ADU Builder in Castro Valley, CA
Get a Free Castro Valley ADU Consultation
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.
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.
New for 2027: Two Detached ADUs in Castro Valley (AB 956)
AB 956 lets Castro Valley homeowners build two detached ADUs from January 1, 2027. See Alameda County ADU rules on size, height, parking and separation.
Read the full guide: Two Detached ADUs in Castro Valley | AB 956 overview for every city we serve