Bay Area region / California
San Mateo ADU rules
& permit guide.
Planning an ADU in San Mateo? Review local size, setback and parking requirements, then confirm the details for your property with the permitting agency.
Local source collected 2026-05-20 · View sources
Local requirements at a glance
Min/max ADU size
Min: None Max ADU: No max size limit, dependent on underlying zoning FAR (allowing at least an 800 sq ft) Max JADU: 500 square feet
Height
Detached ADUs: 16 feet to building plate; 24 feet max Attached ADUs: 24 feet to building plate; 32 feet max *Two story units are allowed provided they meet these standards.
Setbacks
Side and rear: 4 feet Front: Set by underlying zoning Separation from main residence (detached): 4 feet Daylight Plane: meet R1 daylight plane from side and rear property line
Parking
JADUs: No parking required ADUs: One spot unless ADU meets one of the State law exemptions
Process
Confirm your parcel and ADU type with the local planning department, prepare the required plans, and follow the city's building permit checklist. Ask which utility, fire and site reviews apply before submitting.
Your next three steps
- Confirm the property's jurisdiction, zoning and site constraints with San Mateo's planning department.
- Discuss the ADU type, dimensions and utility needs with your designer before finalizing a layout.
- Ask the permitting agency for its current application checklist, fee estimate and review process.
2026 state-law context
ADUs and JADUs have different occupancy rules
California generally prohibits local owner-occupancy requirements for ADUs. A narrow exception applies to certain separately sold affordable ADUs. For JADUs, owner occupancy generally applies when the bathroom is shared with the main home; separate-bathroom JADUs are treated differently under 2026 law. Check the HCD handbook and your project's permit conditions.
Older local summaries may not reflect the latest state changes. Read HCD’s March 2026 handbook, pages 32–33.
Questions for your first planning conversation
- Which approval path applies to my building and ADU type?
- Do easements, trees, slope, access or fire conditions affect placement?
- Which utility upgrades and agency reviews should be included in my budget?
- Are preapproved plans available, and what site work still needs review?