A straight-answer guide from Southcoast Building LLC — Westport, MA.
Most of the accessory dwelling units we build aren’t investment properties. They’re for a parent. A homeowner calls because Mom shouldn’t be living alone two towns over, or because the in-law apartment is the difference between aging at home and aging in a facility. That’s the real reason ADUs have taken off across the South Coast — and it’s why the first question is almost always the same: what is this going to cost me?
Here’s the honest answer, without the runaround.
What an ADU costs to build around here
For a well-built accessory dwelling unit in Southeastern Massachusetts, plan on a range of roughly $200,000 to $375,000. On a per-square-foot basis, that lands most projects between $275 and $400 a foot, depending on size and finish level.
That’s a wide band, and it should be — anyone who quotes you a single tidy number before seeing your property is guessing. A compact 400–600 square foot unit built off an existing structure sits toward the bottom of that range. A full 800–1,000 square foot detached cottage with its own foundation, systems, and finishes sits toward the top. Where your project lands depends on a handful of real factors, which we’ll walk through — because understanding them is how you keep the number in your control.
A note on this number: it’s a planning range to help you budget, not a quote. Final pricing always comes after we walk your site.
Why the range is so wide
Four things move an ADU budget more than anything else:
Attached vs. detached. Adding a unit onto or above an existing structure — over a garage, off the back of the house — is generally less expensive than a standalone cottage, because you’re sharing a wall, a roof, or a foundation. A detached ADU is its own little building, with its own everything.
Site conditions — especially septic. This is the one that surprises people. If your property is on septic (as much of Westport, Dartmouth, Acushnet, and Freetown is), a second dwelling can trigger a septic upgrade or a whole new system, and that’s a significant line item before a single wall goes up. Ledge, grading, and how far utilities have to run all push the number too.
Foundation and utilities. A new detached unit needs a foundation poured and power, water, and often gas run to it. An attached or over-garage unit may piggyback on what’s already there.
Finish level. Standard, upgraded, or high-end kitchen and bath selections can swing the total more than any other interior choice.
What’s the most expensive part of building an ADU?
People assume it’s the kitchen or the finishes. It usually isn’t. The costliest surprises live below grade — the site work, the septic, and the foundation. Those are the parts you can’t see in the finished photos, which is exactly why they get underestimated. When we price an ADU, we start there, because getting the site and systems right is what protects the rest of your budget.
Are ADUs even allowed where I live?
Good news on this front. As of February 2025, Massachusetts law allows accessory dwelling units by right in single-family zoning districts statewide — meaning your town generally can’t ban a modest accessory unit outright. That’s a major change from just a couple of years ago, when it was a town-by-town patchwork.
The by-right protection covers units up to 900 square feet; a larger ADU can still require a special permit. And “by right” doesn’t mean “no rules” — setbacks, septic capacity, and the permitting process still vary by community, and your local building department has the final say. Part of our job is knowing how Westport, Dartmouth, Acushnet, Mattapoisett, Fairhaven, Freetown, Fall River, and New Bedford each handle it — so you’re not the one untangling zoning bylaws.
How long does it take to build an ADU?
For a typical project, plan on four to five months of construction once permits are in hand. Permitting and design add time on the front end, and site work (again, especially septic) can extend the schedule. We give you a real timeline before you commit — not an optimistic one.
Is it worth it?
For the families we build for, the math is rarely just about resale. It’s about keeping a parent close, safe, and independent — and doing it in a space that’s genuinely theirs, not a converted basement. An ADU also adds real, lasting value to your property and flexibility for whatever comes next: a returning adult child, a home office, eventually rental income.
Built right, it’s one of the most useful things you can add to a property. Built cheap, it’s a headache you’ll pay for twice.
What an accurate number looks like for your property
The ranges above will get you in the ballpark for budgeting. To get from a ballpark to a real number, we need to see your lot, your septic situation, and what you’re picturing.
The fastest way to start is our Cost Guide for a self-serve estimate, or Start Your Project to give us the details and get the conversation going. When you’re ready to talk it through, we’ll walk the site with you — no pressure, no punch-list surprises.
We’ve been building on the South Coast since 2020, and we run this like the business it is: a firm scope, a firm price, and steady communication with Mike and Roy from first walk to final walkthrough.
Southcoast Building LLC — new construction, ADUs, and additions across Southeastern Massachusetts. Built right. Run like a business.
Pricing figures are 2026 planning ranges for Southeastern Massachusetts and are not a quote; final pricing follows a site evaluation. ADU zoning is governed by your municipality and the local building inspector.