ADU Building Permit and Energy Code Requirements in Massachusetts
Since February 2, 2025, Massachusetts zoning lets one accessory dwelling unit (up to 900 sq ft or half the main house, whichever is smaller) be built by right in single-family districts (M.G.L. c.40A s.3). Zoning got easier, but the building and energy code still apply: under the Stretch and Specialized codes an ADU needs a HERS rating of 52 or less if mixed-fuel or 55 or less if all-electric.
ADU HERS targets
Mixed-fuel 52, all-electric 55, solar electric 55, solar plus all-electric 58 (Table R406.5). The existing house is not included in the ADU's rating.
What the ADU Law Changed
- An ADU is a self-contained unit with its own sleeping, cooking and sanitary facilities and a separate entrance, no larger than half the main house or 900 sq ft, whichever is smaller (M.G.L. c.40A s.1A).
- Towns can't prohibit, unreasonably restrict or require a special permit for one ADU in a single-family zoning district (c.40A s.3).
- Towns can't require owner occupancy, and can require no more than one extra parking space (none within half a mile of transit).
- State regulations (760 CMR 71.00, "Protected Use ADUs") took effect January 31, 2025, and don't override the building code.
ADU Energy Code Targets
From Table R406.5 of 225 CMR 22 (Stretch Code) and Appendix RC (Specialized Code).
| ADU type | Max HERS |
|---|---|
| Mixed-fuel | 52 |
| Solar electric | 55 |
| All-electric | 55 |
| Solar + all-electric | 58 |
DOER's 2025 training slides apply the 52-58 range to both stand-alone ADUs and ADUs attached to an existing house, and note that the existing house is not included in the ADU's HERS rating. ADUs still need a blower door test, duct testing, ventilation and the other mandatory items.
ADU Permit Checklist
- Building permit application, scaled plans and a certified site plan showing setbacks.
- Many towns run ADUs through site plan review (for example Malden) or publish an ADU guide (for example Dedham).
- Utility capacity: sewer and water permits for added flow in some towns (for example Dedham).
- Energy code documentation: projected HERS report for the ADU, then the confirmed rating at final.
- Fossil-fuel-free towns may require the ADU to be all-electric.
If an ADU is created by converting space inside an existing house, ask us early: the energy path can follow the alteration rules instead, and the building inspector decides.
FAQ
Do ADUs need a HERS rating in Massachusetts?
Under the Stretch and Specialized codes, yes: Table R406.5 sets ADU targets of HERS 52 mixed-fuel and 55 all-electric (55 solar electric, 58 solar plus all-electric).
How big can a by-right ADU be in Massachusetts?
Up to half the gross floor area of the main house or 900 sq ft, whichever is smaller (M.G.L. c.40A s.1A).
When did the Massachusetts ADU law take effect?
The by-right ADU provisions took effect February 2, 2025, and the 760 CMR 71.00 regulations took effect January 31, 2025.
Is the main house included in an ADU's HERS rating?
No. DOER's 2025 Stretch Code slides say the existing house is separate from an attached ADU and is not included in the HERS rating.
Does the ADU law override the building code?
No. The zoning protection doesn't change the building code; the ADU still needs a building permit and energy code compliance.
Town-by-Town Permit Checklists
Local building department details, checked on each town's own website:
Sources: M.G.L. c.40A s.3 · Mass.gov accessory dwelling units · 760 CMR 71.00 · DOER 2025 Stretch Code webinar slides. Code text current as of October 2026; DOER has proposed updates to the Stretch and Specialized codes, so confirm before you file.
Related: Stretch code report for permitting · What triggers a HERS rating · REScheck and COMcheck · Phius verification
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