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MASSACHUSETTS · STATE ADU LAW, IN PLAIN ENGLISH

Can you build an ADU on your property?

Since February 2025, 350 Massachusetts cities and towns must allow one accessory dwelling unit by right. Answer a few questions and we'll tell you what the state guarantees you — including the square footage you're actually allowed, which is usually not the 900 you've read about.

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A plain-English reading of M.G.L. c. 40A and 760 CMR 71.00 — not legal advice.

A New England colonial with a gray cottage ADU behind the house at golden hour, string lights, birch trees and a golden retriever on the lawn
By right since Feb 2, 2025
SAMPLE ANSWER Yes — up to 900 sq ft Worked out in your browser
350

cities and towns must allow one ADU by right (everyone but Boston)

Feb 2025

when the state law took effect, everywhere, all at once

0

special permits or discretionary hearings — that's what "by right" means

60 sec

to read the law against your house — free, no email

SIX WAYS IN

Where could yours go?

An ADU doesn't have to mean new construction. Most start with space you already own.

A bright finished basement apartment with a kitchenette and sitting area

Inside the house

A basement, the attic, or a carved-out corner of the floor plan.

An attached garage converted into an apartment with tall black-framed windows

The attached garage

Convert the bays you never park in anyway.

A farmhouse with a gray side addition containing an apartment

An addition

Built onto the house, with an entrance of its own.

A red barn converted into a dwelling with large glass windows

The detached garage or barn

Four walls you already own, put to work.

A carriage house with an apartment above a two-car garage

Above the garage

A second story over the cars — the classic carriage house.

A white board-and-batten cottage among birch trees

A new cottage on your lot

A small home of its own, out in the yard.

THE OTHER REASON TO BUILD ONE

A second paycheck, right on your own property

An ADU doesn't have to be for your mother-in-law. Rent it out, long-term, to anyone — that's exactly what the law allows.

Rent it — to anyone

No family-member requirement anywhere in the state law. A long-term tenant at market rent is a fully legal use of your ADU.

No strings on you, either

The state bars owner-occupancy requirements for by-right ADUs. You don't have to live in either unit for the arrangement to work.

Income that outlasts the build

Monthly rent against your mortgage, added property value, and a space that can flex later — in-law suite, office, or rental again.

See if your lot qualifies — free Short-term rentals are different — your town may restrict those.
THE 60-SECOND CHECK

Read the law against your house

A few quick questions. Your answers never leave your browser — the result is worked out right here on the page.

Only used to word the answer. Nothing is looked up and nothing is stored.

Boston plays by a different rulebook

The 2025 state ADU law works through Chapter 40A, the state zoning act — and for historical reasons Chapter 40A has never applied to Boston. Boston writes its own zoning code and runs its own ADU program, with its own rules about who qualifies, what size is allowed and what approvals you need. Nothing this screener checks would tell you the truth about a Boston address.

If your property is in Boston, start with the City's own ADU information rather than the state rules. If it's in a town that borders Boston — Brookline, Newton, Milton, Quincy, Everett and the rest are all ordinary Chapter 40A municipalities — type that town instead and the screener works normally.

What kind of home is on the lot?

The state protection follows the zoning district, not the kind of building. Most homes in Massachusetts sit in a district that qualifies — this answer just changes what else we have to tell you.

An ADU has to be accessory to something

The state rule defines an accessory dwelling unit as a unit on the same lot as a principal dwelling — a main house that already exists. On vacant land there is nothing for it to be accessory to, so the ADU route doesn't open until the main house is built or approved. Build the house first, and the ADU question becomes a live one.

If you're planning both at once, that's a real question and the order matters: email us about your property and we'll tell you honestly whether it's worth pursuing.

How many separate homes are on the lot right now?

Count whole homes — the main house, a second house, each side of a two-family. Don't count a unit here if it's an in-law apartment; that's the next question.

Is there already an accessory apartment on the lot?

An in-law apartment or ADU: smaller than the main house, with its own kitchen, bathroom and entrance. This is a different question from the one above — a second full house, or the second unit of a two-family, is not an accessory apartment and doesn't use up your ADU.

A rough number is fine — your listing, tax bill, or assessor's card will have one. This is the number that sets your size limit, so leaving it blank means we can't give you one. Don't count the garage.

How many bedrooms does the house have now?

Bedrooms, not rooms. If you're on a septic tank this is the number that decides whether your apartment can have a bedroom at all — the two get counted together against what your system was designed for.

