Acushnet will offer you an accessory apartment. You don’t have to take it
Acushnet’s bylaw has no accessory dwelling unit provision — not one occurrence in the adopted text. What it has instead is a 1990s-style accessory apartment: special permit only, capped at 25% of your house’s habitable floor area, and no more than minimum exterior alterations. That is not a protected-use ADU, and it cannot be applied to one.
An accessory dwelling unit is what the state regulation calls it. An in-law apartment is what most people in Acushnet mean, and a backyard apartment is what a detached one looks like. Acushnet’s bylaw names none of the three — which is the whole problem below.
An accessory apartment by special permit, not exceeding 25% of the habitable floor area of the entire single-family dwelling, with not more than minimum exterior alterations.
On a 1,800-square-foot house that provision yields 450 square feet, by special permit, with the outside of the building essentially untouched. A protected-use ADU under 760 CMR 71.00 is as of right at up to 900 square feet, detached if you want, with no special permit at all. The two are different animals and the older one has no authority over the newer. Expect to have this conversation at the counter, because Sec. 3.4.B is what the bylaw puts in front of anyone who looks.
And the published text is out of date
This one is confirmed by date, not inferred. The Acushnet text we hold is stamped “updated as of 06-01-2025”. The Annual Town Meeting was held 16 June 2025 — fifteen days later — and the Attorney General approved three zoning articles from it, Warrant Articles 35, 36 and 37, plus a map amendment, in case 12015 decided 24 November 2025.
AG MLU case 12015, decided 2025-11-24; Acushnet held edition dated 2025-06-01So there are three approved zoning articles that cannot be in the published text, whatever they contain. The AG letter is a bare approval and does not describe them. Before anything is designed in Acushnet, those three articles have to be read at the Town Clerk’s office — they may or may not touch ADUs, and assuming either way would be guessing.
The setback trap: a gate at 120 square feet
Acushnet writes a ten-foot accessory setback and then gates it to buildings not exceeding 120 square feet — a shed, a pool. An ADU is nowhere near that, so it takes the principal setbacks instead:
| Yard | Accessory relief | What an ADU takes |
|---|---|---|
| Front | None — the gate reaches side and rear only | 25 ft |
| Side | 10 ft, but only up to 120 sq ft | 15 ft |
| Rear | 10 ft, but only up to 120 sq ft | 20 ft |
This is the Acushnet fact that decides where a detached unit can physically go. Twenty feet off the rear line and fifteen off the side is a materially different footprint from the ten-and-ten a quick read of the table suggests.
Altering an older house: the 25% of assessed value cap
If your house is nonconforming — and many in Acushnet are — an extension or alteration needs a Zoning Board special permit, and the Board may not grant it unless the cost does not exceed 25% of the assessed value of the nonconforming structure at the time of application.
Sec. 5.2 with Sec. 3.4.COne- and two-family houses are excepted from that machinery where the work will not increase the nonconforming nature of the structure. So an over-the-garage or attached unit that does not push further into a deficient yard generally avoids the special permit and the 25% cap entirely. Which side of that line your design falls on is a drawing decision, and it is worth making deliberately rather than discovering it at the counter.
Measuring the size cap
The state definition at 760 CMR 71.02 governs the size of a protected-use ADU, and a local definition cannot shrink a state entitlement. We compute the half-GFA figure on the state definition and carry the local number alongside it, because a local official may well reach for the local one.
760 CMR 71.02With no local ADU provision, the state figure governs unopposed: 900 square feet or half your house’s gross floor area, whichever is smaller. Work your own number on the ADU size calculator.
Septic and the well
Acushnet is largely on private septic. Title 5 tests design flow on the bedroom count of the whole property, so the question is whether your existing system carries the added bedroom. That answer — not the zoning — is usually what decides whether an Acushnet ADU is worth building.
310 CMR 15.000Planning ranges are in our cost guide.
We will tell you whether it works before you spend anything
Southcoast Building is an approved Feasibility Study Provider under the Massachusetts Housing Partnership ADU Incentive Program. A licensed builder walks your property, reads your parcel against the bylaw, checks the septic and the grading, and tells you whether an ADU works here and roughly what it would cost. Thirteen of our studies have been approved by MHP to date.
Request your free study$0 to you under the MHP ADU Incentive Program · One study per property · Michael Bicho · (508) 269-2131
Where these figures come from
- Town of Acushnet Zoning Bylaw, held edition stamped “updated as of 06-01-2025” —
Sec. 1.2(definitions),Sec. 3.3.1.B(1)andSec. 3.3.1.E(3)(RA district dimensional),Sec. 3.4.B(accessory apartment),Sec. 3.4.CandSec. 5.2(nonconforming). Searched in full: zero occurrences of “accessory dwelling” and zero of “ADU”. - Massachusetts Attorney General, Municipal Law Unit, case 12015, decided 24 November 2025 — approving Warrant Articles 35, 36 and 37 (zoning) from the Annual Town Meeting of 16 June 2025, plus a map amendment. None of these can be in the held edition.
- 760 CMR 71.00 — the operative rules for a protected-use ADU, which the Sec. 3.4.B accessory apartment provision does not govern.
- 310 CMR 15.000 (Title 5) for the design-flow and bedroom-count test.
Zoning changes, and a building inspector’s reading of it is the one that governs your permit. Nothing here is a determination about your property. It is what the adopted text and the Attorney General’s decisions say as of 8 September 2026.