Massachusetts ADU Questions, Answered

An ADU, or Accessory Dwelling Unit, is a second self-contained home on the same lot as a single-family house. It has its own kitchen, its own bathroom, its own sleeping area and its own entrance. Massachusetts law caps a protected-use ADU at 900 square feet. People build them for aging parents, for adult children, for a family member who needs to stay close but wants their own front door, and sometimes for rental income.

Mostly they are the same thing called different names. "ADU" is the legal and zoning term Massachusetts uses, so it is the phrase on your permit paperwork. "In-law suite," "granny flat," "granny pod," "mother-in-law apartment," "backyard cottage," "casita" and "carriage house" are all everyday names people use for the same structure. There is one real distinction worth knowing: an in-law suite is sometimes just a bedroom and bathroom added to the main house with no separate kitchen or entrance, which makes it an addition rather than an ADU. If it has its own kitchen and its own entrance, it is an ADU in the eyes of your town, whatever you call it at the dinner table.

A BuildX ADU home package runs from about $284,301 to $525,861 depending on the floor plan you choose. On top of the home package you should budget roughly $50,000 to $150,000 for site work, which covers excavation, foundation, utility connections and septic. Site work varies more than the house does, because it depends entirely on your lot. See our ADU pricing page for the full breakdown by plan.

BuildX builds with a 100-day guarantee once construction starts. Across our 14 completed ADUs the average build has been 82 days. Permitting and design happen before that clock starts, and how long they take depends on your town and whether your project is by-right or needs a special permit.

Usually not. Under the Affordable Homes Act and 760 CMR 71.00, a protected-use ADU of up to 900 square feet is allowed by right in any zone that permits single-family homes. By right means no special permit and no owner-occupancy requirement. Some towns still have older bylaws on their books that conflict with the state rule, and state law overrides those for a protected-use ADU. Anything larger than 900 square feet, or a second ADU on the same lot, falls outside the protected use and goes back to local rules. Our town-by-town ADU rules pages cover the specifics where you live.

This is the question that most often decides a project, and it is worth answering early. In Massachusetts, septic is governed by Title 5, and capacity is calculated by bedroom count rather than by square footage. If your existing system was designed for a three-bedroom house and you add a one-bedroom ADU, you now need four bedrooms of capacity. Sometimes the existing system already has room. Sometimes it needs to be upgraded or replaced, which is a real cost worth knowing about before you fall in love with a floor plan. In towns with public sewer this is not a concern at all.

An ADU does not automatically need separate utilities. A separate electric meter is optional in most cases and is usually driven by whether you intend to rent the unit and bill utilities separately. Water and sewer connections are typically shared with the main house. Septic is the exception: the system has to have enough permitted capacity for the total bedroom count across both dwellings, whether that capacity is in one shared system or an upgraded one.

Yes. Under the state by-right provisions you can rent a protected-use ADU as a long-term rental, and there is no owner-occupancy requirement for the protected use. Short-term rentals are a different matter and many towns restrict them separately, so check your local rules before counting on that income.

Generally yes, though how much depends on your town and your lot. An ADU adds permitted, finished living space with its own kitchen and entrance, and in a market with very little housing supply that is a meaningful asset. It also creates rental capacity, which some appraisers treat as income potential. What we tell homeowners honestly is that most people build an ADU because they need it for family, and the value is a real but secondary benefit.

Neither. Every BuildX ADU is custom stick-built on your property by our own crews, framed wall by wall the way a house is built. We do not use modular boxes, shipping containers or prefab panels. It is a real home, built on site, and it is built to last as long as the one you already live in.

Most of the homeowners who come to us choose detached, and the reason they give is almost always privacy, for both households. A detached ADU is its own small building in the yard. An attached ADU is built into or onto the existing house, which can be less expensive because it shares walls and utility runs, but it means shared walls. Under the state by-right rules both are allowed in zones that permit single-family homes. The right answer depends on your lot, your setbacks and how much separation the family actually wants.

We build across Southeastern Massachusetts and the South Shore from our offices in Carver and Hanover, including Plymouth, Duxbury, Marshfield, Pembroke, Kingston, Scituate, Norwell, Hanover and the surrounding towns. We publish detailed ADU rules for 40 Massachusetts towns. If you are not sure whether you are in our service area, see our service area or just call and ask.

Start with a conversation about your property and what you are trying to accomplish. There is no charge and no obligation. We look at your lot, your septic situation and your town's rules, and tell you honestly whether an ADU makes sense for you. You can also walk through a finished one first at a private showing, which most people find more useful than any brochure.