ADU rules in Westport, and the words the Attorney General deleted
Westport allows one accessory dwelling unit by right — a backyard apartment, an in-law apartment, a garage or barn conversion. The bylaw as adopted said “single-family”, and the Attorney General struck that word out in October 2025. If you own a two-family or a multi-family here, the town’s ADU section now reaches you too.
People here call it different things. A backyard apartment for a grown child, an in-law apartment for a parent, a barn or garage conversion for either. In the bylaw they are one thing — an accessory dwelling unit — and Westport’s Sec. 9.5 treats them alike.
“single-family” — in Sec. 9.5.2(a) and Sec. 9.5.2(c)
The Attorney General disapproved and deleted the single-family references in both subsections, because a protected-use ADU is allowed on any lot holding a Principal Dwelling — including a two-family and a multi-family. Westport’s held edition is the 5 May 2026 Annual Town Meeting text, so the deletion is already incorporated. The practical point stands: owners of two- and three-family houses in Westport routinely assume the ADU section is not for them. It is.
What Westport does not ask of you
Two things Westport leaves out that neighbouring towns do not, and both are worth money.
- Owner occupancy
- Not required. Sec. 9.5 imposes no owner-occupancy condition at all.Sec. 9.5 — no provision present
- Special permit
- Not required. One protected-use ADU is as of right.Sec. 9.5.2
- Short-term rental
- Restricted. The bylaw does carry a rental condition, so plan on a long-term tenant.Sec. 9.5.2(g)
- Parking
- One space, and only where the whole lot sits outside a half-mile radius of a Transit Station. Westport has no such station, so plan on one.Sec. 8.3; 760 CMR 71.02
How big it can be
900 square feet, or half the gross floor area of your house, whichever is smaller — and Westport writes the measure by reference to the state definition rather than inventing its own.
Sec. 9.5.2 — gross floor area per 760 CMR 71.02That is unusually clean drafting and it removes the argument most towns leave open. Work your own number on the ADU size calculator.
Setbacks, and the coverage rule that catches people
Westport sets its yards building-by-building rather than by structure class, so the same numbers reach a detached ADU:
| Yard | Requirement |
|---|---|
| Front | 25 ft clear depth from the street line — but no more than the average of the buildings within 250 ft on both sides, which on a built-up road can be less |
| Side | 10 ft clear width; 25 ft from the street line on a corner lot |
| Rear | 6 ft, or 25 ft from a rear street line |
Sec. 7.4 caps what your buildings may cover — 50% of an ordinary lot and 65% of a corner lot, measured at ground level, and expressly counting the house together with every other building on it. On a small lot with a house, a garage and a shed already standing, the ADU has to fit inside what is left of that percentage. This is the Westport constraint that most often decides a detached unit.
If your lot is smaller than the zoning minimum
Westport carries two separate protections and they run on different dates. Section 7’s frontage and area requirements do not apply to a single-family lot that on 24 August 1982 was not held in common ownership with adjoining land and had at least 20,000 square feet and 100 feet of frontage, recorded by that date. Separately, a lot created by an 81P endorsement dividing land that had two or more substantial buildings standing on 27 October 1971 is treated as a lawful pre-existing nonconforming lot.
Sec. 7.6; Sec. 7.8Westport is old, and a great many parcels here qualify under one or the other. It is worth checking which before anyone tells you the lot is too small.
Septic is the number that moves
Most of Westport is on private well and septic. Title 5 tests design flow on the bedroom count of the whole property, not of the new unit alone, so the question is whether your existing system carries the added bedroom. Where it does, an ADU can often go in with no septic work at all. Where it does not, the system is the largest line in the budget.
310 CMR 15.000Knowing which of those two you are in is most of what a feasibility study is for. Planning 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 Westport Zoning By-Law —
Sec. 9.5(accessory dwelling units),Sec. 7.4(coverage),Sec. 7.6andSec. 7.8(pre-existing lots),Sec. 7.7(yards),Sec. 8.3(parking). Held edition is the text approved at the 5 May 2026 Annual Town Meeting. - Massachusetts Attorney General, Municipal Law Unit, case 11800, Art. 28, decided 17 October 2025 — single-family references in
Sec. 9.5.2(a)and(c)disapproved and deleted. - 760 CMR 71.00 — §71.02 definitions and gross floor area; §71.03 parking and dimensional relief.
- 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.