An accessory dwelling unit sits on Boston's City Hall Plaza on May 15, 2026 as part of a showcase put on by city officials. Photo by James Sanna | Banker & Tradesman Staff
Massachusetts’ accessory dwelling unit success story is being stymied by inconsistent local regulations, a new report from The Boston Foundation argues.
“We’ve created a puzzle in Massachusetts that no one can comprehend,” researcher Amy Dain, a senior fellow at TBF’s Boston Indicators think-tank, said at an event unveiling the report Wednesday morning.
Towns and cities across Massachusetts might have permitted around 1,200 new ADUs in the first 12 months after legalization, but that’s just a drop in the bucket of what’s actually needed to keep abreast of demand, Dain said.
“Massachusetts has more than 3 million dwelling units. Twelve hundred ADUs is a hundredths of a percent of that. With further reform, we could get tens of thousands of ADUs,” she said.
The crux of the problem, Dain’s research found, is that the state ADU law leans on Massachusetts’ patchwork of local rules around things like stormwater and setbacks to fully govern where an ADU can be built.
And most of those have no consistent definition from town to town. In some cases, like stormwater or wetlands rules, many towns go beyond what the relevant state laws and building codes require. Sometimes towns laws even lack definitions for key terms like flooding – something Dain found in 24 towns’ wetlands regulations.
“The state fire code, the state energy code, the state building codes: They’re all super complex and it’s difficult enough to have just one set” for builders to get their arms around, Dain said. “But when you add onto that regulations that volunteers wrote or [were written by] towns that don’t have a lot of staff capacity…you wind up with something that’s not viable.”
The upshot, Dain said: The system is typically too complex for most ADU builders to work in more than a few towns, because it takes so long to master their unique zoning and other rules.
That’s preventing Massachusetts from experiencing a flowering of ADU companies that other states, like ADU poster child California, have seen and keeping a lid on how many units get built to meet the vast need for housing, she said. It also has implications for efforts to build more small-scale “gentle density,” middle-income housing in residential neighborhoods, Dain said.
Research by the Pioneer Institute free-market think tank, released in February, drew similar conclusions about barriers created by complex and inconsistent local rules.
Chris Lee, head of design and development at Backyard ADUs, said in a panel discussion following the report that his experience confirms what Dain’s report uncovered.
“What it means for us on the ground: we’ve had to fight tooth and nail to build a design-build company that can service the entire state. And it just barely works,” he said.
He estimated it costs his company around $20,000 per ADU to track and meet the disparate local construction rules. With the economics of an ADU project often the biggest barrier to construction, as builders told Banker & Tradesman earlier this year, that can make or break a project’s budget.
In one of his company’s first projects, on a “suburban, manicured-lawn” property in Lexington, local wetlands laws meant an additional $100,000 in testing, analysis, grading and infrastructure to build just one ADU to protect a wetland located in a power line easement.
“We were protecting what were likely man-made wetlands with a $30,000 stormwater system that’s usually used for multifamily development,” he said.
Dain’s report argues that state legislators and local leaders need should come up with clear, uniform, statewide rules for rules governing ADUs, and bar towns from innovating beyond those. Part of that, she said, should be an effort to figure out where state environmental laws fall short of what’s needed to protect vulnerable locations like vernal pools or Cape Cod’s troubled ponds and bays – shortfalls that in many cases drove towns to enact harsher laws, she acknowledged.
State housing officials should also take another look at their ADU regulations, Dain said, looking for places they’ve created confusion, redundancy or barriers to ADU production, some of which Dain’s report identifies, and improve coordination across state departments to make the system easier for people with ADU projects to navigate.
And Massachusetts should investigate regionalizing the permit review process “to reduce the burden on applicants and the staffing and technical expertise requirements for cities and towns,” The Boston Foundation said.
