What Is the Massachusetts ADU Law? MGL c. 40A §3A Complete Guide.

What MGL c. 40A §3A Actually Says

The 2021 Massachusetts ADU law (Section 3A of the Massachusetts Zoning Act, MGL c. 40A) made ADUs permitted as-of-right in any district that allows single-family residential use. The law's text is short; the implications are significant.

What's Permitted As-Of-Right

• One ADU per single-family lot, by right (no special permit, no variance, no public hearing required)
• Detached, attached, or interior conversion configurations all permitted
• Municipality cannot impose substantive owner-occupancy requirements (some local bylaws still try; Land Court rulings pending)

What Local Can Still Regulate (Dimensional)

• Setback requirements (typically 5-15 ft side, 10-25 ft rear)
• Maximum size (most MA towns: 900-1,200 sq ft, or 50% of primary, whichever less)
• Maximum height for detached ADUs (typically 25-30 ft)
• Maximum lot coverage (varies; commonly 25-40%)
• Parking: maximum 1 off-street space required

What Local Cannot Do

• Prohibit ADUs outright
• Require minimum lot size beyond underlying zone
• Require parking beyond 1 space
• Require special design review beyond what applies to primary

Boston vs Cambridge vs Other Major Cities

Implementation varies by municipality:

Boston

ADUs already permitted under Article 2A of Boston Zoning Code prior to 40A §3A. Owner-occupancy of primary required; ADU may be rented to non-family. Max ADU size 900 sq ft. STR of standalone ADU prohibited.

Cambridge

Article 4 of Cambridge Zoning updated 2023 to align with state law. No owner-occupancy requirement post-amendment. STR not permitted; long-term rental (12+ month leases) only.

Newton

Owner-occupancy required of primary dwelling. Rental of ADU permitted with deed restriction. STR limited to 60 days/year owner-occupied.

Brookline + Somerville + Worcester

No owner-occupancy requirement. Long-term rental permitted at market rates.

780 CMR Construction Requirements for ADUs

40A §3A handles zoning. 780 CMR handles construction quality. Every MA ADU must satisfy:

• Egress (Chapter 10): separate exterior egress door OR shared with rated separation
• Fire-rated separation (R302): 1-hour wall + ceiling between ADU and primary
• Plumbing (248 CMR): independent kitchen + bath; Master Plumber permit
• Electrical (527 CMR): separate sub-panel feed; Master Electrician permit
• Mechanical: independent heating + ventilation per ASHRAE 62.2
• Accessibility (521 CMR): full code on new construction; existing conversions exempt

Mass Save Treats ADU as Separate Dwelling

Critical financial implication: Mass Save rebates apply per-unit, not per-property. A property with primary + ADU captures rebate stack TWICE — separate HEAs, separate insulation, separate heat pumps.

Worked example: ADU heat pump install qualifies for $10K standard / $16K income-eligible Mass Save rebate as its own filing, completely independent of the primary dwelling's heat pump rebate. Income tier verification uses ADU occupant income, not property owner income. ADU + primary together can capture $48,000-$76,000 in stacked rebates under aggressive electrification.