Can you build an ADU in Portola Valley? (94028)

What the parcel data shows in Portola Valley

We ran every single-family parcel in 94028 — 2,342 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. 94.9% can fit a detached ADU. That puts Portola Valley in the top quarter of everywhere we have analyzed — it is an unusually workable neighborhood for a backyard home.

Underneath that headline: 77.6% of lots come back clearly eligible, 17.3% are eligible with a caveat we would want to confirm, and 5.1% 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 Portola Valley is 31,534 square feet, but lot size is not buildable size. After setbacks and the footprint of the existing home, the median parcel here has 25,616 square feet of genuinely usable rear-yard area — roughly 19% of the lot is spoken for before you start.

Of the lots we analyzed, 99.1% have room for at least a compact detached unit, and 92.8% can take a full two-bedroom-plus home. The most commonly recommended size in Portola Valley is the EVO Two Plus, which is the best fit on 92.8% of lots.

Because most parcels here clear the bar for a larger unit, the binding constraint in Portola Valley is usually preference and budget rather than geometry.

What holds lots back in Portola Valley

17.1% of lots in 94028 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.

1.2% are inside a FEMA Special Flood Hazard Area, where the finished floor has to sit at or above the base flood elevation and flood insurance applies.

What an ADU could earn in Portola Valley

HUD puts the fair market rent for a one-bedroom in 94028 at $3,190 a month — a conservative 40th-percentile figure, so treat it as a floor rather than an asking price.

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 2,342 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 Portola Valley

Can you build an ADU in Portola Valley (94028)?

Yes. Of the 2,342 single-family lots we analyzed in 94028, 94.9% can fit a detached ADU under California state law — 77.6% clearly eligible and 17.3% eligible with a caveat to confirm. The median lot has 25,616 square feet of buildable area left after setbacks and existing structures.

How big of an ADU fits on a typical Portola Valley lot?

99.1% of lots in 94028 fit at least a compact detached unit and 92.8% fit a full two-bedroom-plus model. The most commonly recommended size here is the EVO Two Plus, best fit on 92.8% of lots. Your own maximum depends on setbacks, the position of your existing home, and access.

What is the average lot size in 94028?

The median single-family lot in 94028 is 31,534 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 25,616 square feet of usable rear-yard area.

Is Portola Valley in a fire hazard zone?

17.1% of lots in 94028 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.

Does flood zoning affect ADUs in 94028?

1.2% of lots here sit in a FEMA Special Flood Hazard Area. On those parcels the finished floor must be at or above the base flood elevation and flood insurance applies, both of which affect foundation design and cost.

How much rent could an ADU earn in Portola Valley?

HUD's fair market rent for a one-bedroom in 94028 is $3,190 a month. Actual rent depends on unit size, finish, parking and privacy — treat these as an area expectation rather than a quote for your unit.

Is Portola Valley a good neighborhood for an ADU?

On the physical measures we can check, Portola Valley ranks higher than 94% of the neighborhoods in our data, with 94.9% 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.