ADU Regulations in San Mateo, California
San Mateo County · CA · Pop. 105,000
Max ADU Size
1,200
sq ft detached
Rear Setback
4 ft
Permit Timeline
60 days
Build Cost
$220,000 – $430,000
Zoning
Allowed ADU Types
State law preempts local owner-occupancy requirements. One ADU plus one JADU permitted per single-family lot. Multifamily lots may allow multiple detached ADUs in existing non-livable space per state law.
Source: City of San Mateo — Accessory Dwelling Units · San Mateo Municipal Code Chapter 27.19
Dimensions
Size & Setback Requirements
Detached
1,200
sq ft max
JADU
500
sq ft max
Attached
50% of primary home, max 1,200 sq ft
Setbacks
front
Same as primary dwelling
side
4 ft
rear
4 ft
Max height: 16 ft
Lot coverage: Per underlying zone
ADUs built within the envelope of a legally established structure require no additional setbacks. Two-story ADUs may be permitted with design review in some zones; check with Planning for height exception process.
Permitting
Permit Process
Department
City of San Mateo Community Development Department
Review Timeline
60 days
Permit Fees
$11,000 – $22,000
Impact Fees
Impact fees waived for ADUs under 750 sq ft per state law
Required Documents
- ›Site plan
- ›Floor plans
- ›Elevations
- ›Title 24 energy compliance
- ›Structural drawings
- ›Geotechnical report (if required by slope conditions)
San Mateo permit fees reflect Peninsula market rates and robust city services. The city offers an online permit portal for ADU pre-screening. School fees and park fees may apply for ADUs over 750 sq ft. Early pre-application meeting strongly recommended.
Visit City of San Mateo Community Development Department →Infrastructure
Utilities & Requirements
Solar
Required
Parking
Not required
Separate Meter
Optional
Fire Sprinklers
See note
Financials
ADU Costs in San Mateo
Typical Build Cost
$220,000 – $430,000
Cost per Sq Ft
$300 – $385
Monthly Rental
$2,300 – $3,700/mo/mo
Property Value Added
Adds $350,000 – $600,000 to property value
Available Grants & Programs
- ★CalHFA ADU Grant Program available
- ★San Mateo County ADU loan programs (check county housing office)
San Mateo sits at the heart of Silicon Valley's northern corridor. Caltrain Baby Bullet service makes ADUs attractive to San Francisco and South Bay tech workers. High property values mean ADU ROI is among the strongest in California. Rental demand is consistent year-round with very low vacancy.
Use the free ADU Cost Calculator →Free Service
Get Contractor Quotes in San Mateo
Connect with licensed local ADU contractors. Takes 2 minutes, no obligation.
San Mateo Quick Facts
- Max size1,200 sq ft
- Rear setback4 ft
- Permit timeline60 days
- Build cost$220,000 – $430,000
- Rental income$2,300 – $3,700/mo/mo
- Owner occupancyNot required
Official Source
City of San Mateo — Accessory Dwelling UnitsSan Mateo Municipal Code Chapter 27.19
Last verified: March 14, 2026
Free Tools
Stay Updated
ADU rules change often.
Get notified when San Mateo's regulations update, plus guides on permitting and financing.