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Additional Dwelling Units

We want to streamline the process for homeowners looking to create an additional unit.

The Additional Dwelling Unit (ADU) pilot program allows owner occupants in the City of Boston to carve out space within their homes. They can create smaller, independent units, known as Additional Dwelling Units (ADUs). In 2019 the City of Boston scaled the ADU pilot program citywide and made it permanent. Boston Zoning Commission approved a citywide zoning text amendment. For more information on the pilot program, please read about the ongoing ADU program.

Early Adopter Cohort Pilot

During the summer of 2021, the Housing Innovation Lab in collaboration with the Department of Neighborhood Development, Boston Planning & Development Agency, and the Inspectional Services Department co-hosted a pilot program for homeowners in Dorchester, Jamaica Plain, Mattapan, and Roslindale interested in turning existing detached exterior structures like carriage houses, accessory buildings and underutilized garages into livable space. As an initial step, we partnered with individual homeowners to better understand the barriers, gaps, and opportunities of transforming existing exterior structures to an ADU. This exploration will build the foundation for future citywide ADU programming and efforts across Boston. 

Over the course of four months, participants met with representatives from the collaborating departments to, through a series of workshops, begin to prepare a set of materials necessary for seeking building permits with the Inspectional Services Department and requesting variances (if needed) from the Zoning Board of Appeals.

What the Homeowner Pilot Program did not do
  • We did not fast track or streamline the approval process for ADUs in existing exterior structures
  • We did not waive or bypass existing regulations around building and fire code
  • We did not offering financial assistance for the creation of this type of ADU


There are many existing structures that are underutilized in our neighborhoods that could play a role in achieving some of our citywide goals to reduce carbon emissions, promote resilient infrastructure and foster the creation of new affordable housing opportunities. This pilot aimed to create a pathway to turn these structures into livable spaces that could provide:

  • More housing opportunities that are affordable
  • Additional income for homeowners to be able to age in place 
  • Preservation opportunities for historic structures  
  • Create and green reuse of existing structures


The goal of participating in this four month program was to gain the following:
  • Better understanding of project feasibility
  • Better understanding of zoning and variance process for small scale residential projects
  • List of zoning variances needed for ADU project in existing structure
  • Preliminary plans for renovation of existing exterior structure
  • Budget for renovation of existing exterior structure



Homeowners of 1, 2, or 3 family buildings in Mattapan, Dorchester, Jamaica Plain, and Roslindale with carriage houses, accessory buildings and underutilized garages.

  • Property with site access to allow for fire truck/emergency vehicles
  • Owner occupied 1, 2, or 3 family home 
  • Exterior structure larger than 150 square ft (minimum size based on sanitary code)

early adopter cohort


Since the summer, we have begun to carve out time during our monthly ADU Workshops where homeowners interested in transforming their existing exterior structures can ask questions from the various department representatives in the space. Sign up to attend the monthly ADU workshops by visiting the following link.

A Look Back at ADUs in Boston

The goal of our initial ADU pilot program (pilot ran Oct 2017-May 2019) was to develop more natural affordable housing options. We wanted to do this while creating rental income for homeowners. We wanted homeowners to take advantage of the existing space in their homes. We believe this program can help people age in place and prevent displacement


We wanted to see if these units increase affordable housing options in Boston.

We hoped to create safe living environments and support family arrangements. We also hoped this would help aging homeowners remain in their homes. Based on the results of the pilot, we hoped to find out the impact these units have on affordability and displacement. We would also get a better idea of the impact of expanding the program. Our final presentation to the Zoning Commission has more information.


Pilot Zoning and Eligibility

We crafted the zoning language for the program with the help of the Boston Planning & Development Agency. The agency's board approved the amendment, which you can read online. The Mayor signed off on the language in November 2017.

To qualify for a unit, your home must be:

  • three units or less
  • owner-occupied, and
  • in the City of Boston.

Additional Dwelling Unit Toolkit

As part of this program, we developed an Additional Dwelling Unit Toolkit. The toolkit is a guide for homeowners who would like to build their own unit. We walk homeowners through the:

  • permitting
  • design
  • financing, and
  • construction processes.


Additional Dwelling Unit Loan Program

We also plan to offer limited financial support to homeowners looking to build an Additional Dwelling Unit. A zero-interest, deferred equity loan up to $50,000 will be available for eligible homeowners. The loan is through the Boston Home Center's Additional Dwelling Unit Loan Program.

Results and lessons learned

The Additional Dwelling Unit (ADU) program has helped many residents. That's especially true for older adults interested in aging in place and staying in their neighborhoods. We're providing new housing for family members and renters. These ADU program participants ranged in age, income, race, and ethnicity. In addition, many residents want the program to expand to other Boston neighborhoods. You can learn more about the ADU Program and what's next in the ADU Program Report.

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