New Massachusetts Accessory Dwelling Unit (ADU) Laws

Accessory Dwelling Units (ADUs) in Massachusetts: A New Era of Home Expansion

What Just Changed — ADUs Now “By Right” in MA

As of February 2, 2025, a major shift took effect in Massachusetts. The Affordable Homes Act (Chapter 150 of the Acts of 2024) now allows Accessory Dwelling Units (ADUs) of up to 900 square feet to be built by right in single-family zoning districts across the state. Mass.gov+2Mass.gov+2

This means homeowners no longer have to jump through complicated hoops — like requesting special permits or zoning variances — just to build a small, secondary home on their property. mcgregorlaw.com+2mcgregorlaw.com+2

Under the law:

• An ADU must be no larger than half the gross floor area of the main house or 900 sf, whichever is smaller. Moriarty Bielan & Malloy LLC+3Mass.gov+3Mass.gov+3

• The ADU must have a separate entrance, either directly from outside or via a corridor meeting building code egress requirements. Mass.gov+2Mass.gov+2

• Municipalities may impose reasonable regulations (setbacks, height limits, design constraints), but they cannot require owner-occupancy, demand special permits for protected-use ADUs, or “unreasonably restrict” their creation or long-term rental (unless used as short-term rentals) AARP States+4Mass.gov+4Mass.gov+4

In short: for many Massachusetts homeowners, adding usable living space just got easier.

Why Build an ADU? (More Than Just Extra Square Footage)

Here are compelling reasons homeowners are turning to ADUs:

• Multi-generational living made easier — an aging parent, adult child, or guest can live nearby but still have privacy.

• Rental income & investment returns — ADUs offer an opportunity to offset mortgage costs or boost cash flow.

• Maximize your land’s potential — especially for properties with room to expand or unused backyard space.

• Flexibility for changing needs — an ADU can later be used as a home office, guest suite, or even as a home gym or studio.

• Increased property value — a well-designed ADU can raise your home’s resale appeal, particularly in a market with housing constraints.

Because the permitting hurdle is lower under the new law, it’s a particularly attractive time to consider this kind of addition.

What to Know Before You Build (Key Constraints & Local Variations)

While the state law paves the way, every project still has to comply with local rules and building codes. Here are things to watch out for:

• Municipal zoning/local bylaws — some towns may still enforce stricter dimensional standards (setbacks, height, bulk). While they can’t impose unreasonable restrictions, local rules may slow down approval or create limits. mcgregorlaw.com+3Accessory Dwellings+3Backyard ADUs+3

• Short-term rental restrictions — many municipalities may restrict or prohibit ADUs being used as short-term rentals or Airbnbs. Mass.gov+2Mass.gov+2

• Utilities & infrastructure — plumbing, septic (or sewer), electric, and HVAC must meet building code and site constraints.

• Title V / septic issues — for properties on septic, capacity and health code compliance matters may come into play.

• Parking requirements — the law limits parking mandates: towns may require one parking space unless the ADU is within ½ mile of public transit, in which case no additional parking is required. AARP States+3Mass.gov+3mcgregorlaw.com+3

• Gross floor area calculations — how “gross floor area” is defined (which parts of the home count) can affect allowable ADU size. Mass.gov+1

Because so many details hinge on local regulations, consulting your town’s building / zoning department is a critical first step.

How B2 Builders Can Help You Add an ADU — End to End

At B2 Builders, we specialize in taking your idea from concept through completion. Here’s how we support ADU projects:

1. Site evaluation & feasibility — checking lot size, setbacks, utilities, and whether your property qualifies.

2. Design & permitting — creating plans that comply with state ADU rules and local bylaws, submitting for permits, and coordinating with inspectors.

3. Construction & coordination — foundation, framing, systems, finishes — managed with quality and efficiency.

4. Customized finishes that match your existing home — so your ADU looks like it belongs, not an afterthought.

5. Guidance through regulations — staying current with changing laws, code standards, and local requirements.

If you’ve been wondering whether you can build extra living space, now is the moment. The legal barriers are lower — and the potential is greater than ever.

Want to Learn More?

• Read more about the law and implementation on Mass.gov’s official ADU page Mass.gov+1

• See the final regulations summary published with the ADU rollout mcgregorlaw.com+2Moriarty Bielan & Malloy LLC+2

• For town-specific specifics, check your municipal planning / zoning department for ADU rules.