Since February 2, 2025, every Massachusetts town outside Boston has to allow one qualifying accessory dwelling unit by right on a single-family lot - no special permit, no zoning hearing. If you are thinking about a backyard unit or an in-law conversion for a parent, the question is no longer whether the town allows it. It is which parts of the process the town still controls, and which parts the state has taken off the table.
What "by right" actually changed
The Affordable Homes Act legalized ADUs statewide, and the Executive Office of Housing and Livable Communities wrote the rules that took effect February 2, 2025. The state now says a qualifying unit is legalized "by-right in every zoning district subject to the Zoning Enabling Act," a change that "excludes the City of Boston" (Mass.gov, Executive Office of Housing and Livable Communities). By right means a homeowner with plans that qualify "may take their plans directly to the local building inspector for a building permit" - the building inspector checks the plans against the code, rather than a zoning board deciding whether to allow the unit at all.
The unit has to be a "Protected Use ADU" to get that protection. That means it stays within the state's size limit - broadly, 900 square feet or half the primary dwelling, with the exact ceiling set by the regulation (760 CMR 71.00) - and meets the building code. Boston runs its own separate ADU ordinance and is not covered by the statewide framework.
What the state took away from towns
Two things a town used to be able to do, it no longer can.
It cannot require a special permit or public hearing for a qualifying unit. That is the whole point of by-right: the discretionary approval that used to take months and could end in a denial is gone for units that qualify.
And it cannot require you to live on the property. The regulations state a Protected Use ADU "may not be subject to owner-occupancy restrictions". That matters for the aging-parent case specifically: a family building a unit for a parent, or a parent building one and moving into the main house, does not have to structure who lives where around a town occupancy rule.
What a town can still require
By right is not no rules. The state left towns room, and this is where a South Shore project still varies by town. Municipalities "may adopt reasonable regulations governing ADUs, with administrative approval". In practice that means a town can still apply reasonable design, dimensional, and safety standards through the building-permit process, can set stricter rules in a historic district, and can regulate or bar short-term rental of the unit.
The other thing that has not gone away is cost, and here the state is blunt about the number. EOHLC reports that building-code fire-separation requirements, septic upgrades, and labor and materials can, per some building inspectors it surveyed, "drive the total cost of an ADU to $500,000". That is a high-end figure for a difficult detached build, not a typical one, but it is a state-sourced warning that a detached unit on a lot with an old septic system is a different project from converting a basement.
What this means if you are planning one for a parent
The permit is now the predictable part. The variables that decide your project are the ones the state left with the town and the site: the building code, whether your lot is on town sewer or septic, whether you are converting existing space or building new, and any local design rules. Before you design anything, the two questions worth answering are which of those apply to your specific lot, and whether the unit is an internal conversion or a detached build, because that split drives both the cost and which local rules bite.
Related: how an ADU project works, and how we match you with a builder who has done them on the South Shore.