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…