Under the Affordable Homes Act (Chapter 150 of the Acts of 2024), Massachusetts allows one Protected Use accessory dwelling unit by right in single-family zoning districts as of February 2, 2025. The unit may be up to 900 square feet or half the principal home, whichever is smaller, with no owner occupancy required.
Last verified: September 25, 2026 against the Massachusetts EOHLC ADU guidance and FAQs on mass.gov and the statutory text of Chapter 150 of the Acts of 2024, Sections 7 and 8.
An accessory dwelling unit, or ADU, is a second, smaller self-contained home on a lot that already has a house. In Massachusetts the by-right version is called a Protected Use ADU, which is the single ADU that a city or town must allow without a special permit in a single-family zoning district. This page explains what the Affordable Homes Act allows statewide, and what your city or town may still regulate.
What does the Massachusetts ADU law allow?
The massachusetts adu law allows one Protected Use ADU by right in single-family residential zoning districts. By right means the use is allowed without a special permit or other discretionary zoning approval. The rule comes from the Affordable Homes Act, signed by Governor Healey on August 6, 2024 as Chapter 150 of the Acts of 2024. Sections 7 and 8 of that act amended M.G.L. c. 40A section 1A and section 3.
The by-right provision took effect February 2, 2025. As of that date, any local zoning provisions that are inconsistent with the ADU Law and its regulations are unenforceable. The implementing regulation, 760 CMR 71.00 (Protected Use Accessory Dwelling Units), took effect two days earlier, on January 31, 2025.
The statutory language in Section 8 states that no zoning ordinance or by-law shall prohibit, unreasonably restrict or require a special permit or other discretionary zoning approval for a single accessory dwelling unit in a single-family residential zoning district. Protected Use ADUs are allowed by right in all zoning districts that permit single-family homes to be built by right or by special permit. Only one Protected Use ADU is allowed per lot; a city or town must require a special permit for any additional ADU beyond that one by-right unit.
Massachusetts ADU rules at a glance
Each rule below is a single standard tied to its statute and primary source. A Protected Use ADU is the single ADU allowed by right; the reasonableness test is the standard in the regulation at 760 CMR 71.03(3) that local requirements must survive.
| Rule | Massachusetts standard | Statute or regulation | Source |
|---|---|---|---|
| By-right unit | One Protected Use ADU allowed by right in single-family districts | M.G.L. c. 40A section 3 (St. 2024, c. 150, section 8) | mass.gov ADU Law page |
| Effective date | Took effect February 2, 2025 | St. 2024, c. 150 | mass.gov ADU FAQs |
| Size cap | Not larger than 900 sq ft or half the principal dwelling’s gross floor area, whichever is smaller | M.G.L. c. 40A section 1A (St. 2024, c. 150, section 7) | mass.gov ADU Law page |
| Owner occupancy | May not be required for the ADU or the principal dwelling | M.G.L. c. 40A section 3 | mass.gov ADU Law page |
| Special permit | May not be required for a single ADU | M.G.L. c. 40A section 3 | mass.gov ADU Law page |
| Parking | No more than 1 additional space; zero within 0.5 miles of transit | M.G.L. c. 40A section 3 | mass.gov ADU Law page |
| Short-term rental | Municipality may restrict or prohibit STR use | M.G.L. c. 64G section 1 | mass.gov ADU FAQs |
| Septic | Subject to Title 5 (310 CMR 15.000) if applicable | M.G.L. c. 40A section 3 | mass.gov ADU Law page |
| Boston | State c. 40A ADU law does not apply in Boston | M.G.L. c. 40A | mass.gov ADU FAQs |
How big can an ADU be in Massachusetts?
A Massachusetts Protected Use ADU may be no larger than the lesser of 900 square feet or half of the principal dwelling’s gross floor area. The statutory definition in Section 7 says the unit is not larger in gross floor area than 1/2 the gross floor area of the principal dwelling or 900 square feet, whichever is smaller. The operative measure is whichever is smaller.
