Detached 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

A detached ADU is a standalone building separate from the main house, such as a backyard cottage or a converted carriage house. Homeowners prefer this style because it provides the most privacy for occupants and keeps the main residence quiet. Since it is a completely separate structure, it often requires independent utility connections and foundation work. Exact pricing confirmed free on the call.

Frequently asked in San Mateo

Also serving nearby: Hayward · Daly City · Oakland · Sunnyvale · Fremont

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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: Baywood, Edgewater, Peninsula Point, Shoreview, Serra, Downtown San Mateo — and every surrounding area.