What Triggers a HERS Rating in Massachusetts?
Builders and homeowners frequently ask the same question: When is a HERS rating required in Massachusetts?
The answer depends largely on where the project is located and the scope of work being performed.
First, it’s important to understand that HERS ratings are required only in municipalities that have adopted the Massachusetts Stretch Energy Code or the Specialized Opt-In Energy Code. These performance-based codes rely on a Home Energy Rating System (HERS) score to demonstrate compliance.
Projects located in municipalities that follow the base Massachusetts energy code do not require a HERS rating.
If you’re unsure which energy code applies to your project, you can check your municipality here:
https://www.spectrumnrg.com/energy-codes#codebytown
Because most Massachusetts communities have adopted either the Stretch Code or the Specialized Code, many residential projects now require energy modeling, field inspections, and blower door and duct leakage testing as part of the permitting and occupancy process.
While new construction always requires a HERS rating, renovations and additions can trigger one as well depending on the size and scope of the project.
New Construction Projects
For residential construction located in Stretch Code or Specialized Code communities, a HERS rating is required for new dwelling units.
This includes:
New single-family homes
New multi-family residential buildings
New accessory dwelling units (ADUs)
These projects must meet the applicable maximum HERS rating target and complete required field verification testing, including blower door testing and duct leakage testing where applicable.
Because these requirements are known early, HERS modeling is incorporated during project design and permitting.
Renovations That Trigger a HERS Rating
Renovation projects can also require a HERS rating when they meet specific thresholds defined in the Massachusetts code amendments.
For one- and two-family homes or townhouses, a HERS rating is required when the renovation qualifies as an Extensive Alteration under the International Residential Code (IRC).
This occurs when the renovation affects:
More than 1,000 square feet, or
More than 100% of the existing conditioned floor area
When either threshold is met, the entire dwelling unit must receive a certified HERS rating, not just the portion being renovated.
For multi-family residential buildings, similar thresholds apply under the International Existing Building Code (IEBC).
If work within a single dwelling unit qualifies as a Level 3 Alteration and affects:
More than 1,000 square feet, or
More than 100% of the existing conditioned floor area
then that individual dwelling unit must receive a HERS rating.
Importantly, this requirement applies only to the unit being renovated, not the entire building. For example, a full gut renovation of one apartment within a multi-family building would require a HERS rating for that unit alone if the thresholds above are met.
Additions That Trigger a HERS Rating
Additions are another area where confusion is common.
In Massachusetts Stretch Code and Specialized Code communities, a HERS rating is required when an addition meets either of the following conditions:
The addition exceeds 1,000 square feet, or
The addition increases the conditioned floor area by more than 100%
When either threshold is met, the entire combined dwelling unit must comply with the HERS rating requirements for alterations, additions, or change of use.
In practice, this means the existing home and the new addition are modeled and tested together as a single building.
Because of this, larger additions may require improvements to parts of the existing home such as:
insulation levels
air sealing details
windows and doors
ventilation systems
mechanical equipment efficiency
Planning for these requirements early can help avoid changes later during construction.
Important Exception: Converting Basements or Attics
There is an important exception that often applies to renovation projects involving unfinished space.
A HERS rating is not required when an existing unconditioned basement or attic is converted into conditioned living space, provided the project simply moves the home’s thermal boundary within the existing structure.
This typically means:
The foundation footprint is not expanded, and
The roof structure is not expanded or altered (such as adding dormers)
If the work is limited to finishing existing space inside the current building shell, the project does not trigger a HERS rating.
However, if the project includes structural expansion—such as extending the foundation, enlarging the building footprint, or adding dormers—the work may qualify as an addition and a HERS rating would then be required.
It’s also important to note that converting an attic or basement into an Accessory Dwelling Unit (ADU) would typically trigger a HERS rating because it creates a new dwelling unit.
Real-World Examples
Understanding the thresholds becomes easier when looking at typical project scenarios.
Example 1: Renovation That Triggers a HERS Rating
A homeowner renovates a 1,200 square foot single-family home and the renovation affects approximately 1,050 square feet of the building. Because the project exceeds the 1,000 square foot threshold, it qualifies as an Extensive Alteration.
In this case, the entire dwelling unit must receive a HERS rating, including energy modeling and blower door testing.
Example 2: Addition That Triggers a HERS Rating
A homeowner adds a 1,100 square foot addition to an existing 1,000 square foot home. Because the addition exceeds 1,000 square feet, the project triggers the HERS requirement.
The entire 2,100 square foot combined home must now meet the applicable HERS rating target under the Stretch Code.
Key Takeaways
In Massachusetts communities that have adopted the Stretch Energy Code or Specialized Opt-In Code, a HERS rating is required for:
New construction
New single-family homes
New multi-family residential buildings
New accessory dwelling units (ADUs)
Renovations
Renovations affecting more than 1,000 square feet, or
Renovations affecting more than 100% of the existing conditioned floor area
For multi-family buildings, these thresholds apply to the individual unit being renovated, not the entire building.
Additions
Additions larger than 1,000 square feet, or
Additions that increase the conditioned floor area by more than 100%
Exception
Converting an existing unconditioned basement or attic to conditioned space without expanding the foundation or roof structure
Understanding these triggers early in the design process can help builders and homeowners avoid delays during permitting, inspections, and final occupancy.
