ADUs in San Mateo County (Peninsula): Rules by City

From Daly City down through Redwood City, the Peninsula is one of the most active ADU markets in the state — strong rents, great schools, and families who want a place for parents or adult kids close by. The catch: ADU rules in San Mateo County depend entirely on whether your home is in the unincorporated county or inside one of its 20 cities. Here’s how to figure out which applies to you. (For the statewide baseline, see our Bay Area ADU guide.)

First, find your jurisdiction

San Mateo County’s own ADU ordinance governs only unincorporated areas — the pockets outside any city’s limits. If your address is inside a city, that city’s rules apply instead. A great free tool for checking your exact standards is the San Mateo County ADU Resource Center (smcadu.org), which publishes the current rules for the unincorporated county and every city.

Unincorporated San Mateo County

  • Size: Up to 800 sq ft “by right” with 4-ft setbacks and a 16-ft height; larger ADUs (up to 1,000 sq ft for 2+ bedrooms) are allowed within the standards. JADUs up to 500 sq ft.
  • Setbacks: At least 4 ft from rear and interior side property lines. Converting an existing legal structure into an ADU is exempt from those setbacks.
  • Ministerial approval & parking waivers follow the state baseline — 60-day review, and no added parking required near transit.

The Peninsula cities

Each city — including the ones we work in most, like Daly City, South San Francisco, San Bruno, Burlingame, Pacifica, and Brisbane, as well as San Mateo, Redwood City, Belmont, San Carlos, Millbrae, Foster City, Menlo Park, and the rest — has its own ADU ordinance. All of them must honor the state minimums (ministerial 60-day approval, 4-ft setbacks, guaranteed sizes, parking waivers near transit), but maximum size, height, and design details vary city to city. Check your city’s planning department or look it up on smcadu.org — and we’re glad to point you to the right contact.

This page is general information, not legal or construction advice, and rules change. Confirm specifics with your city or the county before you build. We’re always happy to refer trusted ADU architects and contractors who work across the Peninsula — just ask.

Thinking about an ADU on the Peninsula?

We’ll help you confirm your city’s rules, weigh the cost, and understand the value an ADU adds. Start with a free home valuation or call (415) 407-5324. See also: ADUs in San Francisco, Marin County, and the East Bay. — Beatrice Kopilenko, REALTOR® · DRE #01970797 · Primavera Realty