Frequently Asked ADU Questions

Internal ADUs may be built on all owner-occupied lots that contain 1-, 2-, or 3-family homes. Detached ADUs and ADUs built as external additions to the existing home (usually called Attached ADUs) are only allowed to be built without special zoning approval in Mattapan. Learn more on our Zoning for ADUs page.

Use the City's Zoning Viewer tool to look up your zoning. 

The timeline to design, permit, and build an ADU will depend on a variety of factors specific to your property, as well as how quickly you are able to obtain financing for your project and when you decide to work with building professionals.  As a general guideline, you can expect the following timeline, with some variation:

Feasibility & Design phase (up to 3 months)

During this phase, you will be researching the type of ADU that best fits your needs and your existing property's characteristics. You may decide to hire an architect or builder to help you clarify your design and get a cost estimate for the scope of work your project may require. This cost estimate will help you solidify your budget and obtain any outside financing you need, such as loans from your banking institution or the Boston Home Center's ADU Loan.

permitting phase (up to 3 months)

During this phase, you and/or your general contractor will submit your building permit application to the Inspectional Services Department for review. If all documents are submitted in full and the Plans Examiner doesn't flag any code violations, review could take about 5 weeks. Review could take longer if you need to update plans or submit more documentation, or if the project requires zoning relief from the Zoning Board of Appeal.

construction phase (6-12 months)

During this phase, your contractor and building team will work on building your ADU. The construction timeline greatly depends on the type of ADU you've opted to build and the scope of work on your property. 

The answer to this question primarily depends on your budget and comfort level or knowledge with major home improvement projects! 

when to hire a pro for the Design of your ADU

You will need a licensed architect to stamp your ADU plans before you submit them as part of a building permit application. Some homeowners have a firm idea of the design of their ADU and only hire an architect at the very end of the design process to finalize their idea. Other homeowners prefer to work with an architect early in the process to brainstorm the feasibility of building an ADU on their property. 

when to hire a pro for ADU Construction

Generally, under Massachusetts Law, you will be required to hire a licensed general contractor to supervise the construction of your ADU.  However, homeowners of single-family homes may be exempt from hiring a general contractor, if they wish to supervise the work on their own. Homeowners who opt for this option will be required to submit a Homeowner Exemption Form alongside their building permit application. This option only applies to properties that have an existing single family home. 

The City does not keep a list of architects or contractors for contact. The following sites can be used as a starting place to begin your research. Be sure to read our page Hiring Architects & Contractors for tips to keep in mind as you begin your search.

Architects

• Boston Society of Architects
• Boston Chapter of the National Organization of Minority Architects

contractors

• Builders & Remodelers Association of Greater Boston
• Associated General Contractors of Massachusetts

The total cost to build an ADU depends both on the type of ADU you will build and the scope and scale of your own unique project. Generally, converting existing space -- such as a basement-- will cost less than building an ADU through new construction. As a rough benchmark, the total cost to build an internal ADU is generally between $75,000 - $100,000; the cost to build a detached ADU can run between $250,000 - $350,000.  Your architect and/or building team will be able to help you estimate the cost of your project. Learn more on Paying for your ADU. 

First, review our fact sheet to determine if you meet the baseline eligibility requirements.

You should apply for the ADU Technical Assistance Grant when you're early on in the process of planning your ADU. If eligible for the program, you'll be paired with staff that can provide important information on the code and zoning requirements for building an ADU. 

You should apply for the ADU Loan once you have begun working with your architect and/or general contractor and have a solid understanding of the design and scope, including estimated cost, of your project. 

You may apply to both programs.

Costs incurred during the design and permitting of an ADU project are eligible for reimbursement, up to a maximum combined amount of $7,500. Common costs that homeowners may submit for reimbursement are:

• A deposit or retainer fee for a general contractor
• Fees associated with architecture/land surveyor/engineer services
• Permit fees associated with a building permit application with the Inspectional Services Department
• Zoning Board of Appeal fees

Homeowners participating in the ADU Technical Assistance Grant program will receive the reimbursement grant after they have 1) received an approved building permit to build their ADU from the Inspectional Services Department and 2)  have enrolled in the Boston Home Center’s ADU Loan Program.

The ADU Loan may only be used for construction costs. It cannot be used to reimburse the homeowner for costs associated with designing or permitting your ADU. Typically, the loan is paid in installments directly to the contractor responsible for the construction work. Payments are made after inspections by the Boston Home Center's Construction Monitoring Unit, in order to determine that all work is going smoothly. We require that the homeowner pay their portion of the construction costs first, and then use the City's ADU Loan as gap financing to finish the project. 

A Plans Examiner from the Inspectional Services Department will review your application documents. They will make sure your plans comply with all zoning, building, and fire codes. Depending on their review, you may be asked to provide further information. You should expect that review will take about 5 weeks. It may take longer if your application package is incomplete. 

You can use ISD's permit finder tool to check the status of your building permit.

Yes. Like any other major home improvement project, an ADU will impact your property taxes once a new assessment has happened. The exact amount is dependent on a variety of factors unique to your property, such as the type of work that has been done. 

Yes. You must still register your ADU even if you are not collecting rent from your tenants or if your tenants are relatives who are 18 years of age or older and not on the deed. Properties that are vacant or being renovated must also be registered annually. Learn more on our page Moving In: What's Next After Your ADU is Built?

Yes, Massachusetts recently passed a comprehensive housing bond bill commonly known as the Affordable Homes Act. One of its provisions allows accessory dwelling units under 900 square feet by right on single-family lots across the state, and to disallow having requirements of owner occupancy for properties with ADUs. This provision functions by amending the Massachusetts Zoning Act, also known as Chapter 40A. Every municipality in Massachusetts except Boston derives its power to enact and enforce zoning through Chapter 40A. Boston is distinct in that its power to use zoning comes from a separate law, Chapter 556 of the Acts of 1956, and so the changes from the Affordable Homes Act do not apply to Boston. This is the same reason that the MBTA Communities multifamily zoning efforts, another amendment to Chapter 40A, did not apply to Boston. Boston was not specifically excluded from either of these two efforts, but Boston has to be specifically included for statewide zoning changes to apply to it. 

However, the fact that ADUs are legal by-right elsewhere in the Commonwealth carries several implications for Boston:

• The state has affirmed that it is in the interest of public policy to allow for fairly unrestricted access to ADUs, and that owner occupancy requirements are recognized as a barrier to this;
• Boston, like many of the surrounding communities where two-, three-, and multi-unit buildings are prevalent, has more work to do to ensure that ADUs will be possible on most lots;
• More non-traditional funding mechanisms will become available for smaller, less expensive accessory units as the market adapts to a new typology of housing across Massachusetts; and
• This will make it easier for banks to offer standardized products for financing small homes and for building companies to design standard, prefabricated small homes.