Accessory Dwelling Units

The Town of Los Altos Hills has adopted an updated Accessory Dwelling Unit (ADU) Ordinance to be consistent with the new requirements under State law. The updated ADU Ordinance can be viewed here.

What is an accessory dwelling unit (ADU)?

An ADU is an accessory dwelling unit with complete independent living facilities for one or more persons.

Types of ADUs

  • Accessory Dwelling Unit:ADU Styles An attached or a detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with an existing or proposed primary residence. It shall include permanent provisions for living, sleeping, eating, cooking, and sanitation on the same parcel as the single-family home. Generally, an ADU falls into one of the following categories:
    • Detached: 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: The unit is attached to the primary residence as an integrated addition with independent entrance
    • Conversion of Existing Space: Floor area within the primary residence that is converted into an independent living unit.
  • Junior Accessory Dwelling Unit: 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.
    • Efficiency Kitchen: A space for cooking and food preparation that includes a sink with a maximum waste line diameter of 1.5 inches, a cooking facility with appliances that do not require electrical service greater than 120 volts, or natural or propane gas, and a food preparation counter and storage cabinets that are of reasonable size in relation to the size of the proposed JADU.

Accessory Dwelling Unit Review and Approval Process

The ADU approval is “ministerial” or administrative. 

Step 1: Pre-Application meeting with a Planner. The Planner will provide you with a Checklist that will include information on plans and documents including geotechnical report that you need for applying for a Building Permit.

Step 2: Gather all the plans and Documents and apply for a Building Permit.

Development Standards

Allowed Uses

One ADU and/or one JADU are allowed on any property, regardless of lot size, in the R-A (Residential Agricultural) District with an existing single-family house.

Development Area and Floor Area

A new ADU, attached or detached, is allowed to be up to 800 square feet in size and is exempt from a lot's maximum development area (MDA) and maximum floor area (MFA).  All floor area over 800 square feet is counted toward a lot's MDA and MFA.


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.  


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.  

Size Limits

Every property is allowed to have an ADU that is up to 800 square feet in size.  If a property's MDA and MFA allow it, an ADU can be up to 850 square feet (studio and one-bedroom) or up to 1,000 square feet (two-bedroom) in size.  If one onsite parking space and 30-foot side and rear setbacks are provided, an ADU may be up to 1,200 square feet in size.

A JADU is allowed to be up to 500 square feet in size.  If an onsite parking space is provided, a JADU can be up to 650 square feet in size.

Access and Parking

Access and parking for a new ADU shall utilize the same driveway as the primary single-family dwelling and shall not result in the creation of a new driveway on the property.

One (1) on-site parking space, in addition to the primary residence’s required parking, is required for a new ADU, with the following exceptions:

  • The parking space for the ADU can be tandem and/or located within a setback area;
  • No additional onsite parking is required for a new JADU;
  • No additional onsite parking is required when an ADU is located within one-half mile of a public transit stop or when an ADU is established by converting floor area in an existing primary residence or an existing accessory structure (no new floor area is added); and
  • When a garage or carport is demolished or converted in relation to the establishment of an ADU, the onsite parking spaces do not need to be replaced. Note: If the lost covered parking for a garage conversion is not replaced, this may result in the primary residence becoming nonconforming and limit the ability for future additions to the primary residence without reestablishing the required parking. 


An existing nonconforming structure may be converted into an ADU provided that it was legally constructed at the time it was originally established; and a new ADU may be constructed in the same location and to the same dimensions as an existing structure is considered as a conversion.


An ADU may be rented separately from the primary residence, but it must be rented for a period of at least than 31 days. The ADU cannot be sold separately from the main residence.

Building Design and Placement

An ADU is not subject to the Site Development Review process or any discretionary design criteria, but does need to meet the following objective standards:

  • The design of the ADU shall relate to the design of the primary residence by use of similar exterior wall materials, window types, trims, roofing materials and roof pitch.
  • All exterior lighting shall be shrouded and/or downward facing.
  • Exterior finish colors for new ADUs shall have a light reflectivity value of 50 or less and roof materials shall have a light reflectivity value of 40 or less, per manufacturer specifications. 
  • The placement of a new ADU cannot be:
    • Within the critical root zone of a Heritage Oak;
    • Located on slopes that exceed 30 percent;
    • Within 25 feet of the top of a creekbank; 
    • Within a recorded easement (open space, public utility, etc.); or
    • Within a geotechnical or seismic hazard zone.

For questions or additional information, please Contact the Planning Department at 650-941-7222 or email