Cost to build accessory dwelling unit — San Mateo, CA
Last updated: September 24, 2026
What affects the price
Building an ADU involves several moving parts that shape your total budget. The biggest factor is whether you choose a prefabricated unit or a custom build on-site. Site preparation is also significant; if your land requires extensive grading, utility trenching, or foundation work, costs will climb. Your choice of finishes—like kitchen cabinetry, flooring, and siding materials—also shifts the bottom line. We help you navigate these variables to find a setup that fits your property. Exact pricing confirmed free on the call.
About this service
When calculating the budget for a secondary unit, consider the costs for architectural design, permits, materials, and labor. Custom designs and high-end interior finishes will push your budget higher, while standardized layouts can keep costs more predictable. Exact pricing confirmed free on the call.
Frequently asked in San Mateo
What ADU financing options are available in San Mateo?
How do I find an ADU company in San Mateo?
What is the accessory dwelling unit contractor cost in San Mateo?
Can I build a backyard accessory dwelling unit in San Mateo?
What are ADU cost considerations in San Mateo?
What ADU services are available in San Mateo?
What is a detached accessory dwelling unit?
Also serving nearby: Hayward · Daly City · Oakland · Sunnyvale · Fremont
More on this: HUD ADU resources — ADU policy and financing research.
Service area details
Serving all of San Mateo — population 101,327; about 8,350 residents per square mile across 12.1 sq mi. ZIP codes covered include 94401, 94402, 94403, 94404, 94497.
Neighborhoods served in San Mateo: Peninsula, Bay Meadows, Downtown San Mateo, Horne Place, Edgewater — and every surrounding area.
Areas we serve
Browse by state, then city.
California (4)
All of California · Rialto · San Buenaventura (Ventura) · San Mateo · Vacaville