Accessory Dwelling Units in Massachusetts

What the Affordable Homes Act changed

Governor Healey signed the Affordable Homes Act, Chapter 150 of the Acts of 2024, on August 6, 2024. Section 8 of that law amended the state Zoning Act, M.G.L. c. 40A, to make a “protected use” accessory dwelling unit an allowed use by right in single-family residential zoning districts. The implementing regulation, 760 CMR 71.00, took effect on January 31, 2025, and the zoning change took effect on February 2, 2025.

By right is the part that matters. It means no special permit, no zoning board hearing, and no neighbor veto. A complete application that meets the standards is processed as a matter of course. Your town can still require site plan review, but the standards it applies have to be clear and objective rather than discretionary.

What your town can no longer require

The regulation is specific about what municipalities may not do to a protected-use ADU:

• Require that either the ADU or the main house be owner-occupied
• Require a special permit or public hearing for a first protected-use ADU
• Impose a minimum lot size for the ADU
• Cap, quota, or otherwise limit how many ADUs exist in the town or district
• Dictate whether the unit is attached to or detached from the main house
• Require more than one additional parking space, or any additional parking at all when any part of the lot is within a half mile of a transit station
• Apply dimensional standards stricter than those applied to the principal dwelling
• Apply subjective design review; site plan standards must be clear and objective

Two narrow exceptions survive: a town may still require a special permit for an additional ADU beyond the first on the same lot, and within floodplain or aquifer protection overlay districts.

Up to 900 square feet, or half the gross floor area of the main house, whichever is smaller.

This is the detail most articles get wrong. A number of published summaries say “whichever is greater.” The regulation says smaller. On a 1,400 square foot house, your cap is 700 square feet, not 900. Getting this backwards at the concept stage is how a design gets redrawn.

Where the law applies, and where it does not

A “single-family residential zoning district” is defined broadly: any district where single-family homes are permitted or allowable, whether as of right or by special permit. That sweeps in far more land than most homeowners assume.

Three limits are worth knowing. Boston is not covered, because it zones under its own enabling act rather than c. 40A. Towns may still restrict short-term rental of an ADU. And the ADU regulation does not override the state building code, the fire code, wetlands protection, or Title 5 septic rules, which is where most real-world constraints actually live.

Will it work on your lot? Five feasibility gates

The zoning answer is now usually yes. The useful questions are physical, and they are what we check before anyone draws anything.

• Wastewater. On an unsewered lot this decides the project. See the Title 5 section below. On a sewered lot, confirm capacity and connection requirements.
• Distance to transit. Measured to your lot, not your town. It controls whether any parking has to be added.
• Setbacks and dimensional fit. The ADU cannot be held to stricter standards than the main house, but the main house standards still apply.
• Structure and access. Interior conversion, garage conversion, attached addition, or detached build. Each has a very different cost and permitting path.
• Overlays and resource areas. Floodplain and aquifer protection districts can still require a special permit. Wetlands trigger conservation commission review.

Septic and Title 5: the gate most homeowners miss

If your property is not on municipal sewer, this section matters more than the zoning did.

Title 5, 310 CMR 15.000, sizes a septic system by design flow at 110 gallons per day per bedroom. MassDEP guidance treats the addition of an ADU as new construction for Title 5 purposes. Bedrooms are counted for each unit and then added together for the property: a three-bedroom house plus a two-bedroom ADU is a five-bedroom property.

The consequence is direct. Your existing system has to meet new-construction standards for that combined bedroom count. If it does not, it has to be upgraded or replaced before the ADU can be permitted. On an unsewered lot this is routinely the single largest line item and the longest lead time in the whole project.

The practical move is to price it first rather than last. Pull the system as-built and its rated bedroom count from the Board of Health, and get a soil evaluation, before you commit to a bedroom count or a floor plan. Septic is common in Dover, Weston, Norfolk, Wrentham, and parts of Medfield, Foxborough, and Sharon, though it varies parcel by parcel.

What permitting actually looks like

• Confirm the zoning district and that the unit qualifies as a protected use
• Resolve wastewater: Title 5 review with the Board of Health, or sewer connection capacity
• Design to the size cap and to the dimensional standards that apply to the main house
• Site plan review, where the town requires it, against clear and objective standards
• Building permit, plus electrical, plumbing, and mechanical permits
• Inspections, certificate of occupancy, and decisions on separate utilities and addressing

Timelines vary by town and by whether wastewater work is involved. A straightforward interior conversion on sewer moves quickly. A detached unit on a septic lot that needs a system upgrade is a different project on a different calendar.

What an ADU actually costs in 2026

Greater Boston market ranges by approach:

Site work, meaning utility runs, grading, access, and drainage, commonly adds $20,000 to $80,000. On an unsewered lot a Title 5 upgrade can exceed all of it. As a worked example, a 900 square foot detached ADU at $300 to $450 per square foot is roughly $270,000 to $405,000 before site work.

These are 2026 Greater Boston market ranges, not a quote. Your number moves with the five feasibility factors above, the finish level, and the condition of what is already there. We define a fixed scope before demolition rather than discovering it during construction.

Town by town

Fourteen towns we build in. Commuter rail matters because of the parking rule; wastewater matters because of Title 5.

A station in town does not by itself exempt your lot. The parking exemption applies within a half-mile radius measured to the lot. The wastewater column describes a general pattern, not a determination for your parcel: confirm with the town Board of Health. Local rules change, and the building department and Board of Health are the authority for your property. We verify current requirements with them before design.

Sources

Verified September 2026 against the following official sources:

• Mass.gov: Accessory Dwelling Units
• Mass.gov: Accessory Dwelling Unit FAQs
• 760 CMR 71.00: Protected Use Accessory Dwelling Units
• MassDEP: Title 5 compliance guidance for ADUs
• Mass.gov: ADUs officially allowed statewide

Planning note: Zoning, permitting, building-code, and program requirements may change and can vary by property. Homeowners should confirm current requirements with their municipality, the Commonwealth, and qualified design and construction professionals before beginning work.