State ADU law
Massachusetts ADU laws: one ADU by right since 2025
The 2024 Affordable Homes Act changed the state Zoning Act. Protected ADUs became by right on February 2, 2025.
What to do next: use this research to check your own lot and bids, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsWritten by HyreADU Research Desk Primary-source statute research
The short answer
Massachusetts ADU laws let owners build one accessory dwelling unit by right in any single-family zoning district. Chapter 40A, section 3, as amended by the Affordable Homes Act (Acts of 2024, Chapter 150), bars zoning that prohibits, unreasonably restricts, or requires a special permit for a single ADU.
The ADU may be no larger than half the main home’s floor area or 900 square feet, whichever is smaller. Towns may not require owner-occupancy.
They may require at most one parking space, and none within half a mile of a commuter rail, subway, ferry or bus station. The state says the by-right rule took effect on February 2, 2025. A second ADU still needs a special permit.
What Massachusetts law requires of cities
Each row is from Acts of 2024, Chapter 150, Sections 7 and 8, as published on mass.gov, and the state’s ADU FAQ.
| Question | What the Massachusetts statute says | Citation |
|---|---|---|
| Must towns allow ADUs? | Yes. No zoning may prohibit, unreasonably restrict, or require a special permit or other discretionary approval for one ADU, or its rental, in a single-family zoning district. | M.G.L. c. 40A § 3 |
| Size? | Not larger than half the principal dwelling’s gross floor area or 900 square feet, whichever is smaller. Towns may add size limits. | M.G.L. c. 40A § 1A |
| Owner-occupancy? | Cannot be required for the ADU or the main home. | M.G.L. c. 40A § 3 |
| Parking? | At most one additional space. None within 0.5 miles of a commuter rail station, subway station, ferry terminal or bus station. | M.G.L. c. 40A § 3 |
| Entrance? | The ADU must have a separate entrance, direct or through a shared hall that meets building code egress. | M.G.L. c. 40A § 1A |
| Short-term rentals? | Towns may restrict or prohibit short-term rental of ADUs. | M.G.L. c. 40A §§ 1A, 3 |
| Second ADU? | More than one ADU in a single-family district needs a special permit. | M.G.L. c. 40A § 3 |
Massachusetts ADU rules set by state law, as read on the official statute page on October 27, 2026.
Who the law covers and when it took effect
The rule applies to every city and town, but only in single-family residential zoning districts. The Executive Office of Housing and Livable Communities may issue regulations; its rules for “Protected Use ADUs” are at 760 CMR 71.00.
Mass.gov states that the law permitting Protected Use ADUs as of right took effect February 2, 2025, and that local zoning inconsistent with the ADU law and regulations has been unenforceable since that date.
What is still up to your city or county
Towns may still apply “reasonable regulations,” including Title 5 septic rules (310 CMR 15.000), site plan review, setbacks, bulk and height. They may set tighter size limits and restrict short-term rental.
The state also published a model ADU zoning bylaw for towns. Your building department applies the building code and can tell you how the town has updated its bylaw.
How to check your own city
Answer these in order, in writing, before you pay for drawings.
- 1 Confirm your lot is in a single-family zoning district
The by-right rule is limited to those districts.
- 2 Measure the main home’s gross floor area
Half of it, or 900 square feet, is your ceiling, whichever is smaller.
- 3 Measure distance to transit
Within 0.5 miles of a listed station, no extra parking can be required.
- 4 Ask whether site plan review applies
It is allowed, but it cannot be a disguised special permit.
- 5 Check Title 5 if you are on septic
Septic capacity can limit bedrooms.
What this page does not cover
This page covers the Massachusetts statute only. It does not cover building codes, septic and well rules, HOA covenants, flood or wildfire overlays, or your city’s fee schedule. Those can still stop a project the statute allows.
We read the statute on October 27, 2026. Laws change, and courts read them.
If your project depends on one clause, have a land-use attorney or the planning department confirm it for your parcel.
For the national picture, see which states have ADU laws; for what to ask at the counter, see the ADU permit guide.
Questions
Can I build an ADU by right in Massachusetts?
How big can a Massachusetts ADU be?
Does Massachusetts require owner-occupancy for ADUs?
Can a Massachusetts town ban short-term rental of an ADU?
Written and audited by
HyreADU Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a builder. We read the permit extract, the statute, the HCD return or the fee schedule ourselves, and publish each figure with its source and retrieval date.
Where a number cannot be traced to a primary source, we leave it out and say what we could not verify. Our store-based claims cover California only.
- CA
- the only state this desk will make store-based claims about
- 5
- jurisdictions with extracted ADU permit evidence
- 735
- CSLB-verified companies in the California store
- 0
- national claims from a one-state store
How this desk works
- Primary sources only. Permit counts come from the city or county that issued the permit. Production counts come from HCD’s Annual Progress Report. Rents come from HUD or the Census. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- This is a California site. The company store is 734 California firms and one New Mexico firm. Permit evidence exists for five named jurisdictions: Los Angeles, San Francisco, Sacramento, San José and unincorporated Marin. A number from that store is titled to those places, never to the United States.
- A permit is not a completion, and a license is not an ADU grade. California licenses no ADU classification. Being named on an ADU permit is evidence of engagement in that jurisdiction, not of quality, completion, or work anywhere else. Owner-builder permits are excluded from contractor counts.
- Calculation is labeled as calculation. Figures we derive are never presented as something HCD, HUD, the Census or a city published. Terner Center research is cited as Terner’s, never restated as ours.
- We do not design, permit or build ADUs, and we take no payment for placement, ranking or a favorable mention. Pages that look like rankings are not: they publish public-record counts and let the reader decide.
- Nothing here is legal, tax or financial advice. Zoning, underwriting and appraisal practice vary by jurisdiction, lender and appraiser. The useful next step on a specific lot is the planning counter and a licensed professional.
Data as of M.G.L. c. 40A §§ 1A, 3 (Acts of 2024, c. 150, §§ 7–8) read on mass.gov, retrieved 2026-10-27.. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold.
Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
- ADU Law: Acts of 2024, Chapter 150, Sections 7 and 8 , Mass.gov, Executive Office of Housing and Livable Communities Retrieved October 27, 2026.
- Accessory Dwelling Unit (ADU) FAQs , Mass.gov; states the February 2, 2025 effective date Retrieved October 27, 2026.
Planning an ADU in Massachusetts?
Tell us about your lot and what you want to build. Matching is not live yet, so this is not a quote. We will send ADU options and the next questions to ask your city.
HyreADU is a publisher. It does not design, permit or build ADUs, and matching with builders is not live yet. This page describes state statutory text as published on the retrieval date.
It is not legal advice. Your city or county applies the rules to your lot, and its planning department is the authority for your parcel.