Boston, MA Accessory Structures: ADU Owner Occupancy (2026)

Article 2, Section 2-1 defines two categories of accessory dwelling unit, and both build owner occupancy into the definition itself rather than into a separate permit standard. A non-detached ADU, created by converting space inside the existing building envelope or adding a bump-out, extension or similar addition, is defined only as available where the Owner is also a principal resident; its additional square footage is capped at 75% of the principal unit's gross square footage or 1,250 square feet, whichever is smaller, it cannot rise taller than the primary structure, and a lot may hold only one.

A detached ADU, built in a separate structure such as a converted garage or a new backyard cottage, is defined as available only where the Owner is also a resident; it is capped at a 900-square-foot building floor plate or the primary structure's floor plate, whichever is less, limited to 1½ stories or the primary structure's height, whichever is smaller, and must sit at least 5 feet from any other structure on the lot. A lot may contain both a detached and a non-detached ADU at once, but never two of the same type.

5, but both carry a separate, non-occupancy condition: the residential structure being converted must be registered in accordance with Ch. 3 of the City of Boston Rental Registry Ordinance at the time of conversion.