For decades, the answer to "can I put an apartment for my mother over the garage" depended on which town you lived in, which board heard your case, and sometimes which neighbors showed up to the hearing. In August 2024, Governor Healey signed the Affordable Homes Act, and Section 8 of that law rewrote the question. As of February 2, 2025, one accessory dwelling unit is allowed by right in single family residential zoning districts across Massachusetts. By right is the phrase doing all the work in that sentence: it means no special permit, no discretionary board vote, no hearing where a project can die because someone objects to the idea of it. If the unit meets the rules, the town's job is to permit it.
The Short Version
The Affordable Homes Act, Chapter 150 of the Acts of 2024, amended the state Zoning Act so that one ADU is a protected use, allowed by right, in single family zoning districts, effective February 2, 2025. The unit must have a separate entrance meeting building code egress requirements and can be no larger than half the gross floor area of the principal dwelling or 900 square feet, whichever is smaller. Towns cannot require owner occupancy, cannot require that the occupant be a family member, and cannot unreasonably restrict creating or renting a long term ADU. Towns can still apply reasonable regulations, including site plan review, dimensional rules, Title 5 septic requirements, and restrictions on short term rentals. EOHLC's implementing regulations took effect alongside the law and supersede conflicting local ADU ordinances adopted before it. The law removed the political barrier. The physical and procedural ones, septic above all on the South Shore, still apply.
What By Right Actually Means
Under the old regime, most towns treated ADUs as a special permit use, which put every project in front of a discretionary board with the power to say no, attach conditions, or simply let an application age. The new law makes the first ADU on a single family lot what the Commonwealth calls a protected use: a homeowner whose project fits the state definition can take plans to the building department for a building permit, the same as a code-compliant addition. Towns may run a site plan review to address legitimate details, but review of a protected use is about how the project is built, not whether it may exist.
Two precision points worth knowing. The by-right protection applies in communities governed by the state Zoning Act, Chapter 40A; the City of Boston operates under its own zoning enabling act and is the notable exception. And the protection covers one ADU per lot; a town may allow more by permit if it chooses, but the statewide right is singular.
The Definition, Translated
The statute defines a qualifying ADU with two structural requirements. First, a separate entrance: either directly from the outside, or through an entry hall or corridor shared with the main house, sufficient for safe egress under the state building code. Second, the size cap, and this is the rule people most often get wrong: the unit may be no larger than half the gross floor area of the principal dwelling or 900 square feet, whichever is smaller.
The law is agnostic about form. A qualifying unit can be internal, like a basement or attic apartment; attached, like a new addition; or fully detached, like a backyard cottage or a converted garage. The EOHLC framework also protects modular and prefabricated units: if a factory-built ADU complies with the state building code, a town cannot restrict it merely for being modular. In other words, the statute drew the box, every yard gets to decide what kind of home fits inside it.
What Towns Can and Cannot Do
The law did not abolish local government. It redrew the line between reasonable regulation and veto power, and knowing where that line sits is most of what homeowners need.
| Towns May | Towns May Not |
|---|---|
| Conduct site plan review of a protected use ADU | Require a special permit or discretionary approval for the first ADU on a single family lot |
| Apply reasonable dimensional and design rules, with stricter reasonable standards possible in historic districts | Require the owner to live on the property, or require the ADU occupant to be a family member |
| Enforce Title 5 septic requirements and the state building code | Unreasonably restrict the creation or rental of an ADU that is not a short term rental |
| Restrict or prohibit short term rental of ADUs | Ban or restrict a code-compliant ADU merely because it is modular or prefabricated |
| Impose additional size limits within reason under the state framework | Enforce pre-existing local ADU ordinances that conflict with the state law and EOHLC regulations, which supersede them |
What the Law Does Not Do
Honest education means saying this part just as clearly. The Affordable Homes Act removed the zoning veto. It did not repeal physics, plumbing, or process. The state building code applies in full, with all its permits and inspections. Title 5 applies on septic lots, and MassDEP issued specific guidance in April 2025 on how Title 5 governs ADUs, which on much of the South Shore makes wastewater capacity the real feasibility test, covered in depth elsewhere in this series. Wetlands, conservation, and health regulations still operate. Short term rental of an ADU can be restricted or prohibited by the town, so this law is about housing people, not listing platforms. And the by-right ADU remains part of the same property: the statute does not by itself turn a backyard cottage into a separately sellable lot or condominium, which is a legal project of its own if an owner ever pursues it.
The one-sentence version to repeat at every kitchen table: the state now guarantees your right to seek the permit; it does not guarantee your lot can support the project. The right is political and it is won. The feasibility is physical and it is checked, septic first, building code second, town details third.
Why It Matters at This Scale
The Commonwealth is not treating this as a boutique policy. EOHLC has estimated that 8,000 to 10,000 ADUs could be built in the five years following the law, the Governor's office projected roughly 10,000 units unlocked by the ADU provision, and the state's housing plan counts ADUs among its core strategies toward the 222,000 homes Massachusetts needs by 2035. Whatever the final count, the structural fact is already true: the legislature attached a potential second dwelling to single family lots across the Commonwealth, and on the South Shore that potential now sits, mostly unexamined, under yards whose owners have not yet heard the news. Delivering that news, accurately, is what this campaign exists to do.
Where the Ecosystem fits: the law is statewide, but the answers are local: your town's site plan process, your lot's septic capacity, your building department's requirements. The ADU Exchange connects South Shore homeowners and their REALTORS® with the professionals and town-level knowledge that turn a statutory right into a permitted project.
Sources
- Commonwealth of Massachusetts, Affordable Homes Act, Chapter 150 of the Acts of 2024, Section 8, amending M.G.L. Chapter 40A, Section 3, effective February 2, 2025: malegislature.gov and mass.gov
- Executive Office of Housing and Livable Communities (EOHLC), Protected Use Accessory Dwelling Unit regulations and guidance, published January 31, 2025, and ADU production estimates: mass.gov/info-details/accessory-dwelling-units
- MassDEP, Guidance on Title 5 (310 CMR 15.000) Compliance for Accessory Dwelling Units, April 7, 2025: mass.gov
- Commonwealth of Massachusetts, A Home for Everyone: A Comprehensive Housing Plan for Massachusetts (February 2025): mass.gov/a-home-for-everyone
- Massachusetts Association of REALTORS®, ADUs By-Right: The Final Regulations (February 2025): marealtor.com
Disclaimer: The ADU Exchange is an education and connection platform, not a law firm, and this article is a plain-language educational summary, not legal advice and not a substitute for the statute, the EOHLC regulations, or municipal requirements, all of which govern over any summary and are subject to amendment and interpretation. Requirements vary by municipality and by property. Confirm current rules with your local building department, Board of Health, and a licensed Massachusetts attorney before planning or building. The ADU Exchange is not affiliated with or endorsed by the Commonwealth of Massachusetts, EOHLC, MassDEP, or the Massachusetts Association of REALTORS®. This article is published as part of the South Shore Home Options Campaign in collaboration with South Shore REALTORS®.