Can you build an ADU in San Mateo? (94402)

What the parcel data shows in San Mateo

We ran every single-family parcel in 94402 — 6,481 of them — through the same eligibility engine behind our address checker: real parcel geometry, a 4-foot setback inset, existing building footprints subtracted, then CAL FIRE and FEMA hazard layers applied. 58.5% can fit a detached ADU. That is in the bottom quarter of the neighborhoods we have analyzed. Lots here are tight, and a unit that fits will usually be a small one, carefully placed.

Underneath that headline: 35.5% of lots come back clearly eligible, 23% are eligible with a caveat we would want to confirm, and 41.4% need a closer look before anyone should rely on an answer. The last group is not a rejection — it is where the public data runs out and a person has to look.

How much room is actually left

The median lot in San Mateo is 6,864 square feet, but lot size is not buildable size. After setbacks and the footprint of the existing home, the median parcel here has 3,383 square feet of genuinely usable rear-yard area — roughly 51% of the lot is spoken for before you start.

Of the lots we analyzed, 59.6% have room for at least a compact detached unit, and 25.8% can take a full two-bedroom-plus home. The most commonly recommended size in San Mateo is the EVO Two Plus, which is the best fit on 25.8% of lots.

The gap between those two figures is the story of this neighborhood: plenty of lots can host something, far fewer can host the largest models. Choosing a smaller unit is what converts a marginal lot into a buildable one.

What holds lots back in San Mateo

29.2% of lots in 94402 sit inside a CAL FIRE fire hazard severity zone. Those lots trigger CBC Chapter 7A wildland-urban interface construction requirements, which raise the cost of the exterior envelope. Lots outside the zone are unaffected.

What an ADU could earn in San Mateo

HUD puts the fair market rent for a one-bedroom in 94402 at $3,730 a month, while Zillow's observed index for the same ZIP — all home sizes — sits at $3,693. The first is a conservative, bedroom-specific 40th-percentile figure; the second is a broader market signal. A detached ADU here would realistically rent somewhere between the two, depending on size and finish.

Rent is only half of the return. The other half is what the project costs on your particular lot — the unit price is fixed, but site work, utility runs and permits move with the parcel. Those are the numbers worth pinning down before you decide.

Why your lot may differ from these numbers

Everything above is a neighborhood aggregate across 6,481 parcels in Bay Area. It is a good guide to what is normal on your street and a poor guide to your specific property. Two adjacent lots of identical size can reach opposite conclusions — one has a shallow house and a wide side yard, the other a detached garage sitting exactly where a unit would go.

The analysis also works from public records. It does not know your soil, your utility routing, your easements, or anything a survey would reveal. Run your own address for a parcel-specific answer, and treat both as a screen rather than a permit determination.

Common questions about ADUs in San Mateo

Can you build an ADU in San Mateo (94402)?

Yes. Of the 6,481 single-family lots we analyzed in 94402, 58.5% can fit a detached ADU under California state law — 35.5% clearly eligible and 23% eligible with a caveat to confirm. The median lot has 3,383 square feet of buildable area left after setbacks and existing structures.

How big of an ADU fits on a typical San Mateo lot?

59.6% of lots in 94402 fit at least a compact detached unit and 25.8% fit a full two-bedroom-plus model. The most commonly recommended size here is the EVO Two Plus, best fit on 25.8% of lots. Your own maximum depends on setbacks, the position of your existing home, and access.

What is the average lot size in 94402?

The median single-family lot in 94402 is 6,864 square feet. More useful for an ADU is the buildable remainder: after a 4-foot setback inset and subtracting the existing home's footprint, the median parcel retains 3,383 square feet of usable rear-yard area.

Is San Mateo in a fire hazard zone?

29.2% of lots in 94402 fall inside a CAL FIRE fire hazard severity zone. Building there triggers CBC Chapter 7A wildland-urban interface requirements — fire-rated exterior materials, ember-resistant vents and restricted decking — which add cost the state ADU statute does not mention. Whether your specific parcel is affected depends on its exact location.

How much rent could an ADU earn in San Mateo?

HUD's fair market rent for a one-bedroom in 94402 is $3,730 a month, and Zillow's all-sizes observed index for the ZIP is $3,693. Actual rent depends on unit size, finish, parking and privacy — treat these as an area expectation rather than a quote for your unit.

Is San Mateo a good neighborhood for an ADU?

On the physical measures we can check, San Mateo ranks higher than 19% of the neighborhoods in our data, with 58.5% of lots able to fit a detached unit. That reflects lot geometry and hazard exposure only — not demand, rent growth, or local permitting speed, which vary independently.