Can you build an ADU in Castro Valley? (94546)

What the parcel data shows in Castro Valley

We ran every single-family parcel in 94546 — 10,858 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. 84.8% can fit a detached ADU. That is better than 68% of the neighborhoods in our data: a solid, unremarkable result, which in this context is good news.

Underneath that headline: 68.5% of lots come back clearly eligible, 16.3% are eligible with a caveat we would want to confirm, and 15.2% 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 Castro Valley is 6,360 square feet, but lot size is not buildable size. After setbacks and the footprint of the existing home, the median parcel here has 3,220 square feet of genuinely usable rear-yard area — roughly 49% of the lot is spoken for before you start.

Of the lots we analyzed, 85.4% have room for at least a compact detached unit, and 43.4% can take a full two-bedroom-plus home. The most commonly recommended size in Castro Valley is the EVO Two Plus, which is the best fit on 43.4% 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 Castro Valley

17.7% of lots in 94546 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.

1.6% 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 Castro Valley

HUD puts the fair market rent for a one-bedroom in 94546 at $2,460 a month, while Zillow's observed index for the same ZIP — all home sizes — sits at $2,620. 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 10,858 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 Castro Valley

Can you build an ADU in Castro Valley (94546)?

Yes. Of the 10,858 single-family lots we analyzed in 94546, 84.8% can fit a detached ADU under California state law — 68.5% clearly eligible and 16.3% eligible with a caveat to confirm. The median lot has 3,220 square feet of buildable area left after setbacks and existing structures.

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

85.4% of lots in 94546 fit at least a compact detached unit and 43.4% fit a full two-bedroom-plus model. The most commonly recommended size here is the EVO Two Plus, best fit on 43.4% of lots. Your own maximum depends on setbacks, the position of your existing home, and access.

What is the average lot size in 94546?

The median single-family lot in 94546 is 6,360 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 3,220 square feet of usable rear-yard area.

Is Castro Valley in a fire hazard zone?

17.7% of lots in 94546 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 94546?

1.6% 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 Castro Valley?

HUD's fair market rent for a one-bedroom in 94546 is $2,460 a month, and Zillow's all-sizes observed index for the ZIP is $2,620. Actual rent depends on unit size, finish, parking and privacy — treat these as an area expectation rather than a quote for your unit.

Is Castro Valley a good neighborhood for an ADU?

On the physical measures we can check, Castro Valley ranks higher than 68% of the neighborhoods in our data, with 84.8% 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.