Somerset has no ADU bylaw, and that works in your favour
Search the Somerset zoning by-law for “accessory dwelling unit” and you get nothing. Not a restriction, not a special permit, not a size cap — no local ADU provision exists at all. Which means the state regulations apply exactly as written, with no local layer on top.
Whether you are planning an in-law apartment over the garage, a backyard apartment behind the house, or a full accessory dwelling unit in a converted barn, the same state rules apply here — because Somerset has written none of its own.
Somerset’s zoning by-law contains no accessory dwelling unit provision and no accessory apartment provision.
In most towns the work is figuring out which local conditions survive contact with state law. In Somerset there are none to argue about. 760 CMR 71.00 governs on its own terms: one protected-use ADU as of right on a single-family lot, capped at 900 square feet or half your house’s gross floor area, whichever is smaller, no owner-occupancy requirement, and no more than one additional parking space.
Full-text search of the held 15 April 2023 by-law, checked 2 September 2026What the state rules give you here
With no local provision to navigate, these are your terms:
- Permission
- As of right on a lot in a single-family residential zoning district. No special permit, no zoning board.760 CMR 71.03
- Size
- 900 sq ft or half the gross floor area of your house, whichever is smaller. Basement area at compliant ceiling height counts.760 CMR 71.02
- Parking
- One additional space maximum, and none at all if any part of the lot sits inside a half-mile radius of a Transit Station.760 CMR 71.03(2)(b)
- Owner occupancy
- Not required. No local provision imposes it, and the state rules do not.760 CMR 71.00
- Short-term rental
- No local ADU restriction exists. Check the general by-laws separately.
One honest caveat about currency
The Somerset text we work from is the 15 April 2023 by-law. Three Attorney General decisions postdate it, and we opened all three: every one is a general by-law, not zoning. Case 11466 (August 2024) and case 11869 (December 2025) both concern general by-laws, as does case 12183. So no zoning amendment has slipped past the edition we hold, and the no-ADU-provision finding is confirmed at document level rather than assumed.
AG MLU cases 11466, 11869 and 12183, each opened and read 2 September 2026We still confirm currency with the Town Clerk before a permit application. That is cheap, and an out-of-date reading is not.
Setbacks and the dimensional table
The district setbacks that apply to your parcel are read off the adopted dimensional table for your specific district, and we do that on your address rather than print a number here that may not be yours. Somerset’s Section 5 dimensional requirements are what a structural alteration or enlargement has to comply with, and Sec. 6.1 is explicit that a building which is not a nonconforming use may be altered or enlarged provided the work meets them.
Sec. 6.1; Sec. 5 dimensional requirementsIf your lot is undersized
Somerset carries the standard grandfathering at Sec. 6.2: increases in area, frontage, width, yard or depth do not apply to a lot for single- or two-family use that was separately recorded at adoption, was not held in common ownership with adjoining land, conformed to the requirements then in force, and has at least 5,000 square feet and 50 feet of frontage.
Sec. 6.2There is also a plan-freeze at Sec. 6.2.1 — the by-law in effect at first submission governs, and for eight years from endorsement. Worth knowing if you have an endorsed plan sitting in a drawer.
Sec. 6.2.1Septic is where the money is
With no local ADU conditions to work around, the septic question is the whole feasibility question in Somerset. Title 5 tests design flow on the bedroom count of the entire property. Where your existing system carries the added bedroom, an ADU can go in with no septic work. Where it does not, replacement is the largest line in the budget and it changes the answer on whether to build at all.
310 CMR 15.000That is the single thing we go and check. 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 Somerset Zoning By-Law, held edition dated 15 April 2023 —
Sec. 5(dimensional),Sec. 6.1(nonconforming),Sec. 6.2andSec. 6.2.1(pre-existing lots). Searched in full for “accessory dwelling” and “accessory apartment”: zero occurrences. - Massachusetts Attorney General, Municipal Law Unit, cases 11466, 11869 and 12183 — all three opened 2 September 2026 and all three general by-laws, not zoning.
- 760 CMR 71.00 — the operative rules in Somerset, absent a local provision.
- 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.