San Mateo County, CA Accessory Structures: Tiny Homes (2026)

San Mateo County does not have a stand-alone tiny-house ordinance for unincorporated areas. A small permanent dwelling typically must be built and permitted as an Accessory Dwelling Unit under Development Code Chapter 8.392 (non-coastal) or Chapter 8.396 (Coastal Zone), meeting all ADU and building-code standards. Under California ADU law (Gov. Code 66310-66342), an ADU must be built on a permanent foundation; the statute defines an ADU as attached or detached residential dwelling space with permanent provisions for living, sleeping, eating, cooking, and sanitation. As a result, a movable tiny house on wheels (a recreational-vehicle-type or DMV-registered chassis unit) generally cannot be permitted as an ADU in unincorporated San Mateo County, because it is not on a permanent foundation. There is no County ordinance authorizing movable tiny houses as ADUs, unlike the special movable-tiny-house program adopted by the City of Los Angeles. A detached ADU in the County may be 800 square feet or larger and up to 16 feet in height (the County may not preclude an 800-sq-ft, 16-foot ADU with 4-foot side/rear setbacks), so small permanent cottages are feasible. Living in an RV or trailer as a residence is not an authorized ADU use.