Dartmouth’s ADU bylaw asks who is allowed to live there
Dartmouth adopted its ADU article in June 2024 — eight months before the state ADU regulations took effect. Five of its conditions were flagged by the Attorney General as likely needing amendment, including a requirement that the people in your house and the people in the ADU be related to each other, and a cap of three occupants.
The bylaw calls it an accessory dwelling unit. Around South Dartmouth and Padanaram people say in-law apartment, or a backyard apartment if it is detached. Article 50 reaches all of them, and so do the five flagged conditions below.
Occupancy of the ADU is capped at three persons, and the occupants of each unit must be related to each other.
Dartmouth’s Article 50 was adopted 4 June 2024 and took effect 23 December 2024. 760 CMR 71.00 did not take effect until 2 February 2025. The Attorney General approved the article with the express qualifier that it did not currently conflict with state law, and warned that five provisions may need amendment: the owner-occupancy requirement, the two-space parking requirement, the two-bedroom maximum, the three-person occupancy limit, and the related-occupants rule. None of those five has been tested since the state regulations landed.
The five flagged conditions, as written
These are on the books today. Whether each survives contact with 760 CMR 71.03 is a live question, and it is the reason a Dartmouth ADU is worth a conversation with the building department before a design is drawn.
- Occupants
- Capped at three persons in the ADU, and the occupants of each unit must be related to each other. A caregiver exception sits at H(7).Sec. 375-50.2.H(5); H(7)
- Owner occupancy
- Required in one of the two units, with short-term rental prohibited.Sec. 375-50.2.H
- Parking
- Two off-street spaces for the ADU, in addition to those required for the house. No on-street parking, one driveway, one parking area — though a detached ADU may have its own.Sec. 375-50.2.G
- Bedrooms
- Two maximum.Sec. 375-50.2.C(2)
- Size
- 1,000 sq ft of habitable gross floor area — larger than the 900 sq ft state protected-use threshold.Sec. 375-50.2.C
760 CMR 71.03 caps what a town may require for a protected-use ADU at one additional space, and bars any additional space where the lot sits inside a half-mile radius of a Transit Station. Dartmouth asks for two. That is the flagged provision most likely to come up on a real application, and it is not a number to concede without asking.
The contradiction inside the bylaw
Dartmouth’s own text disagrees with itself about the General Residence district. The Article 5 use table shows an accessory dwelling unit as not permitted in GR. Sec. 375-50.2.A allows GR. And Sec. 375-49.1, a greater-restrictions clause, cuts the other way again.
Art. 5 use table vs Sec. 375-50.2.A vs Sec. 375-49.1If your parcel is in GR, this is not an academic point — it is the first thing to settle, in writing, with the Zoning Enforcement Officer. Nothing else about the project is worth pricing until it is.
Setbacks by district, and the gate that closes on you
Dartmouth writes a generous accessory setback and then gates it by size and height, so it does not reach an ADU. In SRA the five-foot perimeter relief applies only to a structure eight feet or under in height with an average roof height of twelve feet or under, and not exceeding 200 square feet. An ADU is neither. So the ADU takes the full setback:
| District | Front | Side | Rear |
|---|---|---|---|
| SRA | 50 ft from the right-of-way | 20 ft | 20 ft |
| SRB | 60 ft | 20 ft | 20 ft |
| GR | 20 ft | 20 ft | 20 ft |
The 20-foot figure that appears in the SRA front-yard row is likewise gated to an eight-foot, 200-square-foot structure and cannot be read onto an ADU. On an SRA lot the front setback is fifty feet.
Working on an older house
For a structure that legally existed before 26 October 1993, Dartmouth allows the alteration by right — not by special permit — provided the work does not push into a sight triangle, does not come within five feet of any perimeter line, and does not exceed the height of the part being expanded. That is the route for an over-the-garage or attached unit on a house that already sits closer to a line than today’s table would allow.
Sec. 375-7.4D(4)(a)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 71.02; Dartmouth’s own measure at Sec. 375-50.2.C reads ‘habitable gross floor area’Dartmouth’s 1,000 sq ft is above the 900 sq ft protected-use threshold, so for a unit of 900 square feet or less the local cap never binds. Between 900 and 1,000 you are outside protected-use status and every one of the flagged conditions applies with full force. Work your number on the ADU size calculator.
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.
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Where these figures come from
- Town of Dartmouth Zoning Bylaw, Chapter 375 —
Art. 50(Sec. 375-50.1to375-50.3), added by 2024-06-04 ATM Art. 23, effective 2024-12-23;Sec. 375-7.4D,375-8.4D,375-10.4D(dimensional);Art. 5use table;Sec. 375-49.1. - Massachusetts Attorney General, Municipal Law Unit, case 11522, Art. 23 — approved with the express qualifier that it does not currently conflict with state law, and warning that five provisions may need amendment.
- 760 CMR 71.00, effective 2 February 2025 — §71.02 definitions, §71.03(2)(b) parking and §71.03(3)(b)(2) 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.