What is an ADU? Massachusetts’ ADU Law Explained for Greater Boston Homeowners

An ADU, or accessory dwelling unit, is a small, self-contained home on the same lot as a main house, with its own place to sleep, cook, and use the bathroom. It might be an apartment in a finished basement, an addition off the side of the house, or a cottage in the backyard. Since February 2, 2025, Massachusetts has allowed one ADU by right in zoning districts that permit single-family homes, in every city and town except Boston. Below, we explain what counts as an ADU, the different types, what the state law changed, and what your town can still decide.

What Counts as an ADU in Massachusetts

Massachusetts zoning law gives a specific definition. An accessory dwelling unit is a self-contained housing unit, with sleeping, cooking, and sanitary facilities, on the same lot as a principal dwelling. It must also:

• Have a separate entrance. That can be directly from outside, or through a shared entry hall or corridor that meets the state building code for safe egress.
• Stay within a size cap. It can be no larger than half the gross floor area of the main house or 900 square feet, whichever is smaller.
• Follow local rules. Towns may add other reasonable restrictions.

So a guest room with a bathroom isn’t an ADU. Add a kitchen and a separate entrance, and it likely is.

ADU vs. In-Law Suite

People often use the terms interchangeably. “In-law suite” describes who lives there. “ADU” is the legal zoning term for the unit itself. An in-law suite with a kitchen, bathroom, and separate entrance often meets the ADU definition, as long as it stays within the size cap. We cover the design side in our guide to in-law suite additions in Greater Boston.

Types of ADUs

ADUs come in three basic types.

Which type fits depends on your lot, your house, and how much separation you want. A basement conversion may be the simplest path in a house with good ceiling height. On a small lot where setbacks rule out a cottage, an attached addition is often the practical answer.

Essex Restoration builds additions across Greater Boston, including in-law suites and ADUs.

What the Massachusetts ADU Law Changed

The ADU provisions came from the Affordable Homes Act, signed August 6, 2024. They took effect February 2, 2025, 180 days after signing. Here’s what the state’s ADU law now says.

One ADU by right where single-family homes are allowed

Local zoning can’t prohibit, unreasonably restrict, or require a special permit for a single ADU in a zoning district that allows single-family homes. “By right” means you don’t need a special permit or other discretionary zoning approval for that one unit. You still need a building permit.

Size cap: 900 square feet or half the house

The ADU can’t exceed half the gross floor area of the main house or 900 square feet, whichever is smaller. A 1,600-square-foot house could have an ADU of up to 800 square feet. A 3,000-square-foot house tops out at 900.

No owner-occupancy requirement

Towns can’t require the owner to live in either the main house or the ADU. The state’s ADU FAQ adds that towns can’t regulate who lives in the unit.

Limits on parking requirements

Towns can require no more than one additional parking space for an ADU. If the ADU is within half a mile of a commuter rail station, subway station, ferry terminal, or bus station, no additional space can be required.

A second ADU needs a special permit

Only one ADU is protected by right. If a town allows more than one, it must require a special permit for the additional units.

What Your Town Can Still Regulate

The law protects ADUs from unreasonable restrictions. It doesn’t wipe out local rules. Towns can still apply reasonable regulations, including:

• Site plan review
• Dimensional setbacks and the bulk and height of structures. These can be no stricter than the most permissive standards that apply to the main house.
• Title 5 septic rules, where they apply
• Short-term rentals, which towns may restrict or prohibit
• Reasonable fees

Towns can’t impose a minimum lot size for a protected ADU or cap its size below the state limit. And every protected ADU still needs a building permit and has to meet the building code.

If you’re on septic

If your home uses a private septic system instead of town sewer, Title 5 still applies. Under Title 5, each unit’s bedrooms are counted separately, at 110 gallons per day per bedroom. The existing system must be inspected before an increase in design flow. If the ADU shares the system, a multi-compartment tank or two tanks in series is required. Check this early, because septic work can change your budget.

What About Boston?

The state law doesn’t apply to the City of Boston, which isn’t subject to the zoning statute the ADU law amended. Boston runs its own ADU program. Its zoning currently allows one internal ADU in owner-occupied one-, two- and three-family homes; attached and detached ADUs generally need a variance from the Zoning Board of Appeal. Unlike the state law, Boston’s program requires the owner to live on the property.

How Popular is ADU Housing in Massachusetts?

Interest has been strong. As of August 2026, 2,084 ADUs had been approved statewide since the start of 2025, including 854 in the first six months of 2026 alone. Seventy percent of Massachusetts communities have approved at least one. Leaving Boston out of the count, Lowell, Somerville, Plymouth, Lawrence, and Revere have approved the most, and Milton is also in the top 10.

ADU housing appeals to Greater Boston homeowners for a few common reasons:

• A place for aging parents to live close by, with privacy for everyone
• A first home for adult children
• Rental income from a long-term tenant
• A way for owners to downsize into the ADU and rent the main house

Before You Plan an ADU: A Checklist

1. Confirm your zoning district allows single-family homes, and read your town’s updated ADU bylaw.
2. Measure the gross floor area of your house to find your size cap.
3. Check setbacks, lot coverage, and height limits for the main house.
4. Find out if you’re within half a mile of a transit station.
5. If you’re on septic, schedule a Title 5 inspection early.
6. Check for a local historic district or a preservation restriction on your property.
7. Decide between internal, attached, and detached based on budget and privacy.

How Essex Restoration Helps

Essex Restoration has built homes, remodels, and additions across Greater Boston since 1995, including in-law suites and ADUs. Because ADU rules vary by town, the team sorts out zoning and permitting early. That matters in historic towns like Concord and Lexington, where Essex has experience working with local building departments and historic commissions.

If an ADU is part of a bigger project, see our guide on where to start with a whole-house renovation. When you’re ready to talk through your lot and your plans, contact our team.

Frequently Asked Questions

1. What is the difference between an ADU and a guest house?

An ADU has its own sleeping, cooking, and bathroom facilities and a separate entrance. A guest house or guest room without a kitchen usually doesn’t meet the Massachusetts definition.

2. How big can an ADU be in Massachusetts?

Up to half the gross floor area of the main house or 900 square feet, whichever is smaller. Towns can’t cap a protected ADU below that.

3. Can I rent out my ADU in Massachusetts?

Yes. The state law bars towns from requiring owner occupancy and from unreasonably restricting long-term rentals. Towns may restrict or prohibit short-term rentals. Boston has its own rules.

4. Do I need a permit to build an ADU?

Yes. By right means no special permit is needed for one ADU in a district that allows single-family homes, but you still need a building permit and must meet the building code, setbacks, and Title 5 if you’re on septic.