Second units, also known as “accessory dwelling units,” (or “granny units,” “in-law units,” and various other names) are smaller units built on the same property as, or attached to, an existing primary residence. The County of San Mateo regulates the production of second units in the unincorporated areas of the County only (those areas that are not in the jurisdiction of incorporated cities).

On January 10, 2017, the San Mateo County Board of Supervisors adopted an update to the County’s Second Unit Regulations, Chapter 22.5 of the Zoning Regulations. The update formalizes the review and approval of those second units which comply entirely with zoning regulations as a non-discretionary approval, meaning that if the unit complies with applicable regulations, it will not be reviewed at a public hearing, will not be appealable, and will only require a building permit.

The County is also working with Home For All San Mateo County and 21 Elements to provide a Second Unit Resources Center for property owners to promote and ease the construction of new Second Units.