Accessory dwelling units are allowed by right in single-family zones

What changed

The Affordable Homes Act, approved on August 6, 2024, added a paragraph to the state Zoning Act. It took effect 180 days later, on February 2, 2025. From that date, no zoning ordinance or bylaw may prohibit, unreasonably restrict, or require a special permit or other discretionary approval for a single accessory dwelling unit, or the rental of it, in a single-family residential zoning district.

Before this, many towns allowed ADUs only for family members, only with the owner living on site, or only after a special permit hearing. Those local requirements can no longer block the first ADU on a single-family lot.

What counts as a protected ADU

The statute defines an accessory dwelling unit as a self-contained home with its own sleeping, cooking and sanitary facilities, on the same lot as a main home. It must have a separate entrance, directly from outside or through a shared hall that meets the building code for safe exit, and it may be no larger than half the gross floor area of the main home or 900 square feet, whichever is smaller. It can be inside the house, attached or detached.

A second ADU on the same lot, or renting more than one, still needs a special permit.

What towns can still require

Towns keep reasonable control. The statute names site plan review, setbacks, and the bulk and height of structures, and the state septic code where it applies. Towns may restrict or prohibit short-term rental of ADUs, but may not unreasonably restrict renting one long term. They may not require owner occupancy of either unit. They may require at most one additional parking space, and none if the ADU is within half a mile of a subway, commuter rail, ferry or bus station. Building permits and the building code still apply.

Boston, and what it means for buyers and owners

Boston zones under its own code and is not covered by this paragraph of the state law. The city’s Planning Department says it is working to update zoning to make ADUs as-of-right citywide; until then, check the current Boston rules for the specific property.

Outside Boston, a single-family home may carry the option of a legal second unit. Treat that as something to confirm, not assume. Before you rely on an ADU plan, read the town’s updated zoning bylaw and ask the building department about the specific lot: setbacks, height, septic capacity if there is no sewer, and any short-term rental limits. A rented ADU makes the owner a landlord, with the same obligations as any other landlord.