In plain words

Since February 2, 2025, state law has protected one accessory dwelling unit, or ADU, on most lots in Massachusetts zones where single-family homes are allowed. An ADU is a small, complete home with its own kitchen, bathroom and sleeping space on the same lot as a main house. It can be a unit inside the house, an addition, or a separate cottage in the yard. The law makes the first ADU "by right." That means the town reviews it through a normal permit process if it meets the rules, with no special permit (an extra approval a local board can refuse) and no public hearing. The ADU can be up to 900 square feet or half the size of the main house, whichever is smaller, and state guidance says towns cannot set a smaller limit. Towns cannot make you live on the property (an "owner occupancy" rule), cannot choose who your tenant is, and cannot require a minimum lot size. Near a train, subway, ferry or bus station they cannot require any extra parking, and elsewhere they can require at most one space. Towns can still apply building and septic codes, reasonable setbacks (the required distance from property lines) and height limits, and they may ban short-term rentals of 31 days or less. Boston is the one exception and has its own ADU rules. Statewide, towns reported 2,084 approved ADUs from 2025 through mid-2026. Every lot is different, so you can build only if your local rules and your property allow it.

The rules at a glance

State ADU lawYes
Where the state rule appliesAll Massachusetts cities and towns except Boston (350 of 351), in every zoning district where single-family homes are allowed by right or by special permit. Boston zones under its own state act, not M.G.L. c. 40A, so it writes its own ADU rules.
Approved without a hearing?Yes. Qualifying ADUs are approved by staff review, without a discretionary hearing.
Kinds of ADU the state protectsinternal, attached, detached
ADUs per lot1 protected ADU per lot by right (even if the main building has more than one unit). Towns may allow more ADUs, but any extra one needs a special permit.
Owner living on siteTowns can’t require the owner to live on the property.
ParkingIf any part of the lot is within 0.5 mile of a subway, commuter rail, ferry terminal or bus station, a town may not require any added parking space. Everywhere else, a town may require at most 1 added space.
SizeA protected ADU may be up to 900 sq ft or half the gross floor area of the main home, whichever is smaller. The statute's definition says towns may add size restrictions, but the state housing office's FAQ says towns cannot cap a protected ADU below that limit; they may allow bigger ones. The FAQ also says towns cannot limit the number of bedrooms beyond what state building, fire and sanitary codes require.
Lot size and setbacksTowns may not require a minimum lot size or apply density or floor-area-ratio limits to a protected ADU. Setbacks, height, lot coverage and similar rules may apply, but they cannot be stricter than the most permissive standard the town uses for the main home, a single-family home, or an accessory structure in that district, and each rule must pass the regulation's reasonableness test. An existing structure that does not meet current zoning cannot be ruled out for conversion just because of that.
FeesTowns may charge reasonable permit fees, but they may not require impact fees, studies or reports that a new single-family home would not need. Local departments cannot demand a separate water, sewer or electric connection beyond what state or federal law requires; a utility, state or regional board, or court order may require one only if it is reasonable for that project.
Short-term rentalsTowns may restrict or ban renting an ADU as a short-term rental, meaning stays of 31 days or fewer that fall under the state room-occupancy tax. Longer rentals cannot be unreasonably restricted, and towns cannot control who the tenant is.
Can towns opt out?No

What to do next

  1. Call or visit your city or town building or planning department and ask how it is applying the state ADU law and 760 CMR 71.00 to your lot.
  2. Measure your main home's gross floor area to find your size limit: half of it or 900 sq ft, whichever is smaller.
  3. If you have a septic system, talk to your local board of health early; read MassDEP's Title 5 guidance for ADUs.
  4. Check whether any part of your lot is within half a mile of a transit station, which removes parking requirements.
  5. Consider an MHP feasibility study, then look at the MassHousing ADU loan once you have plans and permits.
  6. If you live in Boston, use the city's own ADU program and guidebook instead of the state law.

