Energy Code for Office-to-Residential Conversions in Massachusetts
Converting an office or commercial building to apartments in Massachusetts is a change of use under Chapter 5 of the Stretch Code. Buildings of 3 stories or less follow 225 CMR 22 and need a HERS rating under the change-of-use caps (65 mixed-fuel, 70 all-electric or with solar, 75 both). Taller buildings follow 225 CMR 23 C505, where converted spaces meet new-construction envelope, mechanical and air-leakage rules, or a HERS cap of 55–61. Boston's Specialized Code applies only to new buildings.
Plan this early
The compliance path decides wall insulation, windows, ERVs and air sealing. Pick it before design, especially in historic buildings where exposed masonry may be kept.
Which Code Applies
| Building | Code | Path |
|---|---|---|
| R-2 conversion, 3 stories or less | 225 CMR 22, Chapter 5 | HERS rating under the change-of-use caps (Table R406.5) |
| R-2 conversion, 4+ stories | 225 CMR 23, C505 | New-construction rules for the converted spaces, a HERS cap (Table C407.4), component performance, or EnerPHit |
| Base code town | 780 CMR 10th edition | Chapter 5 of the IECC as amended |
Specialized Code (Boston since Jan 1, 2024) applies to new buildings; existing buildings follow Stretch Code Chapter 5.
HERS Targets for Change of Use
| Building | Mixed-fuel | Solar | All-electric | Solar + all-electric |
|---|---|---|---|---|
| 3 stories or less (Table R406.5) | 65 | 70 | 70 | 75 |
| 4+ stories, HERS path (Table C407.4) | 55 | 58 | 58 | 61 |
The 4+ story HERS path also requires ENERGY STAR Multifamily New Construction. The 12,000 sq ft Passive House/TEDI trigger for new multifamily does not apply to conversions that comply under Chapter 5. Historic buildings, as determined by the governing authority, may be allowed the prescriptive path; confirm with the building official early.
Testing and Ventilation
- Air leakage (C402.5.2): whole building 0.35 cfm/sq ft at 75 Pa, or 0.27 cfm/sq ft of unit enclosure at 50 Pa for Group R. With 8+ units, test the greater of 7 units or 20%, including top, middle and ground-floor units and the largest unit. See commercial air leakage testing.
- Ventilation: 4+ stories need an ERV delivering outdoor air to each unit (C403.7.4.1); 3 stories or less need balanced ERV/HRV with 65% SRE (R403.6.1). See ERV/HRV requirements.
- EV-ready wiring applies only if the project changes the parking (DOER Q&A). Solar-ready applies to new construction.
Boston Programs
| Program | What it offers |
|---|---|
| Office-to-Residential Conversion Program | 75% property tax abatement for up to 29 years; applications through Dec 31, 2026; permit and start construction by Dec 31, 2027; build to the Stretch Code and meet inclusionary rules |
| State funding for Boston conversions (2024) | Up to $215,000 per affordable unit, capped at $4 million per project |
| Commercial Conversion Tax Credit (state) | Awards announced in rounds; $15.3 million for 5 projects in Aug 2026 |
| BERDO | Buildings with 15+ units or 20,000+ sq ft report energy every May 15 and must meet emissions limits stepping down to net zero by 2050 |
How We Help
- Early code-path review of the existing walls, windows and systems.
- Energy modeling to hit the HERS cap with the least invasive envelope work.
- Unit-sampling air leakage tests and ERV airflow verification.
- Final HERS ratings and documentation for the building department.
We are working on a historic office-to-residential conversion in downtown Boston now. Meet Dominic.
FAQ
What energy code applies to an office-to-apartment conversion in Massachusetts?
It is a change of use under Stretch Code Chapter 5: 225 CMR 22 for buildings of 3 stories or less and 225 CMR 23 C505 for 4+ stories.
Does a conversion need a HERS rating?
For 3 stories or less, yes, with change-of-use caps of 65 to 75. For 4+ stories, a HERS cap of 55 to 61 is one option alongside meeting the new-construction requirements for the converted spaces.
Do conversions over 12,000 sq ft need Passive House?
No. The Passive House/TEDI requirement for large multifamily applies to new construction, not to changes of use complying with Chapter 5.
Does Boston's Specialized Code apply to conversions?
No. The Specialized Code applies to new buildings; existing buildings follow Stretch Code Chapter 5.
What are the Boston conversion program deadlines?
Apply by December 31, 2026, and pull a full permit and start construction by December 31, 2027.
Town-by-Town Permit Checklists
Local building department details, checked on each town's own website:
Sources: 225 CMR 22 (residential) · 225 CMR 23 (commercial and multifamily) · DOER 2025 Stretch Code updates: responses to questions · City of Boston: conversion program extended · Boston BERDO · Mass.gov: Commercial conversion awards, Aug 2026. 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
Planning a Conversion? Get the Code Path Right First
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