In Weymouth, your lot size decides whether you get 900 square feet or 750
Weymouth is the only town on our list that tiers the ADU size cap by lot area. At 25,000 square feet or more you get the full 900. Below that you get 750. And the cap is measured against the whole building your ADU sits in, not the finished unit — which on a two-storey barn or garage cuts the footprint roughly in half.
Chapter 120 says accessory dwelling unit. You may be thinking of it as an in-law apartment for a parent or a backyard apartment in the barn. The lot-size tier below applies to all of them, and it is the number that decides the project.
900 sq ft only where the lot meets or exceeds 25,000 sq ft. On any lot under 25,000 sq ft the ADU is capped at 750 sq ft.
Either figure is then further capped at half the gross floor area of the primary structure, whichever is less. And the definition at Sec. 120-6 counts the entire building containing a detached ADU — both storeys, plus any usable below-grade space — against that cap, not the habitable unit alone. Convert a two-full-storey barn on a 9,000-square-foot lot and the maths runs: 750 sq ft cap, whole building counted, so a footprint of roughly 375 square feet. That is the single most consequential fact about building an ADU in Weymouth.
“As of right” here still means a public hearing
Weymouth permits an ADU as of right in any district that allows single-family housing either as of right or by special permit, and the Inspector of Buildings is the permit-granting authority. But as-of-right in Weymouth still carries site plan review and design review with a public hearing. That is a real calendar and a real cost, and it surprises people who read “as of right” and expect a counter transaction.
Sec. 120-135A and Sec. 120-135B; Sec. 120-137BConversions inside the existing house are capped tighter still
An ADU proposed within an existing dwelling is limited to one third of that dwelling’s gross floor area — and is still subject to the 900 or 750 ceiling and the half-of-primary test, whichever is less. No more than one ADU may be created within an existing single-family home.
Sec. 120-136DThree separate caps apply at once and the smallest wins. Work your own number on the ADU size calculator and we will check it against the lot area.
Parking, and the one place Weymouth is generous
One off-street space is required where the ADU is half a mile or more from a commuter rail station. Inside that half-mile radius, no ADU space is required at all. Weymouth has three commuter rail stations — Weymouth Landing, East Weymouth and South Weymouth — so a great many parcels in this town fall inside a radius and owe nothing. Each space, where required, must measure at least nine feet by eighteen.
Sec. 120-136E; Sec. 120-71AThe radius is measured to the parcel, and being just inside or just outside changes the site plan. It is a straightforward thing to establish and it is worth establishing before a driveway is drawn.
Short-term rental is barred on both units
Weymouth’s restriction is unusually broad: neither the ADU nor the principal single-family dwelling may be used as a short-term rental. Most towns restrict the ADU alone. Here, adding an ADU changes what you may do with your own house.
Sec. 120-135DIf any part of your plan involves short-term letting either unit, this provision is the one to read before anything else.
Utilities and the codes that are not preempted
Weymouth states plainly what the state ADU rules do not displace: G.L. c.40A §3 does not supersede building codes, health codes, conservation regulations or Department of Public Works regulations. All ADU proposals must comply with all applicable state and town codes. A detached ADU requires permanent utility connections compliant with the building code and DPW regulations.
Sec. 120-135C; Sec. 120-136OMost of Weymouth is on town water and town sewer, which removes the Title 5 question entirely and replaces it with a connection cost. That is usually the cheaper of the two problems.
Setbacks
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. There is a second reason to be careful here: an older Weymouth house on a corner lot can be nonconforming under Sec. 120-136A(1) in a way the assessor’s record does not show, and conformity is the live question rather than the setback figure itself.
Sec. 120-136A(1)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 Weymouth Code, Chapter 120, Article XXIX —
Sec. 120-134throughSec. 120-138, added 3 February 2024 by Order No. 24-119. AlsoSec. 120-6(ADU definition and the whole-building measure),Sec. 120-71A(parking dimensions) andSec. 120-136A(1)(conformity). - 760 CMR 71.00 — §71.02 definitions, §71.03 parking and dimensional relief.
- G.L. c.40A §3, as amended by the Affordable Homes Act.
- Weymouth is served by town water and town sewer across most of its area; where it is not, 310 CMR 15.000 (Title 5) applies.
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.