LOTCHECK

San Mateo County, California

San Mateo — Development Guide

What can you build in San Mateo? ADU eligibility, SB 9 lot splits, setback requirements, and development potential for every residential property.

San Mateo at a Glance

105.6K

Population

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Parcels

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Median Lot

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Median Assessed

What You Can Build in San Mateo

California state law overrides many local zoning restrictions for housing development. Here's what's available to property owners in San Mateo, regardless of what local ordinances say.

Accessory Dwelling Unit (ADU)

State Law

Build a second home on your lot — detached, attached, or converted from a garage. California law (AB 68 / AB 881) guarantees ADU rights for all residential properties. No minimum lot size. Cities must approve ministerially within 60 days.

Max 1,200 sf16 ft height4 ft rear/side setbackNo parking (½ mi from transit)

Junior ADU (JADU)

State Law

Convert space within your existing home into a separate rental unit — up to 500 sf. Can be built in addition to an ADU, giving you two income-producing units on one lot. Owner occupancy required for either the JADU or primary home.

Max 500 sfWithin existing structureOwner-occupiedNo additional parking

SB 9 — Duplex + Lot Split

State Law

Split your lot in two and build a duplex on each half — up to four units total on what was a single-family lot. No public hearing, no discretionary review, no environmental review (CEQA exempt). Ministerial approval required.

Up to 4 units totalMinisterial approvalNo CEQAMin 1,200 sf lots

SB 1211 — Streamlined ADU

State Law

Effective January 2025: two-story ADUs up to 1,000 sf are pre-approved statewide. No local design review, no discretionary process. Cities must approve compliant plans within 30 days.

Up to 1,000 sfTwo stories allowed30-day approvalNo design review

AB 1033 — ADU Condo Conversion

Opt-In

If your city has adopted a local ordinance, ADUs can be sold as separate condominiums — independent from the primary home. This unlocks a new path to homeownership and lets ADU builders recoup their investment through sale rather than rental income only.

City must opt inADU sold separatelyCondo map requiredNew ownership path

Housing News in San Mateo

California is so eager for homeowners to build ADUs, it’s helping them save on architect fees

Jul 21 · mercurynews.com
More Housing News →

Frequently Asked Questions

Can I build an ADU in San Mateo? +

Yes. California state law (AB 68/AB 881) allows at least one ADU on any residential lot in San Mateo. ADUs up to 800 sf are approved ministerially within 60 days, regardless of local zoning.

What is SB 9 and does it apply in San Mateo? +

SB 9 (California Housing Opportunity and More Efficiency Act) allows duplexes and lot splits on single-family lots statewide. In San Mateo, this means you can potentially build up to 4 units on what was a single-family lot — no public hearing required.

What are the setback requirements in San Mateo? +

Setback requirements in San Mateo vary by zone. Use LotCheck to check your specific property's setback requirements against San Mateo County's zoning code. For ADUs, California state law allows reduced setbacks of 4 feet for rear and side yards.

How much does an ADU cost to build in San Mateo County? +

ADU construction in San Mateo County typically ranges from $150,000 to $400,000 depending on size, finishes, and whether it's a conversion or new construction. Garage conversions tend to be the most affordable option.