Updated October 2026
If you're thinking about an in-law apartment or ADU (accessory dwelling unit) for a parent, an adult child or a tenant, the first question is usually the same: what does it cost?
National averages don't help much in Greater Boston. Here is what five of our own in-law and ADU projects cost, how long they took, what each one included, and where the money went.
Short answer: On five in-law and ADU projects we built from 2019 to 2026, the in-law or ADU part cost about $240,000 to $415,000. A full apartment with its own kitchen ran about $310,000 to $415,000. An in-law suite that stays part of the main house ran about $240,000 to $380,000.
| Project | In-law / ADU part | Construction |
|---|---|---|
| Full apartment with its own kitchen | ||
| Carriage house conversion, about 495 sq ft (2026) | about $309k | about 14 weeks |
| New garage with an apartment above, about 490 sq ft (2023) | about $390k* | about 19 weeks |
| Two-story addition, apartment on the first floor, about 910 sq ft (2019) | about $415k | about 6 months, whole project |
| In-law suite, part of the main house | ||
| Lower level plus the lower floor of a new addition, about 1,170 sq ft (2021) | about $240k | about 7 months, whole house |
| Lower-level suite with a kitchenette and a small addition, about 1,215 sq ft (2026) | about $380k | about 4 months |
From what homeowners paid on PEGASUS projects, in the dollars of the year each was built. *Without the deck. See every range and get one for your house →
- In-Law Suite or ADU: The Zoning Is Different
- Five PEGASUS In-Law and ADU Projects
- Where the Money Goes
- Why Smaller Units Cost More per Square Foot
- What Drives the Cost
- Is a New Detached ADU Cheaper?
- Frequently Asked Questions
People use the two terms interchangeably, but zoning doesn't. An ADU is a second, separate home on your lot. An in-law suite is extra living space that stays part of your main house. Which one you build decides the size limit, the kitchen you can have, and the permit you need.
| ADU | In-law suite | |
|---|---|---|
| What it is | A separate home with its own kitchen, bathroom and sleeping area | Living space that stays part of the main house |
| Size | Up to 900 sq ft or half the main house, whichever is smaller. | Not held to the ADU limit; set by your town's zoning |
| Kitchen | A full kitchen is part of the definition | Some towns allow only a kitchenette |
| Where | Inside the house, in an addition, or in a separate building (Melrose allows all three) | Inside or attached to the main house |
| Permit | By right on single-family lots since Feb. 2, 2025; a building permit is still required | Depends on your town's zoning |
| Who can live there | Anyone. No owner-occupancy rule; long-term rentals are fine. Melrose bars rentals of 31 days or less. | Check your town |
This shows up in our own projects. Our largest in-law suite is about 1,215 sq ft, more than its town allows for an ADU, so it was built as an in-law suite. That meant a kitchenette instead of a full kitchen. The inspector also wanted to be sure it wasn't a separate living space, so we removed the door between the suite and the rest of the house. If you want a full kitchen, a door that closes and the freedom to rent, design to the ADU rules from the start.
Sources: mass.gov ADU rules, Melrose ADU guide, state ADU regulations (760 CMR 71.00). Rules change and every town is different, so check with your building department before you design.
Each project shows what was built, what the in-law or ADU part cost, how long construction took, and the biggest pieces of the price. Where a project also included work on the main house, we separated that out using our estimate and the plans.
| Size 495 sq ft | ADU cost about $309k | Per sq ft about $625 | Construction 14 weeks |
Proposed plan. The apartment is about 495 sq ft; the patio and trash shed sit outside it.
We turned an old detached carriage house into a full apartment with a bedroom, a kitchen, a bathroom with a tiled shower, and a living area. The price covered gutting and rebuilding the interior, new windows and doors, a two-head split heating and cooling system, its own water heater, a new electrical subpanel, and trenching to run water, sewer and power from the main house. The plumbing fixtures were bought through us and are in that price; the homeowners bought their own appliances.
Biggest pieces of the price
| Electrical | $34,000 | |
| Cabinets and trim | $33,000 | |
| Plumbing | $28,000 | |
| Exterior doors | $27,000 | |
| Framing | $25,000 |
| Size 1,215 sq ft | In-law cost about $380k of about $500k total | Per sq ft about $315 | Construction about 4 months |
Proposed lower level. Bed 1 is the new addition on a slab foundation.
This family needed a home for an aging parent and a family member who uses a wheelchair. We renovated the lower level into a suite with two bedrooms, a living area, a bathroom with a custom shower, a laundry area and a small kitchenette, and added a bedroom on a new slab foundation. Because it was built as an in-law suite, not an ADU, the town allowed a kitchenette but not a full kitchen, and no door closing it off from the main house.
A family room for the main house sits above the new bedroom. The whole project is on track to finish at about $500,000; the family room and a new deck are about $120,000 of that.
Biggest pieces of the in-law part
| Foundation and excavation | $56,000 | |
| Electrical | $43,000 | |
| Framing | $28,000 | |
| Plumbing | $28,000 | |
| Plaster walls | $27,000 |
| Size 1,170 sq ft | In-law cost about $240k of about $858k total | Per sq ft about $205 | Construction 7 months whole house |
Lower level, about 1,170 sq ft of living area. The two bedrooms are in the new addition, on a slab; the garage is not part of the suite.
