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Accessory Dwelling Units (ADUs) in Boston

Learn more about Accessory Dwelling Units (ADUs) and how to build one in Boston.

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Boston allows homeowners to build independent living units, also known as Accessory Dwelling Units (ADUs), on the property where they live once the design is approved. These additional units can be built inside of an existing home. In some cases, ADUs can also be built outside of an existing home, such as in a backyard. For homeowners who qualify, we offer interest-free loans to aid in the construction process.

What is an ADU?

ADUs can support households as they grow and change over the years.

An Accessory Dwelling Unit (ADU) is an additional residential unit that can be built within an existing home or outside of it. In Boston, homeowners of 1-, 2-, and 3-family homes may be able to build an ADU when they live on the same parcel where the ADU will be built. 
 
Internal ADUs are additional units built within an existing home, such as by converting a basement or attic into a separate unit. External ADUs are units that are built as additions to existing homes or as fully detached, smaller structures. 

Image of an internal ADU in the converted basement of a three decker

Diagram showing a basement conversion ADU with an external addition attached to the exterior of a three decker home

Image of an external ADU attached to the exterior of a home

Image of a detached external ADU in the backyard a three decker

 

ADU Questions? We Can Help!
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We'll answer your questions at any stage of the process, email ADU@boston.gov

Process Overview

Graphic showing the process to build an ADU starting with Learning, then Planning, Permitting, Building, and Moving In

The process to build an ADU is different for each individual situation, but these are the common steps you can expect.

 

The ADU process generally follows the flow in the timeline shown above. Building an ADU involves consideration of many different factors, such as feasibility of building an additional unit on your property, the cost of the project, and who to hire to assist you with design and construction. 

The ADU Toolkit below outlines the steps and information available to guide you in the process.

How Do I Build an ADU?

Step 1: Research What You Can Build

What you can build depends on zoning and building code regulations applicable to your lot. Some requirements apply to all ADUs. Learn more about what’s possible for your home.

What Can I Build?

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Step 2: Plan for Your ADU

Learn about planning the design of your ADU, hiring contractors, and ways to pay for your project. You can also learn more about resources Boston offers to support homeowners' ADU projects, like the ADU Workshop and ADU Loan Program. 

 

How to Plan Your ADU

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Step 3: Secure the Necessary Permits

Before you can build your ADU, you will need building permits from Boston’s Inspectional Services Department (ISD). In some cases, projects may need to be reviewed by the Zoning Board of Appeal (ZBA) before a building permit can be issued. Learn more about how to plan ahead for the permitting process. 

 

How to PERMIT YOUR ADU 

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Step 4: Build Your ADU

Once you have secured your permits, you can begin building your ADU. Learn more about what to expect during the construction phase of your project.

 

build your adu 

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Step 5: Move In

After a final inspection, the occupants of your ADU can move in. If you plan to rent your ADU, you will now be a landlord. As a housing provider, you should become familiar with your responsibilities and with Boston's rental regulations.

 

move in to your adu 

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Still Have Questions?

Mayor's Office of Housing staff are available to answer questions and support you throughout the process of building your ADU.

Reach out to us at or complete our contact form to have someone reach out to you.

Connect with the ADU Team.

Contact Us

Boston’s 0%-interest construction loan program for homeowners

ADU Loan Program

ADU Workshop

The ADU Design Workshop is an opportunity for Boston homeowners who are interested in building an ADU in their home or on their property to get feedback from City staff before they submit a building permit application to the Inspectional Services Department.

Workshops are held virtually (via Zoom) on the first Thursday of each month, from 4-5:30 PM.

Register to attend an ADU Design Workshop.

Benefits of ADUs

ADUs can expand lower-cost housing options, empower residents to build wealth, and foster diverse, multi-generational living spaces in Boston. 

ADUs allow homeowners to utilize their property to its fullest potential and support a household as it grows and changes over the years. Homeowners may opt to build an ADU so that family members can move close by, or to generate extra income through a rental unit.

Homeowner unlocks the door of his detached garage ADU.

Benefits For the Community
  • Efficient strategy for adding housing stock
  • Allow long-time residents to age in place
  • Expand lower-cost housing options in a neighborhood
Benefits For Homeowners
  • Provide flexibility for diverse family arrangements
  • Provide housing and care for a loved one
  • Generate extra income by renting

About ADUs in Boston

Boston is committed to supporting the production of ADUs in our City.

ADUs are an important part of Boston’s housing production toolbox and can help ease Boston’s affordable housing crisis. The Planning Department is publishing an ADU Pattern Book in October to assist homeowners interested in building an ADU. It will offer tips and ideas supported by architectural expertise, and feature ADU designs recommended for approval through the ZBA process. In addition, the Planning Department is working on updating the zoning code for small-scale residential buildings to better accommodate ADUs. Learn more about zoning updates here.

 

Outside of a detached ADU, looking upwards toward the balcony.

ADU Resources

Take advantage of our resources for homeowners interested in ADUs.

Frequently Asked Questions

Find answers to the most common questions about Accessory Dwelling Units
ADU Design Workshop

Public Workshop with Boston's ADU Staff to help homeowners building ADUs
ADU Loan Program

City's loan program to help homeowners finance their ADUs
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