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Additional Dwelling Unit toolkit

The program offers more housing options while creating rental income for homeowners. 

The Additional Dwelling Unit (ADU) program is for homeowners in the City of Boston.  Through the ADU program, they can carve out a space in their home to create smaller, independent units.

Are you eligible?

If you answer yes to ALL of the questions in this section, you are eligible.

  • Do you live in the City of Boston?
  • Are you the owner of a one-, two-, or three-family home?
  • Do you live in the property where you want to build the unit?
  • Do you plan to build the unit inside the footprint of your home?
  • If your home is in the FEMA Flood Zone and you want to build a basement unit, is the basement or first floor elevation ("Top of Bottom Floor" on an elevation certificate) above the Base Flood Elevation (BFE)?

Designing Your ADU

The City of Boston Staff who run the ADU Workshop have put together an Informational video about the program, which can be found online.

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There are many ways to design and build your ADU. This section includes some important design tips and strategies. Our goal is to make this design process accessible to all homeowners through some general advice and guidance.

To help with your planning, view the checklist:

design ADU checklist

Building Code Requirements Basics

Since an additional dwelling unit is classified as an independent unit, it must be inspected as such. A unit should comply with all building and sanitary codes. Learn more about Boston’s Housing Code. A professional architect or contractor can help you at any stage of the process. See these tips for hiring a contractor.

Below are some important code and other design considerations for ADUs. There must be:

  • a full unit, which includes a bathroom and kitchen
  • two means of egress, which are ways for people to exit the building
  • water and electricity shutoff valves accessible to all units
  • compliant ceiling height, especially for basement and attic units, and
  • a sprinkler added in the new unit if you have 2- or 3-family home.

The unit cannot involve any bump out, extension, or construction to the existing envelope of the structure that results in the addition of gross floor area. Exterior work is allowed as long as the interior building square footage stays the same.

We can discuss code and design considerations particular to your home at the ADU Design Workshop (see how to apply section to sign up).

How to apply

Step 1 - Plan for your unit

Before you can apply for an additional dwelling unit permit, you need to develop plans for the unit you want to build. Check out the design section above for advice.

  • Create preliminary designs or drawings of your proposed unit
  • Prepare a description of your additional dwelling unit
  • Take pictures of the current space
  • Get a cost estimate
Step 2 - Attend ADU Design Workshop (Optional)

The ADU design workshops are currently being held digitally on the first Thursday of each month, from 4 - 7 p.m. To receive the link to the virtual meeting, please click on the “Sign Up To Attend” button below and make sure to enter your email address.

The beginning of the workshop will include a brief time for general questions. This will be followed by a review of applicants' proposed plans for their own ADUs in 15-minute time slots. This limits the number of applicants to about 10 per workshop.  After signing up, you will be notified if you are in the first 10 or not. If not, you can still attend to listen to the conversation (since many projects face similar challenges) and fill in if someone else doesn't show up, or you may request to attend a future workshop.

Plans will only be reviewed if they have been submitted online through the “Sign Up To Attend” button below.  Workshop attendees are encouraged to view this 50-minute ADU information session before the workshop.



Before you can start construction, you will need to get a long-form permit from the Inspectional Services Department. Click the how-to guide below for more information.

Permit How-To Guide

Step 4 - Register Your Unit

An additional dwelling unit used as a rental must be registered in accordance with Ch. 9-1.3 of the City of Boston Rental Registry Ordinance at the time of conversion.

Boston Home Center ADU Loan

The Boston Home Center will offer a loan that provides gap funding for qualified program participants.


The loan is zero-percent interest (0%), deferred payment loan of up to $30,000 for home renovations. The loan has no monthly payments. It also does not come due for repayment until the owner sells or transfers ownership, or undertakes a cash-out refinance of the home.


To qualify for the loan, you need to be approved for an Additional Dwelling Unit permit. Your household income also needs to be at or below 135 percent of the Area Median Income. If your income is between 120 percent and 135 percent of the Area Median Income, you will have to provide a dollar-for-dollar match of the loan amount.

For more information, and to apply, visit the how-to webpage below:


Common questions

Why DID the City start this program?

Additional dwelling units will increase affordable housing options in the City. We want to create safer living arrangements and support multigenerational family arrangements. Our goal is to provide more chances for homeowners to age in place and stay in their homes. An additional dwelling may provide needed income, or a place for a supportive relative to live, or a place with more accessibility for the homeowner. These units offer an opportunity to use existing infrastructure to achieve the City’s housing goals. Learn more about the start of the program on the ADU Program page.

What if I sell my home?

If the new owner would like to use the additional dwelling unit as a rental unit, they will need to live on the property. They also must continue to register the unit with the rental registry.

What if there is already an unpermitted additional unit in my home?

We want to help homeowners bring existing units up to code. If you are eligible for the program, and your current unit can meet all of our criteria, you can apply to have it permitted as a rental unit.

What do I do if my property is in the FEMA Flood Zone?

You must be able to show that the lowest elevation of the building (the "Top of Bottom Floor" on your elevation certificate) is above the Base Flood Elevation (BFE) to be eligible for the ADU program. An Elevation Certificate will show the BFE. You may want to check your mortgage documents for this information.  If you need to obtain an Elevation Certificate for your property, Elevation Certificates are prepared and certified by a Licensed Land Surveyor, Registered Professional Engineer or Registered Architect who is authorized by state or local law to certify elevation information. There is a useful FEMA website that explains (1) what an Elevation Certificate is and (2) how to get one for your property. Once you obtain an Elevation Certificate for your property, please include it with your application. If you need additional advice, attend an ADU design workshop.

Who can I talk to if I have additional questions?

For general questions about the design of your ADU, sign up to attend an ADU design workshop (see the How to Apply section to sign up.)

For questions about getting an ADU permit, contact the Inspectional Services Department.

For questions about getting an ADU loan, contact the Boston Home Center.

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