Somerville, Massachusetts, has long embraced both triple-deckers and accessory dwelling units as practical ways to make housing more attainable.
But a quirk in the state’s updated accessory dwelling unit rules could force the city to allow something far stranger: three-story backyard homes that function like tiny triple-deckers.
Here’s what to know
Since 2025, Massachusetts has required municipalities to permit accessory dwelling units in areas zoned for single-family homes, so long as the units satisfy state standards.
Somerville’s zoning makes the issue more complicated: the same districts permit duplexes and triple-deckers, which city officials said could mean an accessory dwelling unit in the rear yard qualifies for the height and story allowances that apply to a triple-decker, The Boston Globe reported…