ADU Builder in Castro Valley, CA
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Alameda County ADU Rules
Castro Valley ADUs follow the Alameda County ADU ordinance, updated in January 2026:
- Detached: 850 sq ft (studio or one bedroom) or 1,200 sq ft (two or more bedrooms).
- Attached: 50% of the main home, capped at 850 or 1,000 sq ft; 850 sq ft always allowed.
- JADU: 500 sq ft.
- Height: detached 16 feet, 18 near major transit, up to 20 to match the roof pitch; attached up to 25 feet.
- Setbacks: 4 feet side and rear; ADUs over 800 sq ft use the main home’s front setback.
- Parking: one space with many waivers; none for JADUs.
ADU Construction
Permits
Other agencies: Castro Valley Sanitary District (21195 Center Street, (510) 537-0757) for sewer, Alameda County Fire for access and water supply (roads at least 20 feet wide and no steeper than 14%), and Castro Valley Unified for school fees on additions over 500 sq ft.
Alameda County Building Inspection (Public Works Agency)
399 Elmhurst Street, Room 141, Hayward, CA 94544 · (510) 670-5480 · pe*******@***wa.org
Permit processing Monday, Tuesday, Thursday and Friday 8:30 a.m. to 4:30 p.m., Wednesday 9 a.m. to 4:30 p.m.; over-the-counter plan check by appointment at (510) 670-5440. Inspections online, at (510) 670-5434, or by email.
Alameda County Planning (CDA): 224 W. Winton Avenue, Room 111, Hayward · (510) 670-5400.
Applications run through the Alameda County Permit Portal.
399 Elmhurst Street, Room 141, Hayward, CA 94544 · (510) 670-5480 · pe*******@***wa.org
Permit processing Monday, Tuesday, Thursday and Friday 8:30 a.m. to 4:30 p.m., Wednesday 9 a.m. to 4:30 p.m.; over-the-counter plan check by appointment at (510) 670-5440. Inspections online, at (510) 670-5434, or by email.
Alameda County Planning (CDA): 224 W. Winton Avenue, Room 111, Hayward · (510) 670-5400.
Applications run through the Alameda County Permit Portal.
Explore More
Other Castro Valley services: Bathrooms · Kitchens · Additions · New Homes · Castro Valley overview
Same service nearby: Hayward · Dublin · San Ramon
See how to build an ADU.
Castro Valley ADU Builder FAQ
Who approves ADUs in Castro Valley?
Alameda County, under its ADU ordinance.
How large can a detached ADU be?
850 or 1,200 sq ft depending on bedrooms.
Is parking required?
One space, waived in many cases; none for JADUs.
When were the rules updated?
January 2026.