An ADU in Plymouth County, MA costs $180,000 to $350,000 for a detached 600 to 900 sq ft unit with full utilities — the most common project type across South Shore towns. Garage conversions and attached ADUs run 30% to 40% less. Nationally, ADU construction averages $180,000, with most projects falling between $40,000 and $360,000 depending on ADU type and finish level.[1]
US ProPaint & Renovation, a licensed Massachusetts general contractor (License 186517) based in Hingham and serving Scituate, Norwell, and Duxbury, has built additions and ADU-style living spaces across Plymouth County for more than 23 years. This guide reflects both local project experience and verified Massachusetts data — not national averages alone.
Nationally, the average ADU costs $180,000 to build, with $150 to $300 per sq ft as the typical range.[1] In Massachusetts, the Greater Boston area starts higher — Boston.gov cites costs beginning at roughly $275 per sq ft as a baseline for the metro.[4] For Plymouth County specifically, $180,000 to $350,000 covers most detached ground-up builds, while conversions of existing structures start lower.
Here is how ADU construction cost breaks down by type for Plymouth County projects:
ADU Type | Description | Estimated Cost (Plymouth County) |
Garage Conversion | Existing garage shell converted to livable space with plumbing, electrical, insulation, and finishes | $90,000 – $160,000 |
Attached ADU / Addition | New living space added to the primary home, sharing a wall and often utilities | $140,000 – $250,000 |
Detached ADU — Standard | Freestanding 600 to 800 sq ft cottage or backyard unit with full utilities | $180,000 – $280,000 |
Detached ADU — Custom | Larger or higher-finish 800 to 900 sq ft detached unit with site work and custom features | $280,000 – $350,000+ |
Attached ADUs converted from existing garages in Plymouth County run 30% to 40% less than ground-up detached construction — the savings come from using an existing foundation, roof, and shell rather than starting from bare ground.
Building an ADU in Plymouth County typically costs $180,000 to $350,000 for a detached 600 to 900 sq ft unit with full utilities. Plymouth County zoning recently expanded ADU allowances under the Massachusetts Affordable Homes Act, but Hingham, Scituate, and Norwell each have unique setback and parking requirements that affect both where you can build and what it costs.
A local worked example: A 750 sq ft detached ADU on a level Hingham lot — new foundation, stick-built framing, full kitchen and bath, independent electrical meter, and connection to municipal sewer — typically runs $240,000 to $290,000 all-in. That scope includes architectural plans, permits, site work, and a standard-finish interior. A similar project in Norwell with a septic upgrade would add $15,000 to $25,000 to that range.
Governor Healey signed the Affordable Homes Act into law on August 6, 2024. As of February 2, 2025, ADUs up to 900 sq ft are permitted by-right on single-family lots statewide — no special permit, variance, or neighbor approval required in most cases.[2] In the first six months after the law took effect, homeowners filed 844 ADU applications across 170 Massachusetts communities, with 550 approved.[2]
The by-right approval removes the biggest time risk from the ADU process in Plymouth County. Before the law, a project in Hingham or Norwell could face months of zoning board review. That barrier is now gone for projects within the 900 sq ft cap — though each town still enforces its own setback distances, lot coverage limits, and parking rules, which continue to shape where on a given lot an ADU can be sited.
Thinking about an ADU in Hingham, Scituate, or Norwell? Call US ProPaint & Renovation at (617) 922-6305 to schedule a free site consultation. The team evaluates your lot, reviews local setback and utility requirements, and delivers a written estimate before any commitments are made. Request your free ADU consultation online.
The total cost to build an ADU is the sum of five to seven distinct budget categories. Understanding each one prevents the sticker shock that hits when a project estimate arrives without context.
Here is what each category typically costs for a Plymouth County detached ADU:
MassHousing launched a statewide ADU Loan Program in March 2026, offering fixed-rate second mortgages of up to $250,000 for detached ADUs and up to $150,000 for attached ADUs to income-eligible Massachusetts homeowners.[3] US ProPaint & Renovation’s financing program offers additional options for homeowners who need to bridge the gap.
Building an ADU takes 6 to 12 months from initial design through move-in, depending on ADU type and site complexity.[1] The timeline breaks into two distinct phases — and most delays happen in the first one.
Phase 1 — Pre-Construction (Months 1 to 4) Design, engineering, and permit applications consume the first two to four months. Your contractor coordinates the site survey, architectural plans, utility layout, and building permit submission. In Plymouth County towns — where inspectional services departments vary in staffing and review speed — permit processing can run two to eight weeks after a complete application is submitted. Materials with long lead times (windows, doors, kitchen cabinets) are ordered during this phase so they arrive in time for installation.
Phase 2 — Construction (Months 4 to 10) Ground breaks after permits are approved. For a detached ADU, construction follows this sequence: site work and foundation, rough framing, roofing and exterior envelope, rough plumbing and electrical, inspections, insulation, drywall, finishes, and final inspections. A garage conversion moves faster because the foundation and shell already exist — active construction typically runs eight to twelve weeks for a conversion versus fourteen to twenty weeks for a ground-up detached ADU.
What disrupts your property:
What you need to do before construction begins:
Building an ADU costs $40,000 to $360,000 nationally, with an average around $180,000.[1] In Plymouth County, MA, a detached 600 to 900 sq ft ADU with full utilities typically runs $180,000 to $350,000. Garage conversions and attached ADUs cost 30% to 40% less than ground-up detached construction on the same lot.
For most Plymouth County homeowners, yes. A permitted ADU generates rental income, increases property value, and provides flexible housing for aging family members or adult children. Under Massachusetts’ Affordable Homes Act, ADUs up to 900 sq ft are now allowed by-right statewide — making the permitting process faster and more predictable than it was before February 2025.
Yes. A fully permitted ADU adds appraised value to your property and expands your buyer pool — because a legal ADU with rental income potential attracts investors and families seeking multigenerational living options. In Plymouth County, where housing inventory remains tight, an ADU also adds a competitive advantage at resale. Unpermitted ADUs add little or no appraised value and can complicate a sale.
The 50% rule is a common zoning standard that caps the size of an attached ADU at no more than 50% of the primary dwelling’s floor area. In Massachusetts, the Affordable Homes Act supersedes many local restrictions by allowing ADUs up to 900 sq ft by right on single-family lots. Individual Plymouth County towns may still set smaller size limits or apply the 50% cap for attached units, so confirming your town’s current ordinance before designing is a required first step.
Building an ADU takes 6 to 12 months from initial design through final inspection. Pre-construction — including design, engineering, and permitting — runs two to four months. Construction adds another three to six months depending on ADU type and site conditions. Garage conversions move faster; custom detached ADUs with new utility connections take longer.
ADU construction cost in Plymouth County runs $180,000 to $350,000 for a detached unit — higher than national averages because of Massachusetts labor rates, permit complexity, and the frequent need for septic upgrades on lots with private systems. The Massachusetts Affordable Homes Act has removed the biggest permit barrier by making ADUs up to 900 sq ft legal by-right statewide, and MassHousing’s new loan program offers income-eligible homeowners up to $250,000 in fixed-rate financing to close the gap. The homeowners who build successfully are the ones who lock down their lot’s setback compliance, utility plan, and material selections before breaking ground — not after.
US ProPaint & Renovation handles ADU projects across Hingham, Norwell, Scituate, and throughout Plymouth County. Call (617) 922-6305 or schedule a free site consultation online to get a written estimate and a realistic timeline for your property.
