Building Permit for an Addition in Massachusetts: What You Need
An addition in Massachusetts needs a building permit with plans, a certified plot plan, contractor CSL and HIC information (or a homeowner exemption), a workers' comp affidavit and energy code documentation. The big question is the energy path: under the 2025 Stretch Code, an addition over 1,000 sq ft or more than 100% of the existing conditioned floor area needs a HERS rating of the whole home; smaller additions can usually use REScheck or the prescriptive path.
HERS or REScheck?
Over 1,000 sq ft or over 100% of the existing home: HERS rating (65 mixed-fuel / 70 all-electric). Smaller: usually REScheck or prescriptive. Finishing a basement or attic without changing footprint or roofline: no HERS rating.
Addition Permit Checklist
- Permit application with full scope and estimated cost.
- Scaled plans: existing and proposed floor plans, elevations, sections, framing and foundation (stamped engineering for LVLs or steel).
- Certified plot plan from a registered land surveyor showing setbacks and lot coverage (most towns require it whenever the footprint changes).
- Contractor CSL and HIC registration (HIC applies to work on existing owner-occupied 1-4 unit homes), insurance and contract, or the homeowner exemption.
- Workers' comp affidavit and debris disposal facility.
- Energy code documentation: REScheck/prescriptive or a projected HERS report (see below).
- Fire Department review for bedroom additions in many towns; zoning relief if the house is nonconforming.
When an Addition Needs a HERS Rating
From 225 CMR 22 (2025 Stretch Code), which also applies to additions in Specialized Code towns.
| Project | Energy path | Code |
|---|---|---|
| Addition over 1,000 sq ft or over 100% of existing conditioned floor area | HERS rating of the combined home: max 65 mixed-fuel, 70 all-electric (70 / 75 with solar) | R502.1.1, Table R406.5 |
| Extensive alteration over 1,000 sq ft or 100% of the home (1-2 family) | HERS rating to the same targets | R503.1.5 |
| Finishing an existing basement or attic (no footprint or roofline change) | No HERS rating required | R502.1.1 exception |
| Smaller additions and alterations | Prescriptive (R402-R404) or REScheck; a HERS rating is also allowed | R502.1, R402.1.5.1 |
DOER's 2025 training slides put the renovation trigger plainly: the work area is more than 50% of the existing home and it exceeds 1,000 sq ft or 100% of the existing conditioned floor area. The building inspector has the final say on which path applies. In Specialized towns, Appendix RC points existing buildings back to Chapter 5 of the Stretch Code.
Watch Out For Local Rules
- Fossil-fuel-free towns (Cambridge, Brookline, Arlington, Lexington, Newton, Concord and others) restrict new gas or oil equipment in additions over 1,000 sq ft or major renovations.
- Some towns require Fire Department plan stamps for any bedroom addition.
- Large-house or floor-area rules (Wellesley Large House Review, Concord FAR, Newton facade ratio) can add review time.
- Demolition delay can apply when you remove a large share of an older home's exterior.
Check your town's page in the list below.
FAQ
Do I need a HERS rating for an addition in Massachusetts?
Under the 2025 Stretch Code, additions over 1,000 sq ft or more than 100% of the existing conditioned floor area need a HERS rating of the combined home (65 mixed-fuel, 70 all-electric). Smaller additions can usually use REScheck or the prescriptive path.
Does finishing a basement or attic need a HERS rating?
No. 225 CMR 22 R502.1.1 exempts adding an existing basement or attic to the conditioned space when the footprint and roofline don't change.
Can I use REScheck for an addition in Massachusetts?
Yes for smaller additions and alterations: REScheck-Web for the Massachusetts Stretch Code is an approved tool for the prescriptive path. Large additions need a HERS rating.
Do I need an HIC contractor for an addition?
Work on an existing owner-occupied 1-4 unit home is residential contracting under M.G.L. c.142A, so the contractor needs HIC registration. Homeowners who pull their own permit lose access to the HIC guaranty fund.
What HERS score does an addition need?
Table R406.5 sets 65 for mixed-fuel and 70 for all-electric homes (70 with solar, 75 with solar and all-electric).
Town-by-Town Permit Checklists
Local building department details, checked on each town's own website:
Sources: 225 CMR 22 (2025 Stretch Code) · DOER 2025 Stretch Code webinar slides · M.G.L. c.142A s.1. 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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