Somerville residents and councilors are grappling with a possible side effect of the state's new accessory dwelling unit law: the construction of miniature three-story structures behind the city's iconic triple-deckers. Mayor Jake Wilson has proposed allowing three-story ADUs, while residents and councilors have raised concerns about the possibility.
As reported by The Boston Globe, the Massachusetts ADU law allows accessory dwelling units by right in areas zoned for single-family homes and has been in effect since 2025, requiring local governments to permit qualifying units up to 900 square feet of gross floor area or half the principal dwelling’s gross floor area, whichever is smaller. State regulations set dimensional standards no stricter than the more permissive standards for the principal dwelling or an accessory structure. In Somerville, where single-family zones often sit alongside duplexes and triple-deckers, that rule has set off alarm bells.
Somerville City Councilor and soon-to-be state representative Ben Ewen-Campen captured the concern bluntly, according to the Globe's reporting: the law, as written, permits what he called a miniature triple-decker behind each triple-decker. The Massachusetts Executive Office of Housing and Livable Communities reinforced that reading last month, publishing an FAQ in September stating that the ADU law applies to triple-deckers.
No Three-Story ADUs Have Actually Been Built
Despite the fear, no three-story ADUs have been built in Somerville or anywhere else, per the Globe's account. A theoretical configuration could stack three floors, each measuring just 15 by 20 feet including staircases, to stay within the 900-square-foot cap. A spokesperson for the state housing office told the Globe that such structures are highly unlikely given size limits, building requirements, and construction costs — though the possibility alone has been enough to spook city officials shaping local policy.
The state housing office has approved nearly 2,100 ADU permits since January 2025, as of July 2026, according to the Globe. That pace tracks with separate state housing office projections showing the statewide by-right ADU provision could generate between 8,000 and 10,000 new housing units across Massachusetts over five years, a figure Hoodline has reported in connection with similar ADU rollouts elsewhere in the state. The underlying law, Chapter 150 of the Acts of 2024, also bars municipalities from imposing owner-occupancy requirements or mandating off-street parking for ADUs located within a half-mile of transit, according to KP Law, P.C.
Senator Says State Should Fix Its Own Regulations
State Senator Patricia Jehlen, who represents Somerville and voted for the state's new ADU rules, told the Globe that the state housing office should change its regulations and follow the law rather than let the current interpretation stand. Her comments point to a widening gap between the law's intent and how it is now being read in a city already packed with multi-story housing types the statute may not have anticipated.
Somerville's density makes the question particularly salient. The city's built-out urban core highlights the challenges of applying a one-size-fits-all state mandate across communities.
Somerville's Own Cottage Rules Already Stretched Thin
Somerville is not new to backyard housing policy. The city's zoning rules allow backyard cottages by right in Neighborhood Residence districts, after the City Council voted unanimously in September 2022 to remove requirements for two neighborhood meetings and Urban Design Commission review. The rules set a 576-square-foot floor-plate limit for detached cottages.
Even under those rules, the Globe reports that some Somerville backyard cottages have reached nearly 1,800 square feet according to online real estate listings, with some selling for more than $1.3 million. Former Councilor William White and residents have pushed to close perceived loopholes and reduce the maximum size allowed for these structures. The PADU Compliance Amendment, submitted to the Somerville City Council in September, would replace the city's 576-square-foot cottage footprint limit with a 1,750-square-foot maximum gross floor area cap for detached structures, according to Massachusetts ADU Laws & Rules by City.
Mayor Says He Still Backs ADUs, Just Not the Loopholes
Wilson has proposed allowing three-story ADUs, while residents and councilors have raised concerns about that possibility. The dispute reflects questions about how statewide housing legislation applies in Somerville, a densely built city.
The state's broader ADU push comes amid a persistent regional housing shortage.
For now, Somerville remains a city known for its triple-deckers. Whether the state housing office revises its FAQ guidance, or whether the proposed amendment can cap backyard square footage without running into state preemption, remains unresolved as the City Council takes up the proposal.
