Permitting

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.

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

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.

Possibly – rules about parking vary between different cities and towns, but you can’t be required to have more than one space for your ADU. If your property is located within 0.5 miles of a commuter rail station, subway station, ferry terminal or bus station then you can not be required to add an additional parking space for a new ADU. In some circumstances, parking spaces can be tandem (one car in front of another in a driveway or parking space). Talk with local planning staff about what rules apply to your property.

The exact permits required vary depending on location and the details of your project. For example, a property in a historic district, with a septic system, or in a FEMA flood zone will require review and approval from departments or agencies that other projects may not. In general, typical ADUs outside of Boston do not require a specific zoning review and approval, where many ADUs in Boston do. All ADU projects require a building permit to move forward with construction.

ADUs can be charged a variety of fees from your local government, the State, and other agencies. Some fees are based on details of the project while others are fixed. The total amount depends highly on your property and plans, but Planning staff can most likely provide an estimate once you have a good idea of your project details.

See the Budgeting and Finance page for more details.

In most cases outside of Boston, state law no longer allows cities and counties to keep a property owner from building an ADU because of pre-existing issues on the lot that are unrelated to the ADU project. For example, you should not receive comments about correcting the main house or a fence unrelated to the ADU, unless there is an obvious public safety issue.

Water, sewer, electricity, and other potential utility connections are an important part of the permitting and construction process. Outside of Boston, your local government cannot require your ADU to have a separate or shared utility connection, but utilities can require it. You may need upgrades, which can add time and cost to your project. If the project requires trenching across public streets or sidewalks, you may need right-of-way permits that can delay your project even further.

For septic system upgrades and approvals, see FAQ: I have a septic system or well – can I still build an ADU?