Accessory or Junior Dwelling Units (ADUs and JADUs) are residential units located on the same property as a single-family or multi-family residence. 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). Find the regulating ordinance linked below.
Non-Coastal Zone
San Mateo County staff is actively updating its non‑coastal zone ADU regulations to reflect recently enacted state legislation and new statewide interpretations, including statutory changes adopted in 2026. This work ensures the County’s ADU chapter remains fully aligned with evolving state ADU law while maintaining clear, objective standards for ministerial review.
Coastal Zone
- Chapter 8.396 Accessory Dwelling Unit Regulations
- LCP Map 3.2 ADU Special Parking El Granada-Pillar Point
- LCP Map 3.1 ADU Special Parking Moss Beach-Montara
How to Apply
Resources:
- ADU Resource Center
- JADU Deed Restriction
-
ADU Summary and Checklist (To be provided at a later date)
FAQs
Q: Where are they allowed?
- ADUs/JADUs are allowed in all single-family residential, multi-family residential, or mixed-use zoning districts that permit residential uses in non-Coastal Zone areas.
- ADUs/JADUs are allowed in all residential zoning districts in Coastal Zone areas.
- To find out the zoning of your property, view the Zoning Map.
Q: How many are allowed?
- On a property zoned for single-family residential, and not located in the Coastal zone, you can generally build all state-exempt ADU Types: one conversion (or new attached) ADU, one detached, new construction ADU, and one JADU, in combination with one non-state-exempt ADU, for a total of four ADUs. Pursuant to applicable state laws, 66323 (state exempt) and 66314 (non-state exempt).
- On a property zoned for multi-family residential, you can build up to eight detached ADUs with existing multi-family dwellings, or up to the number of existing primary units; if the property only has proposed multi-family dwellings, then you are allowed up to two detached ADUs. And, in combination with at least one conversion ADU, up to 25% of existing multi-family dwelling units in portions of the structure not used as living space—no JADUs allowed on multi-family zoned lots.
Q: What are the required setbacks?
- For ADUs that replace an existing structure (with the same location and dimensions), the setbacks of the existing structure apply. Otherwise, four (4) feet setbacks are required for the side and rear yards.
- Front setback is based on your property's zoning district, but can be reduced as necessary to accommodate an ADU of up to 800 square feet (State Exempt ADU).
Q: What is the allowed height?
- For state-exempt ADUs that replace an existing structure (with the same location and dimensions), the height of the existing structure applies. Otherwise, your structure must be 16 feet or less in height. A height limit of 18 feet, plus two (2) feet, matches the main building roof pitch and is allowed if locked within one-half mile of a BART station.
- For non-state exempt ADU, the height limit is 26’ and the ADU Ordinance includes other provisions related to daylight plane and stepbacks.
Q: What size is allowed?
- For state-exempt ADUs, a new structure can be up to 800 square feet.
- For non-state-exempt ADUs, the maximum allowed size of your structure will depend on the attached/detached type and the proportion of the existing or proposed primary residence.
Q: What are the design standards?
- State-exempt ADUs are exempt from design standards.
- Non-state-exempt ADUs located within Design Review “DR” Districts must comply with the local, objective standards described in the recently adopted Design Review Ordinance.
Q: What are the owner occupancy requirements?
- No owner-occupancy requirements for renting out ADUs.
- For JADUs, if the unit does not contain its own sanitation facilities, then owner occupancy is required via a JADU deed restriction, where it is recorded that the owner must live in the JADU or in the main house containing the JADU.