Wilmington allows one accessory dwelling unit as of right on any single-family lot, up to 1,250 square feet — more than the state requires, and with no half-the-house test. No special permit is needed. The constraint here is not zoning. It is septic.
Town Meeting adopted Article 26 unanimously on 3 May 2025, codified at Zoning Bylaw §4.2. It authorises one attached or detached ADU as an accessory use to a principal dwelling in a single-family residential district, as of right.
The cap sits at §4.2.3: gross floor area not to exceed 1,250 square feet. That is a flat figure. It does not shrink with the size of your house.
That matters, because the state rule is stricter: 900 square feet or half the principal dwelling’s gross floor area, whichever is smaller. On a 1,400 sq ft ranch the state floor is 700 sq ft, not 900. Wilmington sets it at 1,250 either way.
One caveat worth saying plainly: the band between 901 and 1,250 sq ft is Wilmington’s own grant, not a state entitlement. It is real and enforceable today. It is also the town’s to amend at a future Town Meeting, in a way the protected 900 is not.
Also unusual, and genuinely useful: the three residential districts — R-10, R-20 and R-60 — carry no lot coverage cap. On a detached unit that removes a constraint most surrounding towns impose.
§4.2.7 requires only a building permit and an occupancy permit from the Inspector of Buildings. No special permit. No site plan review either — in Wilmington that applies to business, industrial and multi-family only.
One trap: the Building Inspector’s forms page still lists a legacy “Accessory Apartment Application” on the old special-permit track. A protected ADU should not be routed onto it. Filing there concedes a discretionary approval the town cannot require.
Wilmington is mostly unsewered. Municipal sewer runs roughly 34 miles of main with 11 pump stations and just over 1,700 services, discharging to MWRA Deer Island — a minority of parcels in a town of about 23,000. Water is the opposite: municipal supply reaches roughly 99% of residents.
The Board of Health’s regulations go beyond Title 5, and one provision does more damage to ADU projects than anything in the zoning bylaw: a four-bedroom minimum design flow per unit. Counting an ADU as a second unit against that minimum can force a full septic upgrade even when the ADU has a single bedroom.
Other verified local requirements: a minimum leaching bed of 800 square feet; reduced beds (600 sq ft for three bedrooms, 400 for one or two) available only with a deed-recorded bedroom restriction filed at the Registry plus notice to the Board of Health before the occupancy permit issues; a five-foot setback to the property line from water lines; risers to grade.
The practical consequence: on a septic parcel, price a perc test and a Title 5 designer before anyone quotes a build number. The town’s own ADU FAQ warns that septic systems may require upgrading. That is understated.
Wilmington is the only one of the eleven towns we track with no Stretch Code adoption. Base code only.
No mandatory HERS rating. No EV-ready 50-amp circuit. No solar-ready roof zone. The identical unit built in Woburn or Wakefield carries all three. That is a real cost difference, and it is the kind that does not surface until someone prices the job.
Building, wiring, plumbing and gas each run $50 for the first $1,000 of value, then $10 per additional $1,000. Certificate of occupancy $50. Demolition $50. Re-inspection $25.
On septic, add the Health Department schedule: residential disposal system construction $250, upgrade or repair $100, soil testing with perc and deep-hole witnessing $200, septic re-inspection $25, Board of Health variance $25.
Filing is on paper. Cash or check only.
We would rather show you the gap than fill it with a guess. Confirm anything below with the town before you rely on it.
Whether the 2026 Annual Town Meeting amended §4.2.1’s dimensional language — it would change whether lot area and frontage minimums bear on a detached unit. The water and sewer connection fees for an added dwelling unit. Plan review turnaround and the stamped-plan threshold. The Attorney General approval date for Article 26.
The base-code status is also worth re-checking before you rely on it: a town can adopt the Stretch Code at any Town Meeting with a six-month lag, and the state table we read is dated 5 June 2025.
We build accessory dwelling units across the Merrimack Valley — supervised by Matthew Moisan under Massachusetts Construction Supervisor License CS-119855 — and we carry the permit set, the site plan review and the inspections. Tell us the address and we will tell you what the lot actually allows.