The city of Los Altos Hills permits ADUs across most residential zones. Most single-family homes may build one detached ADU and one Junior ADU (JADU) per lot, while multi-family properties may qualify for multiple ADUs depending on site conditions. Los Altos Hills follows California’s ADU laws but has its own requirements for height, setbacks, design compatibility, parking, and utilities.

• Detached ADU: The unit is separate from the primary residence as either a newly constructed detached accessory structure or as the conversion of an existing detached accessory structure (i.e. detached garage).
• Attached ADU: The unit is attached to the primary residence as an integrated addition with independent entrance Conversion ADU: Converted from a garage, basement, or internal space.
• Junior ADU (JADU): A unit that is up to 500 square feet in size and is contained entirely within an existing single-family structure. A JADU must have an efficiency kitchen and a separate exterior entrance, and may include separate sanitation facilities, or may share sanitation facilities with the existing structure.

• Maximum Size:
  
  • Detached: Up to 1,200 square feet (includes internally connected basement floor area).
  • Attached: Up to 50 percent of the habitable floor area of the existing primary residence.
    
• Setbacks:
  
  • A new ADU shall have a front setback (street frontages) of at least 40 feet and side/rear setbacks of at least four (4) feet. If an ADU provides a side and rear setback of at least 30 feet, it can be eligible for additional floor area and building height.
• Height Limits:
  
  • A new ADU has a height limit of 16 feet.  If the new ADU provides an on-site parking space and has a side and rear yard setback of at least 30 feet, it can have a height of up to 19 feet.
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • A Junior ADU (JADU)
    • Located within one-half mile of a public transit stop
    • Converted from floor area in an existing primary residence or existing accessory structure (no new floor area added)
    • Involves the demolition or conversion of a garage/carport (note: losing covered parking may make the primary residence nonconforming)

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To build an ADU in Los Altos Hills, the following approvals are typically required:

• Zoning compliance review with the Planning Department
• Site plan and elevations showing setbacks, height, and lot coverage
• Architectural drawings consistent with town design standards
• Structural calculations and engineering reports (if applicable)
• Title 24 energy compliance documentation
• Specialized reports if required (e.g., geotechnical, arborist, creek setback analysis)
• Building permit submission through the Town of Los Altos Hills’ eTrakit online portal
  

Homeowners should also review the Town’s current permit fee schedule, utility and septic connection requirements, and fire code compliance (sprinklers if required for the main dwelling).

Permit review times vary but are typically between 8 to 12 weeks depending on project complexity, site conditions, and Town workload. Projects requiring corrections or additional approvals, such as a Site Development Permit for slopes, trees, or hazard zones, may take longer. Engaging design and construction professionals familiar with Los Altos Hills’ ADU process and electronic submittal system can help streamline approval.

The relevant zoning district is typically R-A (Residential Agricultural). ADUs are allowed in any lot in the R-A district with an existing or proposed single-family dwelling.

For the most current and complete information, refer directly to the city of Los Altos Hills.