Question 1

How much room is actually left to build in San Mateo County?

Accepted Answer

Very little, and it is close to fixed rather than growing. San Mateo County added about 1,600 net housing units in the year ending January 1, 2026, roughly 0.55%, against a population of 748,182 that barely moved (down 0.03%) over the same period. A buildable lot or a teardown candidate anywhere among the eight cities we serve is competing against a housing stock that has almost stopped expanding, which is why a ground-up or infill project here is underwritten on the lot itself, not on comparable new-build volume nearby.

Question 2

Can I add a new accessory dwelling unit as a ground-up build in San Mateo County?

Accepted Answer

Yes, and the county's own standards for unincorporated parcels are generous. On an eligible unincorporated single-family lot, San Mateo County allows one conversion or new-construction ADU, one detached new-construction ADU, and one JADU, plus one additional non-state-exempt ADU, and up to eight detached ADUs on a multifamily-zoned parcel with an existing building, with no owner-occupancy requirement to rent the finished unit out. Those standards cover unincorporated county land specifically; each of the eight cities we serve, San Mateo, Redwood City, Burlingame, Menlo Park, South San Francisco, San Bruno, Foster City, and Millbrae, sets its own ADU rules inside state law, and we have not verified any individual city's standard, so confirm yours with that city's planning department before you scope the build.

Question 3

How fast does a ministerial approval move on a two-unit or ADU project here?

Accepted Answer

State law sets the clock, not the city. A qualifying two-unit project under SB 9 gets a decision within 60 days, an ADU application gets a completeness review within 15 business days, and an ADU appeal is decided within 60 days. No San Mateo County city has published its own permit timeline, impact fee schedule, or plan-check duration that we could verify, so size your construction draw schedule against the statutory clock above rather than a promised local turnaround, and confirm the current fee and timeline with your city's building department before you close.

Question 4

Does an SB 9 lot split work for a pure investor building on spec?

Accepted Answer

Usually not on its own. The SB 9 lot split path requires the owner to sign a three-year owner-occupancy affidavit, which a pure investor typically cannot do, while the SB 9 two-unit ministerial path carries no such affidavit. At San Mateo County basis, where mid-tier values across the eight cities we serve ran from about $1.2 million in South San Francisco to nearly $2.9 million in Menlo Park as of July 2026, the more useful play for most investors here is adding density on land already paid for, through a second unit or an ADU, rather than splitting the lot.

Question 5

Is there a hidden cost to building near Foster City's levee?

Accepted Answer

Not a hidden one, but a real one on the tax bill. Foster City's levee reached final completion in February 2024, so a parcel behind it sits outside the mandatory flood-insurance zone, but every Foster City tax rate area still carries a city GO bond line of about 0.0285%, roughly $531 a year on the city's mid-tier home value as of July 2026. The city runs a levee bond oversight committee, so that line is very likely the levee's, though we have not read the bond documents to confirm it. Budget that annual line item into your hold cost the same way you would any other assessment; it is a tax bill item, not a construction permit fee, and no county-wide construction impact fee schedule is published for us to cite.

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