Accessory Dwelling Units (ADUs)


planquestion mark

What is an ADU?
How Do I Apply?
Policies & Resources

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

  • Be within a single family home;
  • Be no larger than 500 square feet; and
  • 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.

Policies & Resources

  1. ADU Application 
  2. Building Permit Application
  3. ORD 778 - Amending Sections 17.52.020(E)(l)(a) AND 17.52.020(D)(4) of the HMC regarding maximum size of ADU
  4. ORD 787 - Amending CH 17.52 of the HMC relating to ADU's & JADU's - January 1, 2023
  5. Rental Resources:

How Do I Apply?

Applying for an ADU is quick and easy. Use the San Mateo County ADU Workbook to get started with ideas, tips, and best practices.

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.

Now that you know a little about this application process. Follow the flowchart through to completion for an effective and hassle free experience.

ADU flowchart

Ready To Apply?

If you are to apply for your ADU permit, make sure you have the following items ready.

  • ADU Application
  • Architectural plans, drawn to scale - including but not limited to the site plan, floor plans, all sides elevations with two sections. Please indicate the materials and colors of the ADU on the elevations and note if the ADU will match the materials of the main home.  Staff recommends that all ADU's should match with the existing style, materials, and colors of the residence.
  • Include relevant title report information, survey plans and other applicable documents for the parcel. To enable a smoother review process. 
  • Prepare a site plan showing fire access to allow faster service and feedback from the Town Fire Marshall.
  • ADU Recordation fees are to be paid to the counter Planning staff please email:  for fee amount. Once you have all of those items please contact the ADU Specialist here: 

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.

Contact US

Before you contact the Planning ADU Specialist with your application, make sure you have consulted all of the available resources and have prepared all necessary documents. The Town has a dedicated ADU specialist who can help guide you through the process. 

Mandy Kang - ADU Specialist:

For Additional Answers on your ADU projects please review the Frequently Asked Questions.