2025 Massachusetts ADU Laws and Regulations
Massachusetts recently passed the Affordable Homes Act, a statewide law making accessory dwelling units (ADUs) legal by right across all cities and towns. This marks a major shift, as homeowners no longer have to depend on local zoning boards or special permits to add a second unit to their property. The new law sets a clear baseline: every homeowner in a single-family zoning district can now build an ADU up to 900 square feet, or half the size of the primary home, whichever is smaller.
That said, local governments can still apply certain rules, such as setbacks, height limits, and septic requirements, but they cannot be more restrictive than what’s already required for single-family homes. In other words, towns can shape ADU development, but they cannot block it outright.
This guide will walk you through the key elements of Massachusetts’ ADU law, explain how local zoning bylaws interact with the statewide rules, and highlight what homeowners need to know to get a project approved in 2025 and beyond.
Are ADU’s Legal in Massachusetts
Yes, as of February 2025, accessory dwelling units (ADUs) are legal statewide in Massachusetts. The new housing legislation requires every city and town to allow at least one ADU on any lot zoned for single-family use. This means that a municipality cannot prohibit you from building an ADU if your property is in a single family zone and cannot require that you get a special permit by going through a long zoning process.
Additionally, cities/towns cannot impose stricter building restrictions (such as lot coverage, height and setbacks) than those applied to single-family houses. For example, if a town allows single-family homes to be two stories, it cannot prevent an ADU from being built as a two-story structure.
Some towns have chosen to treat ADUs the same as a single family home while others have adopted specific regulations. Many communities have adopted regulations that encourage ADUs by lessening building restrictions and offering a streamlined process to receive a building permit.
What does “by-right” mean?
Homeowners are now able to build ADUs “by-right”. This simply means that the town cannot require a lengthy process involving approval of the Zoning Board. To receive a building permit for an ADU, the homeowner only has to go through the town Building Department. This process is much shorter than a Zoning approval and will usually take 30-45 from the time documents are submitted.
Quick Summary: What Are Massachusetts
ADU Regulations in 2025?
- ADUs are legal statewide: Every city and town must allow at least one ADU on single-family lots (attached, detached, or interior)
- Size limit: The allowed size for an ADU is up to 900 square feet or 50% of the primary dwelling’s size, whichever is smaller
- By-right approval: No special permit or zoning variance can be required if the ADU meets the state and town requirements
- Parking: Each ADU requires one off-street parking space in addition to what is required for the primary dwelling. No parking space is required for an ADU on a lot that is located within a 0.5-mile of a transit station.
- Occupancy: No owner-occupancy requirement allowed (you can rent both units if desired)
How Local Zoning Still Matters
Even though the new Massachusetts law makes ADUs legal statewide, local zoning still plays an important role in shaping what your project will look like. Towns cannot block you from building an ADU or impose stricter requirements than single-family homes, but they can still regulate certain aspects of your build.
Because of this, while the state law guarantees your right to build, the exact placement, design, and permitting process will vary by town. Homeowners should review both the statewide law and their town’s zoning bylaws to understand what’s possible on their property.
For example, your local zoning bylaws may determine:
- Setbacks: How far your ADU must be from property lines and from the primary house
- Height limits: The maximum allowable height for the ADU
- Lot coverage: How much of your property can be covered by buildings
- Site plan review: Some towns are requiring the site plan and architectural drawings of the ADU be reviewed at a public hearing
- Short term rentals: Most towns are not allowing short term rentals and are requiring ADUs to be rented for a minimum of 28 days
- Utility Requirements: Some towns allow for utilities (water, sewer, electric, gas) to be connected to the primary house while other require the ADU to have its own separate service from the street
- Impact Fees: Most towns are not charging a fee specifically for the ADU but there will normally be fees for the extra water/sewer, electrical and gas hookups
- Separate Sale: The majority of towns are not allowing homeowners to condoize their ADU and sell separately from the primary home
- Septic capacity: In areas without municipal sewer, ADU approval may depend on demonstrating that your system can handle the additional unit
- Design standards: Some towns may encourage ADUs to match the style of the main home for neighborhood consistency
- Historical Districts: If your property happens to be in a historical district (which can be found on your town’s zoning map), building an ADU will require certain design elements and approval from the town’s Historical Board.
Are ADUs Allowed with
Multifamily buildings?
This was a point of confusion that was recently clarified. The state has come out and said that one ADU is allowed by-right on all properties in a single family ZONE. This means that if you own a multifamily property but are in a single family zone, the new state law still applies and you are allowed to build an ADU. We recommend consulting your town zoning map to see if this may be the case.
Boston ADU Laws and
Regulations: A Special Case
While the Massachusetts law allows ADUs statewide, Boston is considered a separate zoning jurisdiction, and the state law does not apply. This means that the baseline “by-right” ADU allowance established under 760 CMR is not applicable in Boston in the same way it is in other towns.
In Boston, homeowners wanting to build an ADU must follow local zoning codes, permitting processes, and any city-specific ADU programs. Because of this, Boston property owners in certain neighborhoods face additional steps before an ADU can be approved.
For a detailed breakdown of Boston’s ADU regulations and how they differ from the statewide law, see our full guide here:
Regulations by City/Town
Each municipality has their own regulations and bylaws governing ADUs. Find your town below for more information on their requirements.