Grants, loans and tax breaks

  • MassHousing Accessory Dwelling Unit Loan Program: A fixed-rate second mortgage, launched March 17, 2026, that pays for construction: up to $250,000 for a detached ADU and up to $150,000 for an attached ADU. It pairs a 20-year loan at 5.25% with extra zero-interest, deferred money. Borrowers must live in their own single-family home, earn up to 135% of area median income, and already have plans and permits. Program page
  • MHP Accessory Dwelling Unit Incentive Program (Phase One): Launched July 21, 2026 by the Massachusetts Housing Partnership with the state housing office. It connects homeowners with professional feasibility studies that check the site, local permits, utilities, early design and rough cost before they commit. The state has committed $10 million over two years to MHP's ADU work. Program page
  • State ADU Design Challenge: A state effort to publish ready-to-use ADU designs that homeowners and towns can use for free. Program page

What your town still decides

Since February 2, 2025, any local zoning rule that conflicts with the state ADU law is unenforceable, even if the town has not rewritten its bylaw yet. Towns still control site plan review, Title 5 septic compliance, setbacks, height and design rules, as long as those rules are no tougher than what applies to a single-family home and are reasonable. Historic districts may set stricter design standards if they stay reasonable. In floodplain or aquifer overlay districts, a town may require a special permit for an ADU only if the main home would need one and the criteria are clear and objective. When a town (not a city) changes its bylaw, the Attorney General's Municipal Law Unit reviews it and has already struck local ADU rules such as minimum lot sizes, floor-area ratios and bedroom caps. The law does not address homeowners' association rules, and selling an ADU as a separate condo is governed by a different law (M.G.L. c. 183A). Boston is not covered and has its own ADU program. In September 2026 the state proposed revisions to 760 CMR 71.00 (public comment open through October 9, 2026); the draft would, among other things, exclude shared hallways, breezeways and exit stairs from the ADU size count, write a ban on minimum lot area into the rules, let designated seasonal communities require year-round occupancy, and stop towns from requiring leases longer than 32 days. Those changes are proposals, not law, as of this date. Separately, the state budget for fiscal 2027 (Chapter 137 of the Acts of 2026) changed Zoning Act rules on variances and nonconforming lots effective July 1, 2026; those changes are not ADU-specific.

Examples of local rules

  • Boston: Boston is outside the state ADU law. Its own program lets owners of 1-, 2- and 3-family homes add an internal, attached or, in some cases, detached ADU, but the owner must live on the same parcel. Some projects still need Zoning Board of Appeal review before a building permit. The city runs an ADU financial assistance program for income-eligible owners. Source

What we’re still confirming

  • Draft changes to the state's ADU rules (760 CMR 71.00) were open for public comment in September 2026 and may change before they're final.
  • Boston's July 2026 proposal to allow backyard ADUs by right in some districts of Hyde Park, Roslindale and West Roxbury hasn't been confirmed as adopted.

