Introduction
Los Altos Hills provides a structured yet accommodating framework for ADU construction, aimed at fostering sustainable living and community growth. These regulations help maintain the town’s aesthetic while allowing homeowners to expand their living spaces. ADU Specialist Bay Area is here to guide you through every step, ensuring your project meets all requirements and is completed smoothly.
Essential ADU Rules for Los Altos Hills
What you can build
Maximum size
Up to 1,200 sq ft, depending on lot size and zoning.
Side / rear setbacks
Minimum 4 feet from the property line.
Two stories
Permitted with height restrictions based on zoning.
Building separation
Must maintain a 6-foot distance from the main dwelling.
Permitting timeline
Standard
Typically 60-90 days for approval.
Coastal
Up to 120 days due to additional review processes.
Zoning Regulations in Los Altos Hills
Los Altos Hills has specific zoning regulations that govern ADU development, ensuring compatibility with the surrounding neighborhood.
ADU Size Limitations
| Lot Size | Single-Family (Maximum Floor Area Allowed) | Duplex and Multi-Family (Maximum Floor Area Allowed) |
| Under 7000 | 700 sq ft | Not permitted |
| 7000 - 9999 | 800 sq ft | 1 ADU + 400 sq ft per existing unit (up to 1000 sq ft max) |
| 10000 - 12999 | 900 sq ft | 1 ADU + 450 sq ft per existing unit (up to 1000 sq ft max) |
| 13000 - 19999 | 1000 sq ft | 1 ADU + 500 sq ft per existing unit (up to 1000 sq ft max) |
| 20000+ | 1000 sq ft | 1 ADU + 550 sq ft per existing unit (up to 1000 sq ft max) |
ADU Height Limitations
The height of an ADU in Los Altos Hills is restricted to a maximum of 16 feet to maintain community aesthetics.
Building Coverage
Building coverage for ADUs must not exceed 50% of the rear yard area.
Location
| Property Type | Location Requirement |
| Single-Family Homes | Attached ADUs: Must be within the primary structure’s footprint. |
| Detached ADUs: Must be located in the rear yard. | |
| Junior ADUs (JADUs): Must be part of the existing primary dwelling. | |
| Duplex and Multifamily | Attached ADUs: Permitted within existing structures. |
| Detached ADUs: Must be located in the rear or side yard. |
Exterior details
ADUs in Los Altos Hills must complement the existing primary dwelling’s design, including matching materials and architectural style.
Parking
One off-street parking space is required per ADU, which can be tandem or in a setback area.
Setbacks and buffer zones
ADUs must have minimum side and rear setbacks of 4 feet and maintain appropriate buffer zones to preserve privacy and light for neighboring properties.
Minimum Lot Area
| Lot Size | Minimum Lot Area for ADU | Description |
| Under 7000 | ADUs not permitted | Lot too small for ADU development. |
| 7000 - 9999 | 7000 sq ft | Adequate space for ADU with all required setbacks. |
| 10000 - 12999 | 10000 sq ft | Sufficient area for larger ADUs with setbacks. |
| 13000 - 19999 | 13000 sq ft | Generous lot size allows for maximum ADU size. |
| 20000+ | No minimum lot area requirement | Ample space for any ADU configuration. |
Connection for utilities
ADUs must have separate utility connections from the primary residence, ensuring independent service and billing.
Fire safety
All ADUs must comply with current fire safety regulations, including the installation of smoke detectors, fire extinguishers, and adherence to egress requirements.
Room specifications
- Sleeping Areas: Must provide adequate space for a bed and storage, typically a minimum of 70 sq ft.
- Kitchen: Requires a sink, stove, and refrigerator, with appropriate ventilation and counter space.
- Bathroom: Must include a toilet, sink, and shower or bathtub, with proper plumbing and ventilation.
- Storage: Adequate storage solutions, such as closets or built-in cabinetry, are necessary for functional living spaces.
Short-term Rentals and Home Occupations Regulations
Short-term rentals are permitted for a minimum stay of 30 days, ensuring neighborhood stability.
Building Codes
ADUs must comply with all local building codes, including structural integrity, electrical, plumbing, and safety standards.
