Hillsborough has created a downloadable Guide for ADUs


There are 3 ways to create an accessory dwelling unit:

  1. Constructing an entirely new accessory dwelling unit which is either attached to, or detached from, the primary dwelling.
  2. Creating a new accessory dwelling unit within existing exterior walls (for example, in the basement or attic)
  3. Approval of an existing structure (or portion of a structure) as an accessory dwelling unit, such as a domestic unit, pool house, recreation room, etc.

To create an Accessory Dwelling Unit, first secure approval from the Planning Division, then secure approval from the Building Division.

Before the unit is approved, a notice must be recorded at the County Recorder’s office to notify future property owners of regulations associated with the accessory dwelling unit including:

  • That the accessory dwelling unit may not be sold separately;
  • Either the accessory dwelling unit or the primary dwelling may be rented, but not both at the same time unless both are rented to the same party.

Parking: Accessory dwelling units with one or two bedrooms require one parking space, and accessory dwelling units with three or more bedrooms require two parking spaces in addition to those required for the primary dwelling. These parking spaces may be unenclosed. Exceptions to the parking requirements may be granted for accessory dwelling units located within one-half mile of a public transit stop or car share vehicle pickup location, or accessory dwelling units located entirely within an existing primary residence or existing accessory structure.

During the design process, please keep in mind the following accessory dwelling unit standards:

  • The accessory dwelling unit must be constructed within the allowable maximum permitted floor area (FAR) and lot coverage, and it must comply with the height and setback requirements as specified in the Hillsborough Municipal Code for all residences (review the Development Standards for key information available online at www.hillsborough.net/planning/informational_handouts.asp).
  • Detached accessory dwelling units may not exceed 1,400 square feet in floor area.
  • Attached accessory dwelling units may not exceed 30% of the floor area of the primary residence (not including the garage area).
  • The lot upon which the accessory dwelling unit is to be located must contain a single-family residence and the property owner must occupy either the residence or the accessory dwelling unit.
  • New construction/exterior changes (if any) should be designed to be compatible with the architectural style and detailing of the primary unit.


Hillsborough City website

Hillsborough Planning Division