Massachusetts’ 2024 Affordable Homes Act made accessory dwelling units buildable by right in single-family zones statewide, and ADU permits have surged since. What many owners and builders discover mid-project: ADUs need their own HERS rating, with targets of HERS 52, 55, or 58 under the 2025 stretch and specialized codes — more forgiving than the 42–45 required of full-size new homes, but still a real requirement with real inspections.
Which number applies to your ADU?
The pattern mirrors the main-house rules: 52 for a mixed-fuel ADU, 55 for all-electric, and a further 3-point allowance (to 58) with the embodied carbon credit — earned through carbon-storing insulation like cellulose or qualifying low-carbon concrete. Detached backyard cottages, garage conversions, and basement or attic ADUs that create a new dwelling unit all fall under the requirement.
Why small units are harder than they look
HERS math is tougher per square foot on small buildings: an ADU has a lot of envelope surface relative to its floor area, so insulation and airtightness weigh heavily. The same disciplines apply as any new home — a blower door test, duct testing where ducts exist, and mechanical ventilation sized by the standard formula (a 1,200 sq ft 2-bedroom ADU needs roughly 59 CFM of continuous ventilation). Compact ducted ERVs and single-head cold-climate heat pumps are the workhorse combination.
The inspection sequence for an ADU
Same as a house, scaled down: preliminary HERS certificate with the building permit, insulation inspection before drywall, blower door and duct tests at completion, final certificate for the certificate of occupancy. See the full walkthrough in our new-construction energy code inspection guide. Budget the rating into the project early — retrofitting airtightness into a finished garage conversion is the expensive way to pass.
Don’t leave rebates on the table
Mass Save new-construction incentives apply to qualifying ADU projects, and all-electric ADUs are strong candidates for the top tiers. The HERS rating you’re required to get anyway is the same one that unlocks them.
Home Energy Efficiency Consultants rates ADUs across Massachusetts — ENERGY STAR® partner, U.S. DOE Efficient New Home partner, 5.0 stars across 135 Google reviews, offices in Somerville, Framingham, Marlborough, and Everett. HERS ratings: (617) 501-6788.
