits converting a garage into an ADU under Development Code Chapter 8.392. California ADU law (Gov. Code 66314) bars the County from requiring replacement parking when a garage is converted to an ADU, and approval is ministerial within 60 days.
In unincorporated San Mateo County, carports are accessory structures regulated by Development Code Chapter 8.348, and required off-street parking may be provided in garages or carports under Chapter 8.344. On steep or elevated front lots, a carport or garage may be built to the front lot line.
Unincorporated San Mateo County has no separate tiny-home ordinance. A tiny home on a permanent foundation can qualify as an ADU under Development Code Chapter 8.392. California ADU law requires a permanent foundation, so movable tiny homes on wheels generally do not qualify as ADUs.
4 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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