San Mateo County Accessory Dwelling Unit (ADU) Standards

        New laws were recently passed, and updates will be coming soon


William Gibson – 650.363.1816

ADU Ordinance

Zones allowed

Bay side: all R zones except R-3

Coast side: R-1

Lot restrictions


Minimum lot size

5,000 sf

Min / Max ADU size

None / 1,200 sq ft

ADU building height

26 feet


5 feet on all sides, unless:

  • Any portion of ADU is in front of primary dwelling: then normal setbacks

  • Garage or other accessory structure conversions: then state law applies

Parking requirements
For 0-2 bedroom units, one space is required and for 3+ bedroom units, two spaces are required, unless state exemptions are met (ADU is within 1⁄2 mile of transit, in existing structures, in historic districts, within one block of car share locations, or in an area with residential parking permit rules that do not allow second units to get a permit). Tandem parking and parking in setbacks is allowed. Permeable paving is required.
Owner occupancy

No requirement

Sample fees from recently built projects*
ADU 1 (626 sq ft, attached): $10,788*
ADU 2 (627 sq ft, detached): $6.577*
*does not include school fee which varies by location and is $1.39-$3.48 / sf
Other requirements
  • ADU must comply with California Building Code fire safety requirements. If the primary dwelling has sprinklers, the ADU must have sprinklers.

  • There are no design requirements, unless the ADU is located in the Coastal Zone, in which case the ADU must be similar in shape, materials and style to the primary dwelling.

  • Tree removals may require a permit per the Heritage Tree Ordinance.

  • Daylight plane and floor area ratio (FAR) requirements may apply.

  • No additional parking is required for garage conversions and replacement parking can be uncovered.

  • Only one ADU per property is allowed.

  • ADUs are only allowed on single family and duplex sites.

  • ADUs must conform to all development standards in the zoning district in which they are proposed.


A planning application is required. Making an appointment to meet with a planner in the initial stages of the project can help avoid challenges and is highly recommended.



Disclaimer: standards and process may vary. Consultation with the contact is recommended before starting any project.

*Fees are specific to each project and will vary. Fees change regularly. These are provided as examples only.

Download ADU Standards PDF

Updated: 12/11/2017