Where would the ADU go?
Would it have its own kitchen?

This is the line between an apartment and a bedroom suite, and the two follow completely different rules.

Town sewer, or a septic tank?

If you've never had a tank pumped, you're almost certainly on town sewer. This is the item that most often decides an ADU project — but a screen can't resolve it.

Nothing is sent anywhere. The answer is worked out in your browser.

Ask about a feasibility report

Your answers above come with it, so there's nothing to repeat. Within 2 business days you'll get an email from Julieann with the price for your property and a sample report — and a link to order if you want to go ahead. No payment until then.

No payment now. Reply by email within 2 business days.

We use this to answer you about this property, and nothing else. We never sell or share your information — no mailing list, no data brokers, no advertising trackers, no cookies on this page. Privacy & security. If your enquiry becomes an order, the Service Terms are the agreement between us — what you get, what you don't, and how refunds work.

What this is: a plain-English reading of Massachusetts state law — M.G.L. c. 40A §§ 1A and 3, in force 2 February 2025, and the regulation 760 CMR 71.00, effective 31 January 2025 — applied to what you told us. It is the same for every lot in your town, because the state floor is the same everywhere. What it is not: legal advice, a zoning determination, or a reading of your town's own bylaw. Your town may be more generous than the state floor; on the things the state fixed, it cannot be stricter. Nothing here is specific to your address.

Your privacy. This screener runs entirely inside your own browser — nothing you type above is sent anywhere unless you choose to contact us. This site sets no cookies and no tracking of any kind, and Bircch never sells or shares what you tell us. Read the full policy.

No cookies. No trackers. Free, and no email required.

THE HONEST VERSION

"By right" — what it does and doesn't mean

The state law did a lot. It did not erase your town's role — and a building permit is still a building permit.

What the state fixed

One ADU allowed by right in any district that allows single-family homes. No special permit, no discretionary hearing, no owner-occupancy requirement.

What your town still controls

Dimensional rules — setbacks, height, lot coverage. Your town can be more generous than the state floor; on what the state fixed, it can't be stricter.

What you still need

A building permit, the state building code, and — on septic — Title 5. By right means no hearings, not no rules.

THE ONE THAT DECIDES IT

On septic? Read this first.

Your system was designed for a set number of bedrooms — and the house's and the ADU's are counted together. It sinks or reshapes more ADU projects than any other single item, and no online screen can resolve it.

THE EXCEPTION

Boston plays by its own rulebook

The state law works through Chapter 40A — which has never applied to Boston. Boston runs its own ADU program. Brookline, Newton, Quincy and the rest of the border towns check out normally.

THE NEXT STEP

Get the full answer for your address

The free check reads the state floor. A feasibility report reads your town's own bylaw and your lot — and tells you what's actually buildable.

  • Your town's bylaw, read against the state floor
  • The size math for your house — the number you can actually use
  • Septic & Title 5 reality check
  • Setbacks, height, and lot coverage on your lot
  • What to file, in what order
  • A photo-real aerial rendering of the ADU on your lot — for new structures and additions
  • If your plan doesn't fit by right: the relief path, spelled out
See a sample report

No payment now. Reply by email within 2 business days.

Ask about a report

WHAT HAPPENS AFTER YOU SEND THIS
  1. Your request comes straight to Julieann.
  2. Within 2 business days, you get an email back with the price for your property and a sample report.
  3. Want to go ahead? That email has a link to order. No payment until then.

A survey, a plot plan, or a hand drawing showing where you want to build. A photo of it is fine. PDF or picture, up to 8 MB total.

Every field is needed so the price fits your property. You’ll hear back by email within 2 business days. We use this only to answer you — no mailing list, no data brokers.

Quick answers

The questions every homeowner asks in the first five minutes.

What this is: a plain-English reading of M.G.L. c. 40A §§ 1A and 3 and 760 CMR 71.00, applied to what you told us. What it is not: legal advice, a zoning determination, or a reading of your town's own bylaw.

BEYOND THE ADU

More from Bircch

Buildability answers for whatever you're planning — and a close eye on the laws about to change the rules.

Feasibility reports The full answer for your address — bylaw, size math, septic, plan. New construction Vet a lot before you buy, build, or divide what you own. Additions How big, and where — before you pay for drawings. What's coming Question 7 and the duplex provision — explained in plain English.