Arlington, MA Accessory Structures: ADU Rules (2026)

Accessory Dwelling Units in Middlesex County are now permitted statewide by-right under the landmark 2024 Massachusetts Affordable Homes Act, which amended MGL Chapter 40A Section 3A effective February 2025. Every single-family zoned lot in all 54 Middlesex municipalities (Cambridge, Somerville, Newton, Lowell, Waltham, Framingham, Medford, Malden, Arlington, Lexington, Concord, Belmont, Watertown, Winchester, etc.) must now allow one ADU by right up to 900 square feet or half the size of the principal dwelling, whichever is less. Key statewide rules: no owner-occupancy requirement, no family-relation requirement, short-term rental restrictions allowed, one ADU per lot, no more than 2 additional parking spaces can be required, and municipalities cannot impose discretionary review. ADUs can be attached, detached, or interior (basement/attic conversion). Local rules govern design, setbacks, and height but cannot exceed underlying zoning. Apply through the local Building Department. Municipalities may still require site plan review for detached structures but not special permit or variance.