Los Altos Hills ADU Permit Guidelines
| Permit Type | Description | Estimated Fee |
| Building Permit | Required for ADU construction. | $2500 - $5000 |
| Electrical Permit | Necessary for wiring and outlets. | $400 - $700 |
| Plumbing Permit | Needed for plumbing installations. | $400 - $700 |
| Mechanical Permit | For heating, cooling, and ventilation systems. | $400 - $700 |
| Grading Permit | Required if site grading is necessary. | $300 - $500 |
| Site Development Permit | Needed for site layout approval. | $300 - $500 |
| Zoning Permit | Ensures compliance with zoning laws. | $200 - $400 |
Property requirements
Properties in Los Altos Hills must meet specific criteria, such as lot size and zoning, to qualify for ADU development.
Parking
Adequate parking must be provided, typically one space per ADU, which can be tandem or within existing driveways.
Front setbacks
ADUs must maintain a minimum front setback of 20 feet to preserve neighborhood character and sightlines.
Side and rear setbacks
Minimum side and rear setbacks of 4 feet are required to ensure adequate spacing and privacy.
Open space and rear yards
A portion of the lot must be dedicated to open space, ensuring a balance between built and natural environments.
Properties that qualify
To determine your property’s eligibility for ADU development, contact the Los Altos Hills Planning Department.
- Verify that your property is within Los Altos Hills’ jurisdiction.
- Check the specific residential zones that permit ADUs. Common zones in Los Altos Hills include R-1, R-2, and R-M.
- Other General Plan designations that permit ADUs:
- Residential Mixed-Use (RMU): Allows a combination of residential and commercial uses, ideal for ADUs.
- Transit-Oriented Development (TOD): Encourages higher density housing near public transit hubs, including ADUs.
- Specific Plan areas: Tailored plans for certain areas that may include provisions for ADUs.
Development standards
Single-family
- Attached: Must integrate with the existing structure’s design and footprint.
- Detached: Positioned in the rear yard, maintaining a harmonious aesthetic with the primary dwelling.
Duplex properties
- Attached: ADUs must be incorporated within the existing duplex structure.
- Detached: Situated in the rear yard, with design elements that complement the duplex.
Multifamily properties
- Attached: ADUs can be built within existing multifamily buildings, adhering to size limitations.
- Detached: Located in the rear or side yard, maintaining compatibility with the multifamily structure.
Junior ADUs (JADUs)
Junior ADUs must be created within the walls of an existing single-family home, limited to 500 sq ft.
Property designations
- Flood Zones: ADUs in flood zones must adhere to specific elevation and flood-proofing requirements.
- Geohazard Zones: Additional geotechnical assessments may be required to ensure stability and safety.
- Historic Designation: ADUs in historically designated properties must maintain the property’s historic character.
- Easements: Development must respect existing easements, such as utility lines or access paths.
Summary
Los Altos Hills provides a detailed framework for ADU development, balancing growth with community integrity. By adhering to these regulations, homeowners can enhance their property value and contribute to the local housing supply. ADU Specialist Bay Area is here to help navigate these requirements, ensuring your ADU project is successful from start to finish.
FAQ
Yes, the maximum size varies based on the lot size, with larger lots allowing bigger ADUs.
ADUs are generally not permitted on lots smaller than 7000 sq ft.
Yes, two-story ADUs are permitted but must comply with height and setback regulations.
Minimum side and rear setbacks are 4 feet, with additional requirements for front setbacks.
Yes, ADUs require separate utility connections from the main dwelling.
Yes, ADUs in historically designated areas must maintain the character of the property.
The standard timeline is 60-90 days, with longer timelines for coastal areas.
Typically, one off-street parking space is required per ADU.
Short-term rentals are allowed with a minimum stay of 30 days.
ADUs must comply with all current fire safety regulations, including smoke detectors and fire extinguishers.
Yes, the maximum height for ADUs in Los Altos Hills is 16 feet.
A grading permit is required if site grading is necessary for the ADU construction.
ADUs must comply with the zoning regulations of the specific residential zone they are in.
Yes, the number of ADUs allowed depends on the lot size and existing structures.
ADUs must match the architectural style and materials of the primary dwelling.
Additional geotechnical assessments and safety measures may be required.
The minimum lot area varies, with smaller lots having stricter limitations.
A portion of the lot must be dedicated to open space, balancing built and natural environments.
ADUs must have independent utility connections to ensure separate service and billing.
Yes, rooms must meet minimum size requirements to ensure livable conditions.
ADUs must not exceed 50% of the rear yard area.
Yes, front setbacks must be at least 20 feet, and side setbacks at least 4 feet.
Yes, but specific elevation and flood-proofing requirements must be met.
Various permits, including building, electrical, plumbing, and mechanical, are required.
