US ProPaint and Renovation handles accessory dwelling unit ADU Cohasset MA homeowners are requesting — detached backyard cottages, attached in-law suites, and garage conversions — from design and permitting through final inspection. Based in Hingham and working across Norfolk County, we deliver a code-compliant, move-in-ready unit backed by a 2-year workmanship warranty and over 23 years on the South Shore.
By JR Reis, Owner · Last updated 6th July 2026
Clients ask what is an accessory dwelling unit before they ask anything else, and the answer shapes every decision that follows. An ADU is a fully self-contained residential unit — with its own kitchen, bathroom, and separate entrance — located on the same lot as a primary single-family home. The three main forms are internal conversions (finished basement or attic), attached additions that share a wall with the main house, and fully detached structures like a backyard cottage or converted garage. Our ADU service page covers each type in detail, including when one form is a better fit than another given your lot.
In Cohasset, where many properties are large Colonial and Cape-style homes on generous lots, all three types are viable. The right choice depends on your existing utility access, setbacks, how much the primary residence can support structurally, and how much separation makes sense for the people who will live there.
Yes — and they are easier to build now than they have ever been. Under the Massachusetts Affordable Homes Act (Chapter 150 of the Acts of 2024), effective February 2, 2025, ADUs up to 900 square feet are allowed by right in single-family residential zoning districts statewide. Cohasset homeowners no longer need a special permit or variance for a standard ADU — the application is processed administratively through the Cohasset Building Department.
Cohasset has also adopted ADU-L (Large) provisions under its own local zoning amendments, allowing units up to 1,250 square feet for qualifying lots. That is a meaningful increase above the state’s 900 sq ft by-right baseline and opens more design options for homeowners who need more functional living space. A few local rules to know upfront: no ADU in Cohasset may be used as a short-term rental (Airbnb, VRBO, or similar platforms), and every unit requires a separate entrance that meets state building code egress standards.
On smaller in-town lots — particularly near the Beechwood neighborhood and along Jerusalem Road — your lot may still be undersized relative to setback or coverage limits, which can require a variance. We assess that at the initial site visit before any money changes hands. For projects that don’t meet the ADU definition but involve adding livable square footage, our home additions service follows a different permitting track. We handle permits ourselves and have worked through the Cohasset Building Department process on multiple South Shore projects.
Clients want to know how much does it cost to build an ADU before they commit to a site visit, and a realistic answer requires knowing the type. Based on current South Shore market pricing:
A detached ADU (400–900 sq ft) typically runs $225,000–$375,000 on the South Shore, covering foundation, framing, mechanical systems, and interior finishes. An attached ADU or converted garage generally costs $150,000–$250,000 because it shares structural elements and some utilities with the primary home. An internal conversion — basement or finished attic — is usually the most cost-effective starting point at $100,000–$200,000, depending on the condition of the existing space.
Several South Shore-specific factors affect cost beyond those national averages. Cohasset properties outside the municipal sewer zone require Title V septic system evaluation; if the existing system cannot support an additional bedroom, upgrades typically add $15,000–$35,000. Rocky ledge is common throughout the coastal South Shore, and if excavation hits granite during site prep, that adds cost and schedule time. The Massachusetts Stretch Energy Code — adopted broadly across Norfolk County — adds roughly 5–8% to insulation and HVAC costs but lowers long-term utility bills in ways that matter for a rental unit generating income. We include all of these variables in our written estimates. Contact us for a free site evaluation and itemized cost breakdown with no obligation.
How long does it take to build an ADU in Cohasset depends primarily on which phase takes the longest — and in Massachusetts, that is almost always permitting. A straightforward detached ADU typically runs 6–10 months from first site visit to certificate of occupancy. Here is how our process breaks down:
Design and permitting (8–14 weeks). We produce architectural drawings and engineering, then submit to the Cohasset Building Department. By-right ADUs receive administrative review, which is faster than the special permit hearings that were previously required, but permit timelines still vary by season and workload.
Foundation and framing (4–8 weeks). Excavation, concrete, and structural framing. Spring and fall are the most productive windows on the South Shore — late-season freeze and summer heat both affect concrete cure times and crew productivity.
