Accessory Dwelling Units

Updated: 1/24/2020

On January 1, 2020, three new State laws (AB 881, SB 13 and AB 68) that establish statewide regulations for the placement and construction of Accessory Dwelling Units (AKA Second Living Units) become effective. Any local rules or regulations that conflict with these new laws can no longer be enforced. Los Altos Hills is moving forward with amendments to the Zoning Code to update the Town’s Accessory Dwelling Unit Ordinance to be consistent with these laws. However, until the new Ordinance is adopted, the applicable rules and regulations for the establishment of new Accessory Dwelling Units, as mandated by State law, are outlined in this handout.


The Town’s General Plan Housing Element encourages property owners to consider building an Accessory Dwelling Unit, as they contribute needed housing types to the community’s housing stock while maintaining the rural, hillside character of Los Altos Hills


  • 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 a proposed or existing 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 ADUfalls 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 secondary dwelling unit that is up to 500 square feet in size and is contained entirely within an existing single-family structure and include the conversion of an existing bedroom. A JADUmust 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

ADU and Apporval Process

Development Standards

ADU lot size

  • 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.
  • Floor Area and Lot Coverage
  • A new ADU shall comply with the applicable Zoning Code and site development standards.


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.


A new ADU shall comply with the applicable Zoning Code and site development standards. 

  • Size Limits

- Detached ADU – Up to 1,200 square feet.

- Attached ADU – Up to 50 percent of the habitable floor area of the existing primary residence, up to 1,200 square feet.

- JADU – Up to 500 square feet.


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 cannot be rented for a period of less than 30 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 ADUshould not:
    • Result in the damage or removal of a Heritage Oak;
    • Be located on slopes that exceed 30 percent;
    • Be within 25 feet of the top of a creekbank; or
    • Be within a recorded easement (open space, public utility, etc.).

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