Laws, bills and citations

The laws behind this (2)
  • State ADU regulations (760 CMR 71.00, Protected Use Accessory Dwelling Units (Executive Office of Housing and Livable Communities regulation), signed January 31, 2025, in effect January 31, 2025). Spells out how the 2024 ADU law works. It lists rules towns may never impose (owner-occupancy, extra parking, caps on how many ADUs a town allows, rules on who may live in the unit, or a requirement that the ADU be attached or detached) and a reasonableness test for everything else, such as setbacks, design rules, septic, site plan review and fees. It also makes towns keep a record of every ADU permit and share it with the state housing office on request. Read it
  • Affordable Homes Act: by-right ADUs (St. 2024, c. 150, ss. 7-8 (Affordable Homes Act), amending M.G.L. c. 40A, s. 1A and s. 3, signed August 6, 2024, in effect February 2, 2025). Requires every city and town covered by the state Zoning Act to allow one ADU per lot without a special permit in any district where single-family homes are allowed. Towns cannot require the owner to live on site and are limited on parking. The ADU can be up to 900 sq ft or half the size of the main home, whichever is smaller. Read it
Proposed and failed bills (7). None of these is law.
  • H.1484 (194th General Court, 2025-2026): pending as of October 3, 2026. Would let each city or town choose to require that the owner live in the main home or the ADU, undoing the statewide ban on owner-occupancy rules. The Housing Committee reported it favorably on November 20, 2025, and it has sat in House Ways and Means since. The two-year session ends in January 2027. Bill page
  • H.4707 (194th General Court, 2025-2026): pending as of October 3, 2026. Would define a movable tiny house (up to 400 sq ft, built on a chassis) and allow it as a home or an ADU, with a state title process that towns opt into, an inspection after 180 days of occupancy, and a ban on local design rules that shut such houses out. The Housing Committee reported it favorably on November 6, 2025, as a new draft of H.1493; it sits in House Ways and Means. Bill page
  • H.1482 / S.969 (194th General Court, 2025-2026): pending as of October 3, 2026. Would create a state Accessory Dwelling Unit Trust Fund that gives grants or loans to owners earning up to 110% of area median income to help build an ADU. The House version was reported favorably on November 6, 2025 and the Senate version on October 27, 2025; both sit in Ways and Means committees. Bill page
  • H.4163 (194th General Court, 2025-2026): died as of October 3, 2026. Would have created a state program letting towns count certain income-restricted, owner-occupied ADUs on the state's subsidized housing list. Sent to a study order on August 13, 2026. Bill page
  • H.4020 (194th General Court, 2025-2026): died as of October 3, 2026. Would have changed the ADU owner-occupancy rule so towns could again require owners to live on site. The committee sent it to a study order on February 12, 2026, which ends a bill's progress. Bill page
  • S.1018 (194th General Court, 2025-2026): died as of October 3, 2026. Would have let towns keep owner-occupancy requirements for ADUs. Sent to a study order on December 4, 2025. Bill page
  • S.1002 (194th General Court, 2025-2026): died as of October 3, 2026. Would have limited ADUs on lots of 10,000 sq ft or less to owner-occupied homes with family members living in the ADU, unless the town voted to waive that. Sent to a study order on December 4, 2025. Bill page

Sources

  1. 760 CMR 71.00, Protected Use Accessory Dwelling Units (Executive Office of Housing and Livable Communities regulation) · official source
  2. St. 2024, c. 150, ss. 7-8 (Affordable Homes Act), amending M.G.L. c. 40A, s. 1A and s. 3 · official source
  3. Mass.gov: Accessory Dwelling Units (EOHLC page with law summary, regulations, permit data, 2026 draft regulations) · official source · checked 2026-10-03
  4. Mass.gov: Accessory Dwelling Unit (ADU) FAQs (EOHLC) · official source · checked 2026-10-03
  5. Informational draft ADU regulation redlines (September 2026) · official source · checked 2026-10-03
  6. M.G.L. c. 40A, s. 3 · official source · checked 2026-10-03
  7. M.G.L. c. 40A, s. 1A (ADU definition) · official source · checked 2026-10-03
  8. H.4020 bill history · official source · checked 2026-10-03
  9. S.1018 bill history · official source · checked 2026-10-03
  10. S.1002 bill history · official source · checked 2026-10-03
  11. FY27 Budget 40A Changes: guidance (EOHLC, 2026-09-18) · official source · checked 2026-10-03
  12. Press release: Governor Healey and MassHousing launch affordable ADU financing (2026-03-17) · official source · checked 2026-10-03
  13. MassHousing ADU Loan Program · official source · checked 2026-10-03
  14. Press release: statewide ADU Incentive Program Phase One (2026-07-21) · official source · checked 2026-10-03
  15. Boston.gov: Accessory Dwelling Units in Boston · official source · checked 2026-10-03
  16. RE117R25: Protected Use ADUs (Mass.gov real estate continuing-education course outline) · checked 2026-10-03
  17. H.1484 bill history (local-option owner occupancy) · official source · checked 2026-10-03
  18. H.4707 bill history and text (movable tiny houses as ADUs) · official source · checked 2026-10-03
  19. H.1482 bill history (ADU trust fund) · official source · checked 2026-10-03
  20. S.969 bill history (ADU trust fund) · official source · checked 2026-10-03
  21. H.4163 bill history (ADUs on the subsidized housing inventory) · official source · checked 2026-10-03

This is general information, not legal advice. Your town, village, city or county decides what can be built on your property. If something here looks out of date, email the team.