Learning the Rules

State law defines “protected use ADU” (PUADU) as a type of ADU that must be allowed anywhere a single-family home can be built, whether by default or by special permit (note that this law does not apply in Boston). This means a municipality cannot require a special permit, waiver, variance, or other zoning relief or discretionary zoning approval for a PUADU that meets that definition of a Protected Use ADU.

PUADUs can be up to 900 square feet or half the gross floor area of the primary dwelling, whichever is smaller, as long as it can fit on the property and meet local regulations about setbacks and other details.* They must have sleeping, cooking, and sanitary facilities and a separate entrance from the primary home. PUADUs are still subject to State Building Code, Fire Code, Energy Code, Massachusetts Lead Law, and all other state health and safety laws, including the State Environmental Code. Every city or town can set their own rules for setbacks (distance from the property line and other structures) and dimensional standards (including size, height, and lot coverage) as long as those requirements are reasonable in accordance with state law and the ADU regulations. For more information on what dimensional standards and setback requirements can be required for a PUADU, visit Accessory Dwelling Unit (ADU) – FAQs | Mass.gov.

ADUs that do not meet the definition of a PUADU may still be allowed but will likely require additional review and permitting.

* Gross floor area is defined in the ADU regulations as the square footage of all stories of the building, including basements and lofts, but excluding garage parking, attics, enclosed porches, and crawl spaces. You should confirm any calculations or numbers with municipalities’ building department and trusted building advisors.

All cities and towns in Massachusetts, except for the City of Boston, are subject to the ADU law because Boston is not subject to the specific General Law (c40A) that the ADU law was written under. Throughout this site we try to clarify where there are differences, but please make sure you verify all local rules that apply to your property.

Boston residents can learn more about Boston’s ADU rules at Accessory Dwelling Units (ADUs) in Boston | Boston.gov.

Even if you’re planning to build a stick-built/traditionally constructed ADU from scratch (rather than a panelized/prefab/modular ADU), you may be able to save significant time and money by using a design that already exists and potentially customizing it from there. You may see these plans described as “ready-made” or “pre-existing.” The use of these plans still requires site planning and permits regarding the placement, utility connections, foundation, and other elements of the plans that are specific to your property. Outside of Boston, state law requires that site plan review be objective and clear, making it easier for property owners to understand what’s required.

As of April 2026, the State of Massachusetts is preparing to provide high-quality, ready-to-use ADU plans submitted by professionals. Check state websites such as the Executive Office of Housing and Livable Communities’ ADU website for more information.

Before committing to an existing ADU plan, be sure to clarify:

• Limitations on what you can customize in the plan, and
• What permits and planning are still required in your local area.

In most cases, you’re not required to tell your neighbors about your ADU. But some projects may require a special zoning variance or other permit that could require a public hearing or notice (note: variances and special permits cannot be applied to a Protected Use ADU).

Regardless, it’s always a good idea to communicate with your neighbors early in the process. Your project will run more smoothly if they are kept informed, and they may have great ideas for your project!

If you live in a Neighborhood or Homeowners Association, talk with your representative or board early in the process because they may have guidelines you’ll need to know for design and construction.

If your property is in a special zone, district, or area, or near certain landmarks or protected areas, you may need to consider additional rules and submit materials for review and approval that can add time and cost to your project. Here are some examples of special locations that require additional permitting:

• Historic districts
• Floodplains/FEMA flood zones/Coastal Floor Resiliency Overlay District
• Located near wetlands, rivers, or other protected areas
• Neighborhood Design Overlay Districts
• Located near public parks or parkways

It is important to understand these rules early on when you talk to local staff about what’s required.

If your property has an onsite septic system and/or well, you will need to hire septic/well professionals to assess the current capacity and any needs for upgrades to support the new ADU. State ADU law does not take the place of state health and safety laws that regulate septic systems, such as the State Environmental Code. Septic system requirements are often based on the total number of bedrooms.

Massachusetts Department of Environmental Protection Title 5 Compliance for Accessory Dwelling Units
FAQs related to Guidance on Title 5 Compliance for Accessory Dwelling Units

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.

There are options for homeowners to convert legally built structures (garages, barns, art studios) into ADUs, but there are many questions to consider, including:

• How to meet State Building Code and other State Code requirements, like the Fire Code
• Extent and cost of upgrades needed to reuse the existing structure
• Approvals and timelines needed to demolish a pre-existing structure
• Requirements to replace parking spots removed by demolishing existing parking
• Whether the project will be considered a detached or internal conversion ADU and the relevant rules

To qualify as a Protected Use ADU (PUADU), the structure converted must meet the definition of a PUADU (FAQ: What is a Protected Use ADU?).

For all cities and towns aside from Boston, if your property is located in a zoning district where single-family homes are also allowed by default or by special permit you are allowed to build a Protected Use ADU (PUADU) of up to 900 square feet or half the size of the gross floor area of the primary residence. (For more details about PUADUs and gross floor area definitions, see FAQ: What is a Protected Use ADU?) If there are multiple primary residences on the property, use the largest one for calculations.

• For example, if your primary residence is 1,100 square feet of gross floor area, then the maximum size of your PUADU may be 550 square feet.
• Or, if your primary residence is 1,900 square feet of gross floor area, then the maximum size of your PUADU may be 900 square feet.

Your municipality may allow ADUs larger than what is allowed by the state law, but cannot restrict you to smaller ones as long as your design meets all other requirements for a Protected Use ADU, as defined in the ADU regulations.

In Boston, maximum sizes depend on the rules of your zoning district and ADU type, but a good rule of thumb is that they are limited to 900 square feet or 30-50% of the primary dwelling’s size.

If you have a small lot without much open space to place even a small attached or detached ADU, consider creating an internal conversion ADU (see the ADU 101 page). Use existing space within the primary residence, such as a basement or attic, to create an ADU.