We completed three ADU feasibility studies in Haverhill in the same week. Each homeowner wanted roughly the same thing: a two-bedroom, one-and-a-half-bath unit at or near the state's 900 square foot limit. The all-in budgets ran from $225,000 to $375,000.
Finish level had nothing to do with the spread. All three were priced with the same mid-grade finishes and all-electric systems. The difference came from what each property already had in place, and from one or two site conditions that the owners could not see from the driveway.
Same city, same unit, three ways to build it
Garage conversion (older two-family with a newer detached two-story garage): about 900 SF, 2 BR / 1.5 BA. All-in cost $225,000 to $275,000, or $250 to $305 per square foot. Projected rent $2,300 to $2,600. Move-in in 7 to 10 months. Feasibility rating 9 out of 10.
Attached addition (1980s ranch on about 7,000 SF): about 800 SF, 2 BR / 1.5 BA. All-in cost $275,000 to $350,000, or $345 to $440 per square foot. Projected rent $2,400 to $2,700 with utilities included. Move-in in 9 to 12 months. Feasibility rating 8 out of 10.
Detached new build (older single-family on more than two acres, private septic): about 900 SF, 2 BR / 1.5 BA. All-in cost $300,000 to $375,000, or $335 to $415 per square foot. Projected rent $2,500 to $2,900. Move-in in 11 to 15 months. Feasibility rating 7 out of 10.
All-in cost includes design, survey, permits, and other soft costs along with construction.
Why the conversion came in lowest
The garage was already a sound two-story shell: poured foundation, framed walls, a second-floor system, and a weathertight exterior. It sat about 20 feet from the house, so the water, sewer, and electrical runs were short. Those are the parts of a new building that cost the most and show the least.
It still was not a light renovation. The budget carries a roof replacement with a shed dormer to make the second floor usable for two bedrooms, an insulated floor over the slab, new window and door openings, and a full set of plumbing, electrical, and heating systems. Even so, it priced roughly $80 to $85 per square foot below what the study estimated for a new detached unit of the same size.
Why the addition costs more per square foot than you might expect
An attached addition skips the long utility trench and the separate service. What it cannot skip is everything a dwelling needs: a full kitchen, one and a half baths, a stair, its own frost-depth foundation, and a one-hour fire-rated wall between the unit and the main house. Packing all of that into 800 square feet drives the per-foot number up, even though the total stays in the middle of the three.
The addition has advantages the numbers do not show. Finished living area attached to the house tends to appraise more consistently than a separate accessory building. With a framed connecting door, the same space can serve family, a tenant, or go back to being part of a larger single-family home without structural work. On a small lot, it also preserves most of the yard.
What pushed the detached build highest
Two things: septic and distance.
The property is on a private septic system. Under Title 5, the state septic code, a system is sized at 110 gallons per day per bedroom. Adding a two-bedroom ADU to a four-bedroom house will in most cases require capacity for six bedrooms, which means upgrading the existing system or installing a second one for the ADU. The study carries a $30,000 to $55,000 allowance for it. A passing Title 5 inspection from a past sale does not settle the question, because an inspection certifies the system's condition and says nothing about whether it can take more bedrooms.
Distance is the other cost. On a large lot the unit sits 150 to 250 feet from the house's utilities. Each additional 50 feet adds about $5,000 to $7,000 across the electric and water lines, so where the unit sits on the lot is a budget decision as much as a design one.
The Haverhill rules that shaped all three
By right, with a building permit. Haverhill amended its ADU ordinance in March 2025 to follow the state's Protected Use ADU framework (M.G.L. c. 40A §3 and 760 CMR 71.00). A Protected Use ADU is a single ADU on a lot in a district that allows single-family homes. It proceeds on a building permit, with no special permit or variance. The application requires a site plan stamped by a registered land surveyor.
The size cap depends on your basement. The state limit is the smaller of 900 square feet or half the main house's gross floor area, which is the floor area measured to the exterior walls. A basement counts toward that total only if its ceiling meets the building code's height for habitable space: 7 feet clear, or 6 feet 8 inches under beams and ducts. In all three studies, that one measurement decided whether the owner could build a full 900 square feet or would be held to something between roughly 710 and 800.
Shared utilities. The ordinance calls for a single utility service to the house and the ADU, with utilities included in the rent when a by-right ADU is rented. In practice that means the ADU is fed from the house's electrical panel, often with an upgrade to 200 amp service, and the rent is quoted with utilities in it. For a detached unit, whether the City will accept a separate meter is a question to settle with the Building Commissioner before design.
Long-term rental only. Leases run 12 months or longer. Weekly and daily rentals are not allowed.
Parking. One off-street space for the ADU, unless the property is within half a mile of transit.
Financing did not follow the price
The lowest-cost project had the hardest financing path. The MassHousing ADU Loan Program is built for owner-occupied single-family homes. Under the program guidelines, a detached unit qualifies for up to $250,000 combined and an attached unit for up to $150,000, with part of each at 0 percent deferred. A two-family does not qualify, so that owner is looking at renovation mortgages underwritten on the value after completion, such as FHA 203(k), Fannie Mae HomeStyle, or Freddie Mac CHOICERenovation. Program terms change, so confirm current limits and eligibility with a lender.
One more point from all three studies: none of these projects is expected to return its full cost in added resale value on day one. The case rests on the rent, or on the use a family gets from the space, and on owning a more flexible property over time.
What this means for your property
Measure your basement ceiling first. It takes a tape measure and five minutes, and it can change the size of the unit you are allowed to build.
If you have a sound garage or outbuilding, start there. An existing foundation and shell is the largest single cost advantage we see.
On septic, get a capacity determination before anything else. A septic engineer's answer sets a meaningful share of the budget and gates the permit.
Talk to a lender before you pay for design. Which programs you qualify for depends on the property type and your current mortgage, and that can decide which project makes sense.
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 the city first. Your MA ADU is a Feasibility Study Provider under the MHP ADU Incentive Program, which pays $500 toward each study. Request a study at yourmaadu.com.
Figures are preliminary planning ranges drawn from recent feasibility studies. They are not a quote, an appraisal, or legal or financial advice. Property details have been generalized to protect homeowner privacy.
