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Homeowners: Big Changes for Small Spaces: What You Need to Know About Massachusetts' New ADU Law
- Tim van der Veen
- Jul 11
- 4 min read
Updated: Jul 14
Massachusetts just made a big move to tackle its housing shortage—and it comes in the form of something small: Accessory Dwelling Units, or “ADUs.” Under the new state law that took effect on February 2, 2025, homeowners in single-family zoning districts now have the right to add one ADU by right, with no special permit required (in most cases).
This is a game-changer. If you’ve ever considered converting a basement, building a backyard cottage, or creating a rental above your garage, it’s time to take another look. Here’s what every homeowner, buyer, seller, and real estate agent in Massachusetts should know.
What Is an ADU?
An Accessory Dwelling Unit is a smaller, self-contained residential unit located on the same lot as a primary home. It must have its own kitchen, bathroom, sleeping area, and entrance—and it can take several forms:
A basement or attic conversion
An addition to an existing home
A detached structure like a tiny home, guest house, or converted garage
The unit must be no more than 900 square feet, or 50% of the gross floor area of the main house—whichever is smaller.
Why This Law Matters
The new law is part of the 2024 Affordable Homes Act, and the goal is simple: to create more affordable housing options throughout the Commonwealth. ADUs offer:
Rental income for homeowners
Housing for aging parents, adult children, or caregivers
Opportunities for downsizing without leaving your neighborhood
Sustainable growth using existing infrastructure
And they’re protected under M.G.L. c. 40A, Section 3, alongside other “Dover Amendment” uses like farming, education, and religious worship. That means cities and towns can’t block you from building one—so long as you meet the basic requirements.
What Cities and Towns Can and Can’t Do
Let’s talk local control. While the state has set the baseline rules, municipalities still have a little leeway—but not much.
Towns CAN:
Ban short-term rentals (under 31 days)
Require 1 additional parking space—unless you’re within 0.5 miles of a transit station
Enforce state health, safety, and environmental laws (e.g. building code, Title 5)
Towns CANNOT:
Require owner-occupancy of the ADU or primary home
Limit who can live in or rent the ADU (again, except short-term rentals)
Ban modular ADUs
Impose stricter design or dimensional standards than they do for other single-family homes
Require special permits or excessive reviews for one Protected Use ADU per lot
In short: if your unit fits within the law’s parameters, your town must allow it.
A Few Caveats (Because There Always Are…)
Title 5 Septic Rules Still Apply: If you’re on private septic, adding a bedroom may require you to upgrade your system. This could make an ADU cost-prohibitive for some homeowners—though it’s less of an issue in sewered communities or for new construction.
New Law Does NOT Apply to Boston: Boston has its own ADU law. More about this can be found at Accessory Dwelling Units (ADUs) in Boston / Boston.gov.
Municipal Pushback: Some towns are trying to find workarounds. They may adopt strict interpretations of building codes, parking, or septic requirements. As always, check with your building department before undertaking any construction project.
One Unit Per Lot: The law protects only one ADU by right. If you want to build more than one, you’ll still need a special permit.
🏡 5 Key Takeaways for Homeowners, Buyers, and Sellers
You Can Add One ADU—By Right
As of February 2, 2025, Massachusetts law allows one ADU per single-family lot without a special permit—as long as it meets the square footage and entrance requirements and complies with state health and building codes.
No Owner-Occupancy Requirement
You don’t have to live in the house or the ADU to build or rent one out. This opens the door to more flexible housing options and investment potential.
Watch the Septic (Title 5)
If your home has a private septic system, you may need to expand or replace it to accommodate additional bedrooms. In many cases, this is the most significant obstacle to building an ADU.
Short-Term Rentals Are Off the Table
ADUs can’t be used for Airbnbs or other rentals under 31 days, UNLESS expressly allowed by municipality. Check with the building department or local zoning by-law.
Check Local Requirements
While towns can’t block ADUs or make the rules overly burdensome, they can require parking, enforce state codes, and regulate design—but only within reason. Always check with your town before you build.
Final Thoughts
This new ADU law opens the door—literally—for homeowners across Massachusetts to create more housing, more income, and more flexibility. Whether you’re planning for your parents, your kids, or your future tenants, now is the time to explore your options.
If you have questions about zoning compliance, septic capacity, or permitting, Vanderveen Law is here to help. We work with agents, buyers, sellers, investors and developers throughout Massachusetts to get you started on the right foot and help avoid costly mistakes.
📞 Have questions? Reach out anytime: www.timvanderveenlaw.com
***This article is intended for general informational purposes and should not be construed as legal, tax or other professional advice. Prior to acting on any information in this article, you should seek legal, tax or other relevant professional counsel.***