The County of Alameda permits ADUs across most residential zones. County of Alameda follows both state and local ADU laws, allowing most single-family homes to build one detached ADU and one Junior ADU (JADU) per lot. Multi-family properties may be eligible for multiple ADUs, depending on site conditions.

County of Alameda aligns with California’s broader ADU legislation but has its own requirements for height, setbacks, and design review.

• Detached ADU: An independent structure built separately in the backyard
• Attached ADU: A secondary unit connected directly to the primary home
• Conversion ADU: A living unit created by converting an existing garage, basement, or interior area
• Junior ADU (JADU): an accessory,second, or secondary unit that is fully contained within a Single-family dwelling or within an attached garage and which provides complete independent living facilities for one or more persons

• Maximum Size:
  
  • Detached: contain 2 or more bedrooms, then limited to 1,200 square feet maximum
  • Attached: Maximum 50% floor area of the Single-family dwelling or 1,200 square feet, whichever is less.(A minimum of 800 square feet is allowed by right regardless of size of the Single-family dwelling.)
• Setbacks:
  
  • Minimum 4 ft from side and rear property lines

• Height Limits:
  
  • Detached: Up to 16 feet
  • Attached: Up to 25 feet or the height limitation in the applicable zoning district that applies to the Primary Dwelling Unit, whichever is lower
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • Site is located within one-half mile walking distance of public transit
    • Site is located within an architecturally or historically significant property or district
    • When on-street parking permits are required but not offered to the occupant

To build an ADU in County of Alameda, the following approvals are typically required:

• Zoning compliance review: Required to ensure the ADU meets all county zoning, setback, height, and development standards
• Site plan and elevations: Needed for permit submission to show the property layout, ADU design, dimensions, and exterior views
• Architectural drawings: Provide detailed construction plans and engineering to confirm safety and structural compliance
• Title 24 energy documentation: Mandatory proof that the ADU meets California Energy Code efficiency standards
  

Homeowners should also review County of Alameda’s current fee schedule, utility connection requirements, and fire code compliance.

Permit review times vary but are typically between 8 to 16 weeks depending on project complexity and city workload. Projects requiring corrections may take longer. Engaging professionals familiar with San Jose’s ADU process can help streamline approval.

ADUs are allowed in all residential zoning districts, including single-family and multifamily zones.There is no minimum lot size requirement for building an ADU in unincorporated County of Alameda. Hillside, wildfire-risk, or environmentally sensitive areas may have added development or safety restrictions.

For the most current and complete information, refer directly to the County of Alameda.