780 CMR 10th Edition: Base Codes, EV Wiring, and Tiny Houses

The tenth edition of 780 CMR, the Massachusetts State Building Code, took effect on October 11, 2024 and became the only code accepted for new permit applications filed after June 30, 2025.¹ The 780 CMR 10th edition changes rebuild the code on the 2021 family of International Codes with heavy Massachusetts amendments, updating structural load calculations, adopting the 2021 energy code, requiring EV-ready wiring in new homes, opening a formal compliance path for tiny houses, and reworking several rules for renovations and change-of-occupancy projects.²

When the Tenth Edition Applies

The Board of Building Regulations and Standards built a concurrency period into the rollout. From October 11, 2024 through June 30, 2025, a permit applicant could choose either the ninth or the tenth edition, but could not mix provisions from both on the same project.³ The BBRS extended the original window to that June 30, 2025 cutoff.

Any application received on or after July 1, 2025 that references the ninth edition will not be accepted and will not receive a building permit.¹ Projects that filed under the ninth edition before the deadline continue under those rules through completion.

Which Base Codes the Tenth Edition Adopts

The tenth edition is built on the 2021 International Codes rather than the older editions underlying the ninth. The primary structure follows the 2021 International Building Code (IBC) across 35 chapters and appendices for commercial and larger residential work. Chapter 51 adopts the 2021 International Residential Code (IRC) with Massachusetts amendments for detached one- and two-family dwellings and townhouses up to three stories. Mechanical systems follow the 2021 International Mechanical Code, with Massachusetts limiting which portions of the IMC’s Chapter 1 apply.²

Plumbing works differently. Massachusetts does not adopt the International Plumbing Code or the International Fuel Gas Code. Any reference to those codes inside 780 CMR is treated as a reference to 248 CMR, the Uniform State Plumbing Code administered by the Board of State Examiners of Plumbers and Gas Fitters.⁴

The state-specific amendments throughout the tenth edition are substantial. The BBRS replaces entire chapters where necessary to accommodate Massachusetts conditions, including snow loading, seismic risk, and energy performance.² Working from an unamended copy of the 2021 IBC or IRC will miss provisions that control plan review.

Structural Load Changes

Chapter 16 governs structural loads, and the tenth edition updates wind, seismic, and snow calculations against ASCE 7-16, a newer reference than the ninth edition used. Design wind speeds generally come down, which translates to roughly a 5 to 15 percent reduction in design wind loads for most projects.

Seismic forces move the other direction. Short-period spectral accelerations, which govern shorter and stiffer buildings, increase significantly across most of Massachusetts. Long-period accelerations, which affect taller and more flexible structures, decrease somewhat. Many low-rise commercial and residential projects therefore face higher seismic design requirements than they did under the ninth edition.

Snow load provisions now account for local elevation within a municipality. A site at a higher elevation than the municipal average may be assigned a greater ground snow load. The code adds requirements for snow accumulation on open-framed structures such as pipe racks and includes prescriptive clearances beneath elevated structures to prevent drift buildup. Foundation provisions add new rules for ground improvement methods such as rammed aggregate piers.

Energy Code, Stretch Code, and Specialized Code

The tenth edition adopts the 2021 International Energy Conservation Code (IECC) as the base energy code, with Massachusetts amendments, setting requirements for insulation, air sealing, and high-performance windows and doors.² For most projects the base code is only the starting point, because most of the state’s 351 municipalities have adopted one of two tougher local energy codes.

The Stretch Code (225 CMR 22.00 for residential, 225 CMR 23.00 for commercial) requires residential builders to use a performance-based compliance path verified by a third-party energy rater. The maximum Home Energy Rating System (HERS) Index score depends on fuel type and whether the home includes on-site solar:⁵

Fossil fuel heating, no solar: HERS 42
All-electric, no solar: HERS 45
With on-site solar PV: HERS 55 (fossil fuel) or HERS 58 (all-electric)

The Municipal Opt-in Specialized Stretch Code goes further. Smaller dwellings under 4,000 square feet mirror the Stretch Code HERS thresholds or follow a Passive House certification path. Larger mixed-fuel dwellings over 4,000 square feet face a HERS 0 requirement or must pursue Passive House certification, which is essentially net-zero energy performance. The Specialized Code also includes a zero-energy pathway that incorporates solar PV.⁵

Check which code your municipality has adopted before design begins. The gap between the base IECC and the Specialized Code is enormous, and discovering the wrong code applies after plans are drawn is expensive to fix. Blower door testing and HERS rating by a third party are standard in Stretch Code and Specialized Code jurisdictions.

