ADU Guide · Verified 2026
Building an ADU in Massachusetts
Massachusetts allows one ADU by right on single-family lots statewide. Under the Affordable Homes Act (Chapter 150 of the Acts of 2024, effective February 2, 2025), every city and town must permit a "protected use" ADU of up to 900 square feet — or 50% of the main home's floor area, whichever is smaller — without a special permit or public hearing. Towns cannot ban ADUs, cannot require owner-occupancy, and limit parking near transit. One key exception: Boston is excluded and runs its own ADU ordinance.
Quick Facts
Statewide floors set by Affordable Homes Act, Chapter 150 of the Acts of 2024, §§ 7-8 (amending M.G.L. c. 40A); regulations 760 CMR 71.00. Your city may be more generous but cannot go below them.
The Three Types of ADU
What you can build by right
Section 8 of the Affordable Homes Act amended the state Zoning Act (M.G.L. c. 40A) to allow one ADU up to 900 square feet by right in single-family zoning districts, in every city and town. "By right" means no special permit, no zoning-board hearing, and no neighbor veto — a compliant application is processed as a matter of course. A town cannot ban ADUs and cannot impose an additional minimum lot size or a separate utility-hookup requirement to block one. Larger ADUs can still be built in many towns, but they fall outside the by-right protection and go through whatever local process remains.
The size limit is two rules, not one
A protected-use ADU may not exceed 900 square feet of gross floor area OR 50% of the gross floor area of the principal dwelling, whichever is smaller. The 50% rule matters for smaller homes: if your main house is 1,600 square feet, your by-right ADU is capped at 800 square feet even though that is under 900; if the main house is 1,000 square feet, the by-right ADU is limited to 500. Individual municipalities may choose to allow larger units, but they cannot set a limit lower than the state standard. The ADU's layout (number of bedrooms and bathrooms) is not restricted as long as it fits the cap and meets code.
You do not have to live on the property
The Act removed municipalities' statutory authority to impose owner-occupancy requirements, so you are not required to live in either the main house or the ADU. That means you can rent out both units on long-term leases. The state also bars towns from "unreasonably" restricting the creation or rental of an ADU that is not a short-term rental.
How approval works
A qualifying ADU is permitted by right, so you skip the special-permit and public-hearing process that used to make in-law apartments a gamble. You still pull a building permit, meet the Massachusetts State Building Code, and obtain a certificate of occupancy — the by-right rule removes the discretionary zoning approval, not the safety review. Because the state regulations (760 CMR 71.00) took effect February 2, 2025 and preempt conflicting local bylaws, your application can be processed under the state framework even if your town has not finished updating its own zoning.
Short-term rentals are excluded
The protections apply to ADUs used as long-term housing, not to short-term (under 31 days) rentals. A town may prohibit or restrict short-term rental of an ADU, and the "no unreasonable restriction" protection specifically does not extend to short-term use. If your plan is an Airbnb-style rental, do not assume the state law protects it — check your municipality's short-term-rental rules.
Boston is a separate world
Boston is explicitly excluded from the statewide by-right ADU framework. Homeowners inside the City of Boston do not get the Affordable Homes Act's by-right path; instead Boston operates under its own ADU ordinance through the Inspectional Services Department. If your property is in Boston, the statewide rules described here do not apply and you should review the city's specific ADU program.
What towns can still require
The law does not strip all local control — towns may still apply reasonable regulations such as setbacks, height limits, and (outside the transit exemption) parking, and can impose reasonable dimensional standards. What they cannot do is ban ADUs, require a special permit or hearing for a qualifying unit, demand owner-occupancy, or set a size cap below the state floor. Near transit, parking requirements are limited: a town cannot require parking for an ADU within a half-mile of a transit stop.
What people get wrong
Two common errors. First, assuming the cap is a flat 900 square feet — for smaller homes the 50%-of-the-main-house rule is stricter and controls. Second, assuming the law covers Boston — it does not; Boston is carved out and runs its own program. A third trap is timing: the ADU provisions took effect February 2, 2025 (180 days after the August 2024 signing), so guidance published before that date describes a law that had not yet started.
From Idea to Move-In: The Timeline
Who Administers It
Counties With Their Own Rules
These Massachusetts counties have verified, county-specific ADU rules that add to the state baseline:
Check Your County
Select your county for local rules, fees, and your building department's contact details:
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
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Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many Massachusetts cities are more generous. Confirm specifics with your local planning department.
Find your county building department →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice — always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated August 2026.