Buyer and Seller Checklist · New England ADU Ecosystem
Since February 2, 2025, an accessory dwelling unit is allowed by right in Massachusetts single-family zoning districts. Whether a specific property can actually take one is a question of paperwork, not opinion. These are the five papers that answer it.
Gather these before you list. They turn "ADU potential" from an adjective into a documented advantage, and they keep you from making a claim you cannot support.
Get all five inside your contingency period. Verifying before you pay is a price negotiation. Discovering after you close is an expense.
Where: your town's Board of Health, by address. Usually free to view.
Look for: the approved bedroom count and design flow in gallons per day. This is the single most expensive variable in an ADU project. Sewer service removes it entirely and is worth saying loudly.
Where: your closing file, the Registry of Deeds, or a licensed surveyor.
Look for: setbacks, lot coverage, easements, and whether equipment can physically reach the rear of the lot. A unit that cannot be built where the plan allows is not potential.
Where: the town clerk, the building department, or the municipal website.
Look for: dimensional limits and any short-term rental restriction. The state removed nearly all municipal authority over ADUs and expressly kept this one. Also ask whether the department has processed ADU permits yet.
Where: the Registry of Deeds. Recorded documents, not the association welcome packet.
Look for: accessory structures, second dwelling units, second kitchens, occupancy and leasing limits, and the recording date. State law restrains municipalities. It does not touch private covenants.
Where: the building department, by address.
Look for: whether an existing in-law suite, basement apartment, or above-garage unit was ever permitted. A septic inspection can put an unpermitted unit in front of the Board of Health in writing, mid-transaction.
| Existing house, 3 bedrooms at 110 gpd | 330 gpd |
| Proposed ADU, 1 bedroom at 110 gpd | 110 gpd |
| Total design flow required | 440 gpd |
| System approved for 3 bedrooms | 330 gpd |
| Result: upgrade needed | Short 110 gpd |
MassDEP assigns no separate design flow to ADUs, so the standard 110 gallons per day per bedroom applies, and an ADU is treated as part of the facility already served by the system. Find the approved bedroom count, not the current one. Confirm with your Board of Health, since local rules and site conditions can be stricter.
New England's ADU Ecosystem. We connect, equip, and guide across MA, RI, CT, NH, VT, and ME. We do not build, design, permit, or construct ADUs, and we are not a brokerage.
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