Permitting Is More Predictable Than You Think.
State law obligates San Mateo to review every ADU permit submission, so the city cannot sit on your application or send it to a discretionary hearing. From your first submittal, plan on roughly three to four months for the full permitting process.
The city publishes its own guidance for homeowners on the City of San Mateo ADU page, which is the best starting point for confirming how the rules apply to your specific parcel. Not sure your lot qualifies? Run it through our free lot check and we will tell you.
Size And Height Limits.
An 800-square-foot ADU is permitted citywide, no matter which zoning district you are in. Beyond that, allowed size depends on your district's Floor Area Ratio: one-bedroom detached units can reach 850 square feet, and two-bedroom detached units can go up to 1,000 square feet.
New detached ADUs can rise to 16 feet at the plate line and 24 feet at the roof peak. Converting space inside an existing home or accessory structure is even more generous: conversions are not subject to size regulations at all, with height allowances of 24 feet to the plate and 32 feet to the peak.
Setbacks, Windows, And Design Rules.
Every San Mateo ADU needs at least 4 feet of setback from the rear and side property lines. Beyond placement, the city cares about privacy: windows must be offset from windows on neighboring properties, and any second-story window within five feet of a residential lot line needs obscured glazing.
Attached ADUs must match the architectural style of the structure they join, using similar materials, finishes, and paint colors. Detached backyard units like Abodu's have more design freedom, and the city's ADU and JADU handout summarizes the standards in one place.
Who Can Build, And How Many.
ADUs are allowed on any residentially zoned property in San Mateo, single-family or multifamily. JADUs are the one exception: they are not permitted on mixed-use properties. Parking for an ADU can be located anywhere on your site, including the driveway or setback areas, and the requirement is waived entirely if you live within a half mile of transit or in a historic district. JADUs never require parking.
Multifamily owners get real scale here: up to 8 detached ADUs per property, plus conversions of non-residential space such as storage rooms or offices, capped at 25 percent of the existing unit count. If that is your situation, see how Abodu handles multifamily ADU projects.
What It Actually Costs.
Custom site-built ADUs in California routinely exceed $250,000, and that is before change orders start stacking up. Abodu works differently: transparent published pricing: homes from $234,800 plus a published installation price, an expected all-in from $298,800 that covers design, permits, the factory build, delivery, and installation. See how the process works from first call to move-in.