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Alameda County, CA Building Safety: Anti-Mansionization (2026)

Some Restrictions
Compiled from the official code textEditor Martyn O'NeillLast verified August 2026

Key Facts

5,000 sf lot
0.5 FAR, 2,500 sf max
10,000-20,000 sf lot
0.40 FAR or 4,500 sf
20,000+ sf lot
0.10 FAR, 10,000 sf cap
Over the cap
requires site development review
Applies to
Castro Valley Urbanized Area only

Summary

Castro Valley limits one-family homes to a sliding floor-area-ratio cap, as low as 0.10 FAR on the largest lots, before extra square footage triggers site development review.

These county ordinances apply to unincorporated areas of Alameda County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

In Castro Valley only (areas within the Castro Valley Urbanized Area) the maximum floor area ratio for a one-family dwelling shall be as follows: At 5,000 s.f.: 0.5 FAR (2,500 s.f.), SDR required to exceed Max s.f. 5,001-9,999 s.f.: 0.3 FAR for every s.f. of lot area above 5,000, plus 2,500 s.f. (3,500 s.f. max). 10,000-20,000 s.f.: 0.40 or 4,500 s.f., whichever is less. 20,000+ s.f.: 0.10 FAR, plus 2,500 s.f., or 10,000 s.f. whichever is less.

View official code

Full Breakdown

Within the Castro Valley Urbanized Area, unincorporated Alameda County's R-1 zoning caps a one-family dwelling's floor area ratio on a sliding scale: 0.5 FAR (2,500 sq. ft.) on a 5,000-square-foot lot, down to 0.10 FAR plus 2,500 square feet (10,000-square-foot cap) on lots of 20,000 square feet or more. New construction or additions that would exceed the maximum FAR or square footage may still be built, but only after site development review. The limit applies only in Castro Valley's urbanized area, not countywide.

Violations & Fines

Building beyond the FAR table without an approved site development review can halt a project at plan check and require the design be scaled back to fit the cap.

Frequently Asked Questions

Can a Castro Valley home exceed the FAR table?
Yes, but only with an approved site development review; the county can still deny or condition approval for oversized new construction or additions.
Does this FAR cap apply to all of unincorporated Alameda County?
No. It applies specifically to one-family dwellings within the Castro Valley Urbanized Area under R-1 zoning, not other unincorporated communities.

Sources & Official References

Other rules in Alameda County

Building PermitsSome RestrictionsGreen Building CodeSome RestrictionsBuilding InspectionsSome RestrictionsElectrical PermitsSome RestrictionsPlumbing PermitsSome RestrictionsLead PaintSome RestrictionsParking Rules: Commercial & Oversized VehiclesSignificant RestrictionsParking Rules: Preferential Parking DistrictsSome RestrictionsParking Rules: Loading ZonesSome RestrictionsLandscaping Rules: Tree Removal & Heritage TreesSome RestrictionsWater Use Rules: Recycled Water RulesLight RestrictionsHistoric Preservation: Historic-Cultural MonumentsSignificant Restrictions

All Alameda County rules

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