Mechanical systems and interior build-out (6–10 weeks). Electrical, plumbing, and HVAC rough-ins run concurrently with insulation, drywall, kitchen, bathroom, and finish work, staged around inspections.
Final inspection and turnover (1–2 weeks). We coordinate the building department final, confirm the certificate of occupancy is issued, and walk through every system with you before we consider the project closed.
Attached and internal ADUs typically run 4–7 months because site work is substantially reduced. We provide a written project schedule at contract signing so there are no surprises about where time goes.
US ProPaint and Renovation is an ADU builder South Shore MA contractors know because JR Reis has been running projects on this coast for over 23 years. When clients search for the best ADU builder on the South Shore MA, our record is what they find: over 2,500 completed projects, a 4.8-star Google rating across 157 reviews, and a specific track record building living spaces — not just structures — for real families. As Kevin Caravella, a South Shore homeowner, put it: “Much more than a painting contractor.”
As an in-law suite contractor Norfolk County families rely on for multi-generational projects, we understand what those builds actually require. A basement ADU for an aging parent needs more than code compliance — it needs proper ceiling height, step-free entry where the lot allows, and noise separation from the main living space. We build to those standards without needing to be reminded. Every ADU includes a 2-year workmanship warranty, and we offer a financing program for qualified homeowners so the investment does not have to be all cash.
We hold Massachusetts Contractor License 186517 and comply with the Stretch Energy Code and all applicable state and local requirements. Foundation and framing are handled by our own crews — we do not subcontract structural work.
When you search for ADU construction near me in or around Cohasset, you are already in our primary service area. When you need a contractor for detached or attached ADU Norfolk County homeowners can reach on the same day they call, that is what our Hingham base makes possible.
We serve Cohasset — including North Cohasset, Beechwood, and the Sandy Beach area — as well as Hingham (our home base), Scituate, Norwell, Marshfield, Duxbury, Hull, Kingston, and Plymouth. We are also a familiar ADU construction company near Cohasset’s neighboring communities in Plymouth County. For homeowners in Cohasset who have worked with us on deck construction or deck repair projects, ADU construction follows the same approach: upfront scope, written schedule, and no surprises at the invoice. Read our home addition guide to understand how ADU projects compare to a standard home addition before your consultation.
 An ADU is a self-contained residential unit with its own kitchen, bathroom, and separate entrance, located on the same property as a primary single-family home. ADUs can be detached (a backyard cottage or converted garage), attached (an addition sharing a wall with the main home), or internal (a converted basement or in-law suite within the existing footprint).
Yes. As of February 2, 2025, ADUs up to 900 square feet are allowed by right in single-family zones under the Massachusetts Affordable Homes Act. Cohasset also permits ADU-L units up to 1,250 square feet under local zoning amendments. All units require a separate entrance and cannot be used as short-term rentals. Setbacks, lot coverage limits, and septic capacity may affect feasibility on specific properties.
Costs range from roughly $100,000–$200,000 for internal conversions, $150,000–$250,000 for attached ADUs, and $225,000–$375,000 for detached builds. South Shore-specific factors — Title V septic, ledge removal, and Stretch Energy Code compliance — affect the final number. US ProPaint offers free estimates with a full written breakdown.
Most ADU projects in Cohasset take 6–10 months from site evaluation to certificate of occupancy. The three main phases are design and permitting (8–14 weeks), foundation and framing (4–8 weeks), and interior build-out (6–10 weeks). Internal conversions typically run 4–7 months because site prep is minimal.
Yes — as a long-term rental under a standard lease of 30 or more days. Cohasset’s zoning bylaws prohibit ADU use as a short-term rental such as Airbnb or VRBO. Under the current state ADU law, there is no owner-occupancy requirement, meaning you do not need to live on the property to rent the ADU to a tenant.
US ProPaint and Renovation 175 Derby St Suite 4 Hingham, MA 02043 Phone: (617) 922-6305 Toll Free: (800) 964-0717 Hours: Mon–Fri 8:00 AM–5:00 PM | Sat 8:00 AM–3:00 PM Serving Cohasset, Hingham, Scituate, Norwell, Marshfield, Duxbury, Hull, Kingston, Plymouth, and surrounding Norfolk County and Plymouth County communities.
