Castro Valley, Alameda County

ADU / Guest House Cost in Castro Valley, CA

Castro Valley remodeling costs are 10% abovethe California state average. Here's what a typical adu / guest house costs in Castro Valley in 2026, based on a 500 sqf project.

Budget

$137,500

$275/sqf

Mid-Range

$220,000

$440/sqf

High-End

$330,000

$660/sqf

Cost breakdown

Foundation & Framing

25%$55,000

Mechanical (HVAC, Plumbing, Electrical)

20%$44,000

Interior Finishes

20%$44,000

Exterior & Roofing

15%$33,000

Kitchen & Bath Fixtures

10%$22,000

Permits, Plans & Engineering

10%$22,000

Breakdown based on mid-range estimate of $220,000

Typical Timeline

4-8 months

Permits Required

Building, electrical, plumbing, mechanical. May require plan check.

Remodeling in Castro Valley

Unincorporated East Bay community with a suburban feel. Many 1950s-60s ranch homes being renovated. Alameda County handles permitting, which is straightforward.

California allows ADUs on most residential lots since 2020. An ADU adds living space and rental income. Cities must approve or deny within 60 days of a complete application.

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