A city or town cannot cap the size of a Protected Use ADU below 900 square feet, but it may allow a larger unit. For measuring the unit, gross floor area includes basements, lofts, and intermediate floored tiers. Gross floor area does not include crawl spaces, garages, attics, enclosed porches, and similar spaces. The statute and regulations also prohibit towns from imposing lot area requirements on Protected Use ADUs, such as calculations based on lot square footage or density.
Does Massachusetts require owner occupancy?
No. The law states that a municipality shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling. You do not have to live on site to build or rent a Protected Use ADU.
The law and regulations also do not allow a municipality to regulate who can live in a Protected Use ADU, so a town cannot require a family relationship between occupants. A municipality may not limit the number of bedrooms beyond what applicable state code requires.
What can Massachusetts cities and towns still regulate?
The state law removes the biggest local barriers but leaves room for reasonable local rules, subject to the reasonableness test at 760 CMR 71.03(3). Section 8 provides that the use of land or structures for an ADU may be subject to reasonable regulations, including site plan review and regulations concerning dimensional setbacks and the bulk and height of structures.
What a municipality may NOT do:
- Require owner occupancy of the ADU or the principal home.
- Require a special permit or other discretionary approval for a single ADU.
- Require more than one additional parking space for an ADU.
- Require any additional parking for an ADU located not more than 0.5 miles from a commuter rail station, subway station, ferry terminal, or bus station. A bus station means a regular and specific location for pick-up and drop-off; flag stops do not count.
- Require a minimum lot size.
- Regulate who can live in the ADU or require a family relationship.
What a municipality may still regulate:
- Reasonable dimensional standards, but any requirements on setbacks, lot coverage, open space, bulk, and height must be the most permissive standard required for the principal dwelling, a single-family residential dwelling, or an accessory structure in the district.
- Site plan review and reasonable fees.
- Short-term rental use of a Protected Use ADU.
- Title 5 septic compliance where a septic system serves the property.
Everything a municipality does must survive the reasonableness test set forth in the ADU regulations at 760 CMR 71.03(3).
Can a Massachusetts town limit parking for an ADU?
A town may require no more than one additional parking space for a Protected Use ADU. Section 8 provides that not more than 1 additional parking space shall be required for an accessory dwelling unit.
Near transit the requirement drops to zero. No additional parking space shall be required for an ADU located not more than 0.5 miles from a commuter rail station, subway station, ferry terminal, or bus station. EOHLC clarifies that a bus station for this exemption means a regular and specific location for pick-up and drop-off, so ordinary flag stops are not bus stations.
Can a Massachusetts town ban short-term rental of an ADU?
Yes. A municipality may restrict or prohibit the use of a Protected Use ADU as a short-term rental, as defined in M.G.L. c. 64G section 1. A short-term rental applies only to stays of not more than 31 consecutive calendar days; exemptions include seasonal-employee lodging and tenancies at will or month-to-month leases.
There is a limit on that power. A town may prohibit short-term rental use, but no municipality shall unreasonably restrict the creation or rental of an accessory dwelling unit that is not a short-term rental. In other words, an anti-Airbnb by-law is allowed, but a by-law that blocks ordinary long-term rental is not.
Does an ADU trigger a septic upgrade in Massachusetts?
It can. Adding an ADU on a property served by a septic system is subject to Title 5, the state septic code at 310 CMR 15.000. Section 8 states that an ADU may be subject to reasonable regulations, including but not limited to 310 CMR 15.000 et seq., if applicable.
Municipalities must comply with applicable Title 5 requirements when they issue permits for Protected Use ADUs, and a homeowner may be required to upgrade a septic system to comply with both Title 5 and local regulations. Any local requirement above the standards set by Title 5, such as additional septic capacity or a separate system, must survive the reasonableness test at 760 CMR 71.03(3)(a). A blanket new-system mandate can be unlawful where some lots are too small to accommodate an additional septic system, which would tend to create a de facto prohibition on ADU development in violation of M.G.L. c. 40A section 3. For property-specific questions, EOHLC directs owners to the local board of health.
Does the Massachusetts ADU law apply in Boston?
