Accessory Dwelling Units (ADUs) in Massachusetts

Building an ADU is an investment of time as well as money. Outside of Boston, most ADUs take 6-10 months from design through construction. In Boston, the process is more like 10-18 months. Of all ADU types, conversions of existing interior space are generally the fastest.

• Planning (setting goals, learning the rules, budgeting): 1-2 months
• Design: 1-2 months
• Permitting: 1-2 months for a Protected Use ADU (see FAQ: What is a Protected Use ADU?) outside of Boston, 3-8 months for an ADU in Boston
• Construction: 4-6 months

Check with your city or town’s building department to confirm timeline for your specific project.

The following are rough estimates for various kinds of projects, including site work and construction:

The final number can vary widely and depends on many factors including:

• ADU size and type
• Design features and fixtures
• Site conditions (slope, fire access, foundation complexity, utility connections)
• Septic and stormwater details
• Geographic location (urban vs. rural, specific location policies)

See the Budgeting page for more detailed information on typical cost ranges and financing.

This site takes you through the ADU process step-by-step, from initial inspiration to construction and renting.

We recommend starting these first steps:

• Use our ADU Wizard by putting in your address and seeing what’s possible. If your property is in the Pioneer Valley, you may be eligible for our program providing professional technical assistance. See program details.
• Read our Getting Started page and then walk through the pages of this site one-by-one for a detailed guide on what to do next.
• Consult with your city or town’s planning and building departments to better understand your local rules.
• Speak to a trusted legal, real estate, or building adviser to understand what options might be available to you.

In most parts of Massachusetts, property owners are allowed to build a Protected Use ADU (PUADU) meeting certain characteristics (see FAQ: What is a Protected Use ADU?). You can build a PUADU in any zone where single-family homes are also allowed, including commercial or multi-family districts where you can build a single-family home – meaning you can add an ADU to a single-family home, a duplex, a triple decker, or a multi-family structure.

Your local community may choose to adopt rules that make some of these rules even easier (for instance, they may let you build a larger ADU). If a city or town chooses to allow more than one ADU on a property, it will still always require a special permitting process.

In most of Boston, ADUs converted from space already inside the home can be added to a 1-, 2- or 3-family home without zoning approval but the owner is required to live on the property. Attached and detached ADUs (including conversions of garages) are technically not considered an ADU by the city and require a much more in-depth zoning and permitting review process. See the Boston ADU website for more details.

In the Mattapan neighborhood of Boston, rules are more permissive. Owners of 1-, 2- or 3-family homes in this area can add up to one “non-detached ADU” and one detached ADU to the property (depending on maximum units per lot that are allowed in your subdistrict). The property owner is required to live on the property. “Non-detached ADUs” are defined as attached and/or conversion ADUs that add some footprint onto the existing home through a bump-out, extension, or other kinds of additions. See the City’s Research Your ADU Options page for more details.