Accessory Dwelling Units (ADUs)

Drawing of Primary Home and Accessory Dwelling Unit

What is an ADU?

ADU is short for “Accessory Dwelling Unit.”  They are also sometimes called “granny flats” or “second units.” To be an ADU, the unit must:

  1. Be on the same lot where there is an existing or proposed single-family home;
  2. Be secondary to that home;
  3. Have a front door, full kitchen and bath that are independent of that home.  

There are three key types of ADUs outlined below:

  • Conversion ADUSpace within an existing or proposed home (like an attached garage or attic) or within the existing footprint of a permitted accessory structure (like a detached garage or outbuilding), can be converted into an ADU, as long as the space has side and rear setbacks sufficient for safety (per the building and fire codes) and it has its own front door, independent of the main home.  If a little extra space is needed to create an independent front door, the existing footprint of the structure can be expanded, up to 150 square feet.
  • Attached ADUA newly constructed unit that is connected to the primary single-family home by at least one wall.
  • Detached ADUA newly constructed unit that is freestanding or separate from the primary single-family home, like a cottage, pool house, or guesthouse.

What is a Junior ADU?

JADU (sometimes also abbreviated a jADU or JrADU) is short for “Junior ADU.”  A JADU is simply a smaller type of ADU.  To qualify as a JADU, the unit must

  1. Be within a single family home;
  2. Be no larger than 500 square feet; and
  3. Have an “efficiency kitchen.” An efficiency kitchen has equipment to cook, at least 15 square feet of counter area and at least 30 square feet of cabinet space.

Bathroom facilities can be independent or shared with the main home.

I'm interested in building an ADU.  What kinds or permits do I need?

Most projects in Town require two permits:

  1. A planning permit, to ensure that the project meets the Town’s zoning and design standards, and
  2. A building permit, to make sure that the project is safe and healthy for people to live in.

However, in 2019 several bills were passed that allow some ADUs to bypass the planning process.  Effective January 1, 2020 these bills became State law.

Is my application eligible to by-pass the planning process?

Great question!  There are three ADU permit types identified in the Town's ADU ordinance:

  1. "Building Permit Only" ADU:  This type of ADU permit bypasses the planning process and is sent straight to the building division for review.  Certain specific criteria have to be met in order for a project to qualify for this type of permit, as outlined in the visual below.*
  2. "ADU Permit" ADU: This type of ADU permit requires review by the planning division to ensure that certain objective zoning ordinance criteria are met, as outlined in the visual below.  Once that review is complete, the project receives an "ADU permit" from the planning division and is sent to the building division for review of the safety of the structure.*
  3. "Exception" ADU: This type of permit applies to any ADU proposal that doesn't meet the rules for a "building permit only" or ADU Permit", and allows for the planning division to apply discretion in reviewing the ADU design.  Once that review is complete and a planning permit is issued, the project is then sent to the building division for review of the safety of the structure.

*Please note that the visual below is intended only as a general guide and does not outline all the requirements for the various ADU types.  Please contact the Building & Planning Department for project-specific information.

Chart Depicting ADU Permit Types

How do I get started?

The 21 jurisdictions in San Mateo County have come together to develop a housing-related resource sharing consortium called 21 Elements.  In partnership with Home for All San Mateo County, 21 Elements has developed an ADU idea book and development workbook to help facilitate Accessory Dwelling Unit development throughout the County.  Both documents may be downloaded by clicking on the links below:

Additionally, the Town has a dedicated ADU specialist who can help guide you through the process.  Please reach out with your ADU-related questions:

Kristina Paulauskaite

ADU Specialist

How do I rent my ADU?

There are various ways that Hillsborough residents can advertise available ADUs to prospective renters.  Since 1979 The Human Investment Project San Mateo County (a.k.a. HIP Housing) has operated their HIP Home Sharing Program to help match individuals seeking housing with people who have a room or ADU to rent.

HIP Housing's Home Sharing Services include:

  • Interviewing and screening applicants
  • Personal Housing Coordinator
  • Background search
  • Facilitation of Living Together Agreement
  • Referrals to community resources
  • Follow up support

For more information on how to participate please visit the HIP Housing Home Sharing website at, or call them at (650) 348.6660.