We completed five ADU feasibility studies in July, in Peabody, Woburn, Danvers, Malden, and the East Boston neighborhood of Boston. All five came back as workable projects. Four were rated 8 or 9 out of 10, and the fifth was rated feasible with conditions.
The most useful finding for other homeowners is about size. State law is often summarized as "ADUs up to 900 square feet," but two of the four studies done under the state law were capped below that by the size of the main house.
The month in numbers
5 studies in 5 communities
3 detached new builds and 2 garage conversions
All-in budgets from about $160,000 to $375,000
Four studies rated 8 or 9 out of 10, and one rated feasible with conditions
Projected rents of $2,500 to $3,200 a month for the two-bedroom units
What we learned
The size cap depends on your house. Under the state's Protected Use ADU rules (M.G.L. c. 40A §3 and 760 CMR 71.00), the limit is the smaller of 900 square feet or half the main house's gross floor area. In Peabody, a house of about 1,300 square feet produced a cap of about 649 square feet. The existing garage fit under it with roughly 85 square feet to spare. In Woburn, the cap worked out to 816 square feet, with the possibility of reaching 900 if a finished, code-height basement counts toward the house's floor area. In Danvers, a larger house meant the 900 square foot figure governed.
An existing garage changes the math. The Peabody conversion carried $150,000 to $225,000 in construction cost for a one-bedroom unit. The garage sat on a sound slab and foundation with a truss roof, so the money goes to interior build-out, systems, and a short utility trench. The study put that at roughly $330 to $355 per square foot, against $400 to $600 or more for a new detached unit. In Malden, a block garage behind a three-family is the base for a two-story, two-bedroom unit of about 800 square feet at $250,000 to $300,000. That budget holds only if an engineer confirms the old walls and footings can carry a second story.
Utilities can come from the house or from the street. Two of the detached studies priced both routes. Running lines from the house is shorter and cheaper, and the ADU shares the house's accounts. Connecting directly to the street gives the unit its own meters and independent services, at the cost of tap fees, a longer trench, and in one case a street opening. In Danvers the difference was about 80 feet of trench against about 146.
Boston plays by different rules. The state by-right ADU law does not apply in Boston. The East Boston study found two possible routes for a detached unit: zoning board relief under the City's ADU program, or a possible as-of-right second dwelling unit under the neighborhood's zoning. The route matters beyond the permit. An ADU can open the door to City financing for income-eligible owners, including a grant of up to $7,500 and a 0 percent deferred loan of up to $50,000, but it carries owner-occupancy conditions. The timeline runs about 12 to 15 months as-of-right and 15 to 18 months or more with a variance.
Study by study
Peabody. A detached garage behind a single-family home, converted to a one-bedroom unit of about 520 to 535 net square feet. Rated 9 out of 10, with an 8 to 12 month timeline.
Woburn. A new detached two-bedroom unit of 816 square feet on a corner lot, for aging parents or rental. All-in budget of $275,000 to $350,000 and a rating of 9. On a corner lot, both street-facing yards carry the full front setback.
Danvers. A new detached, single-level two-bedroom unit of about 900 square feet for the owners' parents, at $275,000 to $375,000 and a rating of 9. The town has not adopted a state-compliant ADU bylaw, and the state law still provides a by-right path up to 900 square feet.
Malden. A two-story, two-bedroom unit of about 800 square feet rebuilt from a block garage behind a three-family, at $250,000 to $300,000. Feasible with conditions, with the structural check on the existing walls as the main one.
East Boston. A new detached unit of about 650 to 750 square feet on a sloped lot, at $225,000 to $325,000 and a rating of 8. The open question is which permitting route applies.
Common threads
Four of the five relied on the state law for a by-right path. That includes the Malden study, where the main house is a three-family. The state regulations look at whether the zoning district allows single-family homes, which is why a multifamily lot can still qualify. The study recommends getting the building commissioner's agreement in writing.
Financing followed the property type. Four of the five properties were owner-occupied single-family homes, the profile the MassHousing ADU Loan Program is built for. The three-family was not expected to qualify and would rely on home equity or a renovation loan. One of the four single-family owners had bought recently, and for that owner the program's 95 percent combined loan-to-value cap, not its loan maximum, was the likely limit.
Three of the five called for a conversation with the building department before any design spending. In each case a single answer from the city or town, such as which review path applies or how a setback is measured, would change the design.
What this means for your property
Find your own size cap. Take half of your home's gross floor area. If that is under 900 square feet, it is your limit.
Look at what is already standing. A sound garage that fits under your cap is often the least expensive starting point.
Decide early who pays the utility bills. Shared services cost less to build. Separate meters cost more and let a tenant hold the account.
If you are in Boston, start with the City. The rules and the financing are different from the rest of the state.
Find out what your property can support
A feasibility study answers these questions for your lot: what you can build, what it is likely to cost, what it could rent for, and what to confirm with your city or town first. Your MA ADU is an approved Feasibility Study Provider under the MHP ADU Incentive Program. Request a study at yourmaadu.com.
Figures are preliminary planning ranges drawn from feasibility studies completed in July 2026. They are not a quote, an appraisal, or legal or financial advice. Property details have been generalized to protect homeowner privacy.