As part of a whole-house renovation, we built a two-level addition of about 21 by 29 feet on a concrete slab. The in-law suite takes in the addition's lower level (two bedrooms and a new bathroom with a tiled shower) plus the existing lower level of the house: a family room, a kitchenette and the existing bath. The addition's upper level belongs to the main house. The whole project was about $858,000.
The in-law part cost less per square foot than the others because much of the space already existed.
Biggest pieces of the in-law part
| Framing | $48,000 | |
| Foundation and slab | $30,000 | |
| Plumbing | $26,000 | |
| Electrical | $21,000 | |
| Plaster walls | $13,000 |
| Size 490 sq ft | Project cost about $427k about $390k without the deck | Per sq ft about $790 without the deck | Construction 19 weeks |
As-built plan. The new deck connects the apartment to the main house.
We built a new garage on a new foundation with a one-bedroom apartment above it: kitchen, tiled bath, bedroom and living space, with its own heating and cooling. A new deck connects the apartment to the main house. The deck was about $39,000 of the price. The garage below is in the price too, which is why this one runs high per square foot.
Biggest pieces of the price
| Framing | $77,000 | |
| Electrical | $35,000 | |
| Foundation and slab | $27,000 | |
| Siding | $24,000 | |
| Plumbing | $23,000 |
| Size 910 sq ft | In-law cost about $415k of about $578k total | Per sq ft about $455 | Construction 6 months whole project |
Proposed first floor. The in-law apartment fills the addition's first floor.
We replaced an old garage with a two-story addition on a full basement. The first floor is an in-law apartment with a kitchen, a bedroom, a living area, a mudroom, a powder room and a full bath with a steam shower, entered through a new covered porch. Upstairs, a new primary suite of about 430 sq ft serves the main house. The homeowners paid about $578,000 in all; the primary suite, a sun room and a few smaller main-house changes were the rest.
Biggest pieces of the in-law part
| Foundation and excavation | $50,000 | |
| Siding and exterior trim | $46,000 | |
| Framing | $37,000 | |
| Electrical | $36,000 | |
| Plumbing | $34,000 |
Plumbing, heating and electrical took 18% to 30% of every job. Here is each project's full quote split by trade:
From PEGASUS signed quotes, by trade, including overhead and profit.
Whatever the size, the systems and the finished interior (plumbing, heating, electrical, insulation, plaster, cabinets, tile and trim) took 41% to 66% of the price. Foundation and framing only become a big share when you build new space, like a garage with an apartment above it.
Every in-law apartment needs a bathroom, heat, power and water, and usually a kitchen, however small it is. In a small unit those costs are spread over fewer square feet. That's why our carriage house conversion runs about $625 per square foot and our lower-level suite about $315. It's also why we don't price in-law apartments by the square foot.
Usually not. A new detached ADU needs everything a home addition needs: a foundation, framing, a roof and all new systems. On our projects, an addition with a kitchen and bath runs $355,000 to $545,000 (see what a home addition costs). Plan on at least that much for a new detached ADU. We price it once the design is drawn.
On five of our projects built 2019–2026, the in-law or ADU part cost about $240,000 to $415,000. A full apartment with its own kitchen ran about $310,000 to $415,000; an in-law suite that stays part of the main house ran about $240,000 to $380,000.
Not under zoning. An ADU is a separate home with its own kitchen, limited in Massachusetts to 900 sq ft or half your main house, and allowed by right on single-family lots. An in-law suite stays part of the main house and follows your town's own rules. Those can limit the kitchen to a kitchenette, and an inspector may require that the suite stay open to the rest of the house, with no door closing it off.
In most cases, yes. Since February 2, 2025, Massachusetts allows one "protected use" ADU by right on lots in zoning districts that allow single-family homes. You still need a building permit, and your town can apply some reasonable rules. (mass.gov)
A protected use ADU can be up to 900 sq ft or half the size of your main house, whichever is smaller. It also needs its own entrance. A larger in-law suite may need extra approval from your town.
Design usually takes 4–6 weeks. On our projects, construction took about 14 weeks for a carriage house conversion, about 4 to 5 months for a lower-level suite or a new garage apartment, and 6 to 7 months when the in-law was part of a larger project. See how long an addition takes for more detail.
Our construction prices include permits, inspections and project management. Design is its own agreement: 1.5% of the construction estimate, with a $1,000 retainer to start. Design usually takes 4–6 weeks. That way you know the plan and the budget before you commit to building.
If you're thinking about an in-law apartment or an ADU, the next step is to get a range for your house or talk through your property. See how we approach ADUs and in-law suites.
Get a cost range for your house, or Book a 15-minute call to talk about what your in-law apartment could look like and what it might realistically cost.
Geoff Shafer has run PEGASUS Design-To-Build in Melrose for 25 years. He is a Massachusetts licensed Construction Supervisor and registered Home Improvement Contractor (HIC 137663).