EV-Ready Wiring in New Homes

New residential buildings must include electrical infrastructure for EV charging. Under Chapter 51, every new one- and two-family dwelling and townhouse must have at least one EV-ready parking space per unit. Multifamily buildings must make at least 10 percent of parking spaces EV-ready.⁶

An EV-ready space is a dedicated branch circuit labeled “EV READY” in the panel directory, terminating at the parking space in either a NEMA receptacle or an SAE J1772 connector. The standard circuit is 50 amps. In one- to four-unit buildings where a 50-amp circuit would force an electrical service upgrade beyond what the dwelling otherwise requires, a 40-amp circuit or a 20-amp 110-volt circuit is permitted. A single 50-amp circuit can also be replaced by three or more 20-amp EV-ready circuits where space allows.⁶ The required number of EV-ready spaces can never exceed the total number of parking spaces required by local ordinance.

A Compliance Path for Tiny Houses

The tenth edition adopts Appendix AQ of the 2021 IRC with Massachusetts revisions, giving tiny houses a defined compliance route instead of a variance request. Habitable spaces and hallways must have a ceiling height of at least 6 feet 8 inches, and bathrooms and kitchens need at least 6 feet 4 inches.

Lofts, meaning floor levels more than 30 inches above the main floor that are open on one or more sides, are permitted with reduced ceiling heights. A loft must be at least 35 square feet in area and no less than 5 feet in any horizontal dimension. The ceiling at the loft’s access point must be at least 3 feet high. Portions of a loft under a sloped ceiling with less than 3 feet of clearance do not count toward the minimum required area.

Renovations, Alterations, and Change of Use

Chapter 34 adopts the 2021 International Existing Building Code (IEBC) with Massachusetts amendments. Several of those amendments matter for retrofit work.

When a building sustains substantial structural damage to elements that carry snow loads, the damaged structure must be evaluated and strengthened to meet current snow load requirements, regardless of whether snow caused the damage. Adding insulation to an unheated roof structure triggers a requirement to check the roof’s structural capacity, because the added insulation reduces latent heat loss and can increase snow accumulation. Major renovations that reconfigure more than 50 percent of the interior area require interior masonry partition anchorages to be evaluated for out-of-plane seismic forces and strengthened if they fall short.

A change of occupancy, such as converting a warehouse into apartments, triggers additional requirements under Chapter 10 of the IEBC. The building official must issue a new certificate of occupancy, and if the new occupancy triggers different fire protection thresholds, those systems must be brought into current code compliance.⁷

Enforcement and Penalties

Municipal building officials enforce the code. On identifying a violation through plan review, inspection, or complaint, a building official can serve a notice of violation or order directing the responsible party to stop the illegal activity and correct the problem.⁸ Stop-work orders are a standard tool on active sites.

Penalties are set by M.G.L. c. 143, § 94. Anyone who violates any provision of the code faces a fine of up to $1,000, imprisonment for up to one year, or both, for each violation. Each day a violation continues counts as a separate offense, so fines can accumulate quickly on an unresolved problem.⁹ The violation penalties section of 780 CMR also references M.G.L. c. 148, § 34C and M.G.L. c. 148A for fire-related violations.¹⁰

1
Mass.gov. Concurrency Period for 9th and 10th Edition of the State Building Code Extended Until June 30, 2025
2
Mass.gov. Tenth Edition of the MA State Building Code 780
3
Legal Information Institute. 780 CMR, Chapter 51, Chapter 1, Part 1, Section 102 – Applicability Concurrency Period
4
Massachusetts State Board of Building Regulations and Standards. 780 CMR – Massachusetts State Building Code Tenth Edition
5
Commonwealth of Massachusetts. Stretch Energy and Municipal Opt-In Specialized Building Code Frequently Asked Questions
6
Legal Information Institute. 780 CMR, Chapter 51, ch. 4, R404, R404.4 – Wiring for Electric Vehicle Ready Spaces
7
ICC Digital Codes. Chapter 10 Change of Occupancy – 2021 International Existing Building Code
8
Legal Information Institute. 780 CMR Chapter 51 Chapter 1 Part 2 Section 114 R114.2 – Notice of Violation
9
General Court of Massachusetts. Massachusetts General Laws Part I, Title XX, Chapter 143, Section 94
10
Legal Information Institute. 780 CMR Chapter 51 Chapter 1 Part 2 Section 114 R114.4 – Violation Penalties