ADU Builder on Cape Cod
Mussato Building Group designs, permits, and builds accessory dwelling units (ADUs) across Cape Cod: detached guest cottages, attached in-law suites, and apartments carved out of a garage or another part of the house. We are a family-owned general contractor founded in 2004, with more than 100 projects completed and a 5.0 rating from 21 Google reviews.
Since February 2, 2025, Massachusetts law allows one ADU by right on most single-family lots, up to 900 square feet or half the size of the main house, whichever is smaller. That has made a backyard cottage or an in-law apartment a realistic project in every Cape town, and we handle the whole job, from the zoning and septic questions to the last piece of trim.
- Family-owned since 2004
- Licensed + insured in MA
- 100+ projects completed
- 5.0 stars from 21 Google reviews
- BBB accredited
Detached ADUs
A detached ADU is a small house of its own: a guest cottage, a studio, or a place for family, with its own kitchen, bathroom, and entrance. We build it from the foundation up, frame to finish, the same way we build our custom homes, and match the shingles, trim, and roofline to the main house.
Attached ADUs and conversions
An attached ADU shares a wall with the main house, as an addition or an in-law suite, or turns space you already have, like a garage or a lower level, into a separate apartment. A conversion is often the simplest path to an ADU, as long as the ceiling height, the egress, and the septic work out.
- Detached ADUs and guest cottages
- Attached ADUs and in-law suites
- Garage and lower-level conversions
- Kitchens and bathrooms sized for small spaces
- Foundations, framing, and exteriors matched to the main house
- Heat, hot water, and electrical for a second dwelling
- Zoning, septic, and permit coordination
- Site work and paths between the houses
ADU rules on Cape Cod
900 square feet by right
State law lets you build one ADU by right on most single-family lots, up to 900 square feet or half the size of the main house, whichever is smaller. Towns cannot require you to live on the property, and they can only ask for limited parking.
Septic comes first
Most Cape homes are on septic, and Title 5 sizes a septic system by bedroom count. An ADU with a bedroom adds to that count, so the Board of Health review is usually the first real hurdle, and sometimes a septic upgrade is part of the project.
Setbacks and site plan
Towns can still apply setbacks, height limits, and site plan review to an ADU, so the lot survey and the town bylaw decide where the cottage can go and how big it can be.
Historic and conservation review
In the Old King’s Highway district along Route 6A, a new ADU that can be seen from the street needs a historic certificate, and lots near wetlands, dunes, or the shore need Conservation Commission review.
Utilities and access
An ADU needs heat, hot water, and electrical capacity for a second dwelling on the lot. We plan the utility runs and the path from the driveway before we break ground.
“Mussato Building Group has been a loyal and trusted partner of ours for over ten years and one hundred projects. Their professionalism and quality of work are second to none. We look forward to working with them far into the future.”
Our Process
Site & Zoning Check
We look at the lot, the main house, the septic system, and the town bylaw to see where an ADU can go and how big it can be.
Design & Permits
A layout, an exterior that matches the house, and a written estimate. We file the building permit and coordinate the Board of Health, historic, or conservation reviews the lot needs.
Foundation & Framing
Foundation, framing, roof, windows, and the utility runs from the main house.
Finishes & Occupancy
Kitchen, bath, flooring, trim, and paint. After the final inspection the town issues the certificate of occupancy, and we walk the finished ADU with you.
ADU Builder by Town
- ADU Builder in Barnstable County, MAAll 15 towns of the county
- ADU Builder in Barnstable, MAHyannis, Barnstable Village, and West Barnstable
- ADU Builder in Bourne, MABuzzards Bay, Sagamore, and Sagamore Beach
- ADU Builder in Brewster, MAWest Brewster, Brewster Village, and East Brewster
- ADU Builder in Chatham, MAChatham Village, North Chatham, and South Chatham
- ADU Builder in Dennis, MADennis Village, East Dennis, and South Dennis
- ADU Builder in Eastham, MAEastham and North Eastham
- ADU Builder in Falmouth, MAFalmouth Village, Woods Hole, and North Falmouth
- ADU Builder in Harwich, MAHarwich Center, Harwich Port, and West Harwich
- ADU Builder in Mashpee, MAMashpee, New Seabury, and Popponesset
- ADU Builder in Orleans, MAOrleans Village, East Orleans, and South Orleans
- ADU Builder in Provincetown, MAthe West End, the East End, and the Commercial Street area
- ADU Builder in Sandwich, MASandwich Village, East Sandwich, and Forestdale
- ADU Builder in Truro, MATruro and North Truro
- ADU Builder in Wellfleet, MAWellfleet and South Wellfleet
- ADU Builder in Yarmouth, MAYarmouth Port, South Yarmouth, and West Yarmouth
Other work on Cape Cod
- Deck Builder on Cape CodNew decks, deck replacement, stairs, and railings
- Deck Replacement on Cape CodTear-out and rebuild of old, soft, or unsafe decks
- Basement Finishing on Cape CodMedia rooms, gyms, bars, guest suites, and egress windows
- Basement Remodeling on Cape CodDated or damp finished basements rebuilt from the foundation in
- Basement Egress Windows on Cape CodCode-size escape windows and window wells for basement bedrooms
- Bathroom Remodeling on Cape CodTile showers, tubs, vanities, and powder rooms
- Kitchen Remodeling on Cape CodLayouts, islands, custom cabinetry, and counters
ADU Builder FAQ
Can I build an ADU on Cape Cod?
In most cases, yes. Since February 2, 2025, Massachusetts allows one ADU by right on lots zoned for single-family homes, up to 900 square feet or half the size of the main house, whichever is smaller. Your town can still apply setbacks, Title 5 septic rules, and historic or conservation review, so we check those for your lot first.
Do I have to live on the property to have an ADU?
No. Under the 2024 state law, towns cannot require the owner to live in the main house or in the ADU for a by-right ADU.
Will my septic system handle an ADU?
It depends on the system’s design capacity and the bedrooms already on it. Title 5 sizes septic systems by bedroom count, so an ADU with a bedroom may need a septic upgrade. We start every ADU with that question.
How much does an ADU cost to build?
It depends on whether it is detached or attached, its size, the foundation, the kitchen and bathroom, and whether the septic system or utilities need upgrades. After a site visit we give you a free written estimate.
Which towns do you build ADUs in?
All 15 towns of Barnstable County: Barnstable, Bourne, Brewster, Chatham, Dennis, Eastham, Falmouth, Harwich, Mashpee, Orleans, Provincetown, Sandwich, Truro, Wellfleet, and Yarmouth.
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