Outwith ADUs

Notes

What does the Massachusetts ADU law actually say?

3 min read

Under the 2024 Affordable Homes Act, accessory dwelling units are now allowed by-right in any zoning district that permits single-family homes across the state (well, everywhere except Boston, due to a quirk of MA zoning law). Before this change, ADU zoning allowances were made town-by-town, zoning-district-by-zoning-district. Many towns also placed restrictions on ADU massing, design, parking, and operation, and these regulations limited ADU development in practice. The state-level ADU law eliminates many of these regulations. Towns can still regulate ADU design and operation, but only if those regulations don't unreasonably restrict ADU development.

Governor Maura Healy at the Affordable Homes Act signing.
(Photo courtesy Joshua Qualls/Governor’s Press Office)

Massachusetts ADU law highlights

  • ADUs can be up to 900 square feet in floor area, or up to 50% of the primary home size, whichever is smaller. Local zoning can allow more square footage, but not less.
  • ADUs can now be internal the original structure (like a basement conversion), attached to the primary structure, or a separate detached building.
  • Zoning's dimensional regulations (for instance, how far back new buildings must be set from the lot line) are still in place, but rules for ADUs can't be more restrictive than those for single-family homes.
  • ADUs are allowed by-right, meaning you don't need to go to your local Planning Board for a Special Permit with a discretionary public hearing. Towns/cities can still require Site Plan Review, which ensures the ADU design follows local regulations. Site Plan Review is administrative. If your ADU checks all the boxes, it must move forward.
  • Owner-occupancy mandates are out. Local zoning used to say that the homeowner must live in the primary home or the ADU. Sometimes, the occupant of the other home had to be related to the homeowner by blood or marriage. These rules made lenders wary and owning an ADU way more complicated. Towns can no longer enforce those rules.
  • ADUs only need one dedicated off-street parking space, maximum. If the home is a half-mile from a transit station, no ADU-specific parking space is needed.
  • Note: Only one ADU per single-family lot qualifies under the state law. For instance, a single-family home with a basement-conversion ADU cannot build a detached ADU in the backyard by-right. That may be allowed under local zoning, but state law requires the ADU to get a Special Permit from the town.

How does the MA ADU law interact with local zoning?

The Massachusetts ADU is the floor of what can be built. Before the law passed, towns could disallow accessory dwellings units entirely, require only certain ADU types (like only internal ADUs), and impose ADU requirments that could kill a project (either outright or through additional expenses). Basically the state ADU law overrides all of those old laws, to the extent that the old laws don't jibe with the state's requirements. Violatory ADU regulations within old zoning bylaws cannot be enforced anymore. Many towns/cities have now passed updated ADU zoning to conform local law with the state requirements, and some have allowed ADUs beyond what is allowed under the state law.

So is an ADU allowed on my Massachusetts lot?

On most single-family properties in Massachusetts, ADUs are now allowed under zoning. Other regulations that stack on top of zoning may still make it expensive or impossible to build the ADU you imagine, but design thinking and feasibliity analysis can often fix these problems.

Read more about the stack of land use laws that affect your ADU here.