No. All cities and towns in Massachusetts, except for the City of Boston, are subject to the ADU law, because Boston is not subject to the specific General Law, c. 40A, that the ADU law was written under. EOHLC frames this as the law applying to 350 of the 351 municipalities. Boston has its own ADU rules, and EOHLC directs Boston residents to Boston’s own guidance.
How Massachusetts compares to other states
Massachusetts joined a wave of New England and other states that made ADUs a by-right use. For a neighboring comparison, see our page on Maine ADU law and our page on New Hampshire ADU law. For a West Coast approach that also limits owner occupancy and parking, see Washington ADU law. If you are weighing a smaller structure, our guide to a tiny house in the backyard explains where that fits. You can also browse the ADU laws library for every state we track.
For what building one actually costs in the state, see our Massachusetts ADU cost guide.
What changed in 2026?
No new statewide Massachusetts by-right ADU statute took effect in 2026. The governing law remains the Affordable Homes Act (Chapter 150 of the Acts of 2024), with the by-right provision effective February 2, 2025 and the regulation 760 CMR 71.00 effective January 31, 2025. EOHLC opened a public comment period on draft revisions to 760 CMR 71.00 on September 11, 2026, with comments accepted through October 9, 2026. As of September 25, 2026, the current in-force regulation is still the version effective January 31, 2025, and any changes from the draft are not yet law.
What changes in 2027?
No further statewide Massachusetts ADU changes are scheduled as of September 25, 2026. We recheck EOHLC guidance and the Massachusetts Register and will update this page when a change is adopted.
Frequently Asked Questions
When did Massachusetts’ by-right ADU law take effect?
February 2, 2025. The Affordable Homes Act (Chapter 150 of the Acts of 2024) was signed on August 6, 2024, the implementing regulation 760 CMR 71.00 took effect January 31, 2025, and the by-right provision took effect February 2, 2025.
How big can a Massachusetts ADU be?
No larger than the lesser of 900 square feet or one-half the gross floor area of the principal dwelling, whichever is smaller. A town cannot cap a Protected Use ADU below 900 square feet but may allow a larger unit.
Can my town require me to live on-site for an ADU?
No. The law shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling. A town also cannot require a family relationship or restrict who lives in the ADU.
Do I need a special permit to build one ADU?
No. For a single ADU in a single-family district, no zoning ordinance may prohibit, unreasonably restrict, or require a special permit or other discretionary zoning approval. A second ADU on the same lot may require a special permit.
How much parking can a town require for an ADU?
At most one additional space, and zero additional spaces if the ADU is within 0.5 miles of a commuter rail station, subway station, ferry terminal, or bus station.
Can my town ban using an ADU as a short-term rental?
Yes. A municipality may restrict or prohibit short-term rental use, meaning stays of 31 days or fewer under M.G.L. c. 64G section 1, but it may not unreasonably restrict ordinary long-term rental or the creation of the ADU.
Does the state ADU law apply in Boston?
No. Boston is the one Massachusetts municipality not subject to M.G.L. c. 40A, so the state by-right ADU law does not apply there. Boston has its own ADU rules.
Will adding an ADU trigger a septic upgrade?
It can. ADUs are subject to Title 5 (310 CMR 15.000) if applicable, and a homeowner may have to upgrade the septic system for the added flow. Local requirements exceeding Title 5 must be reasonable under 760 CMR 71.03(3)(a). Contact your local board of health.
Sources
- Massachusetts EOHLC, ADU Law (Acts of 2024, Chapter 150, Sections 7 and 8): https://www.mass.gov/info-details/adu-law-acts-of-2024-chapter-150-section-7-and-8
- Massachusetts EOHLC, Accessory Dwelling Unit (ADU) FAQs: https://www.mass.gov/info-details/accessory-dwelling-unit-adu-faqs
- Massachusetts EOHLC, Accessory Dwelling Units main guidance: https://www.mass.gov/info-details/accessory-dwelling-units
Disclaimer
This page is general information, not legal advice. ADU law changes often and cities and towns apply it differently. Confirm the current rules with your local planning department or board of health before you build.