How Do the New Massachusetts Energy Code Changes Affect Your ADU Cost?

By Buz Artiano, BuildX | May 2026

Watch: ADUs Explained: Insights into Accessory Dwelling Units w/ Coldwell Bankers | BuildX Podcast

If you have been looking into building an ADU in Massachusetts, you have probably run into different information about the energy requirements. Some articles will tell you that your ADU must be all-electric. But others say gas and propane are also possible. The conflicting information is understandable, because the rules have actually changed. When families are trying to come up with a realistic budget, not knowing what the energy code rules are makes it impossible to compare different prices, pick a heating system, or understand why one estimate is thousands of dollars more than another.

My name is Buz Artiano, and I am the founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We are the only team you need, we handle design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We deal with septic constraints, zoning issues, and the state's new ADU laws.

When the state changed the energy codes for an ADU, we had a meeting with our HERS rater that same week so we would know just exactly what it meant for the cost of the project, the options for heating systems, and electrical panel sizing. Making sure we are aware of all the regulations is part of how we estimate and build every project.

This article outlines what the Massachusetts energy code change means for the budget you've set for your ADU budget in practical terms. We will talk about what changed and why it matters for the cost of your heating and electrical. We also will tell you how to use this information when you are getting cost estimates from any builder.

What Changed in the Massachusetts Energy Code for ADUs?

In the beginning Massachusetts required ADUs to go by the same energy guidelines as new single-family home construction. In July 2024, the state increased the efficiency standard, which lowered the required HERS (Home Energy Rating System) number. A lower HERS number meant that the building would be more energy-efficient, and under those new restrictions, most ADU projects had to use all-electric construction to meet the required rating.

Massachusetts loosened ADU energy code rules so that ADUs don't have to follow the same strict standards as new single-family construction. ADUs can now use mixed-fuel systems, including gas, oil, and propane, that have a HERS rating under 52 (or 55 if there is solar on the roof). This is a big difference because now gas, oil, and propane are all possibilities for heating, cooking, and hot water. The change is because ADUs are smaller buildings with different changes in cost than full-size homes. The rule before was actually keeping some families from building altogether.

You can review the current Massachusetts ADU requirements, including energy code provisions, at mass.gov.

How Does the Energy Code Change Affect Your ADU Budget?

The effect on the budget shows up in three places: your heating system, your electrical panel, and which appliances you choose. With the previous standard of all-electric, ADU projects usually required a heat pump for both heating and cooling, an electric heat pump water heater, an induction cooktop, and an electric dryer. Every one of those appliances pulls power from your electrical panel, which in many cases means you have to upgrade to a larger subpanel or put in heavier wiring from the main house.

Since mixed-fuel is now an option, a gas or propane heating system can deal with the heaviest energy load without affecting your electrical capacity. A gas range and gas dryer will also reduce the demand even more. So because of that, a 100-amp subpanel, which is what is usually installed in garages and additions, might now be enough for the entire ADU. With the all-electric standard, 100 amps was usually not enough to power heat pumps, an electric stove, an electric dryer, and LED lighting all at the same time.

We recently assessed a garage that was being converted where the homeowner wanted to build an ADU for her parents. The structure already had a gas line and a 100-amp subpanel. With the previous rules we would have had to upgrade the electrical service so it could handle all-electric heat, cooking, and hot water. But now, with the updated code, the 100 amps they have now is plenty, because gas handles both the heating and cooking loads, and the electrical panel just needs to support lights, outlets, and small appliances. That one change makes an electrical upgrade unnecessary and they will save thousands on the project.

What Is a HERS Rating, and Why Does 52 vs. 55 Matter?

HERS stands for Home Energy Rating System. It is the way Massachusetts measures how energy-efficient a new building is. The scale works in reverse: a lower number means a more efficient structure. An average new home built to code will score around 100. A net-zero home, one that produces as much energy as it uses, scores 0.

For Massachusetts ADUs, the requirement now is a HERS score of 52 for mixed-fuel buildings (those using any combination of gas, oil, propane, and electric). If you decide to put solar panels on your ADU roof, the requirement goes down to 55. Both numbers apply to a well-insulated, energy-efficient ADU, but they don't make you use a single fuel source to meet the requirement.

This is important because the way to get to a HERS 52 with mixed fuel is simple with proper insulation, good windows, and air sealing. A HERS rater will look at the planning and layout, work with your builder and let them know if it will pass before building starts. That way there is not a pass-or-fail surprise at the end. This is help with the design that your builder should use from day one.

What Should You Ask Your Builder About Energy Code Compliance?

The changes in the energy code are good to know, but it will only help your finances if the builder uses it in their calculations. Here are the questions that separate a builder who understands the current rules from one who is just working from old outdated suppositions.

Does your estimate assume all-electric or mixed-fuel construction? If the builder is still giving you a quote using the old all-electric requirement, their estimate may include an upgrade to the electrical panel heat pump system that you don't need. Make sure to ask them to confirm they are building to the current ADU energy code, not the one for new single-family homes.

Do you work with a HERS rater before construction? A builder who brings in the HERS rater for the design part of the building can enhance the insulation, the placement of windows, and mechanical systems to meet the requirements efficiently. A builder who uses the HERS rating only in the inspection after construction is complete, is likely to over-specify materials or systems as a cushion.

What electrical panel capacity does your plan require? If the builder says you need a 200-amp subpanel for a mixed-fuel ADU, you should ask why. In many cases, 100 amps is enough if gas or propane is handling the heating and cooking. The difference in panel cost, wiring, and possibly even a main panel upgrade at the house can be expensive.

We met with our HERS rater within days of the code change to model what it meant across our standard floor plans. On a typical 900-square-foot detached ADU, the combination of mixed-fuel heating and a properly insulated envelope gets us to HERS 52 without exotic materials or oversized equipment. That is a decision that comes from building enough of these to know where the efficiency gains actually are. The insulation envelope, the window specifications, and the air sealing details matter more than the fuel source. We design for the HERS target from the start, not later as an afterthought.

Does This Change Apply to Every ADU in Massachusetts?

The less-strict energy code is for buildings that are accessory dwelling units under the Massachusetts Affordable Homes Act. If your project is permitted as an ADU, it will be qualified for the mixed-fuel HERS 52 rule. If your project is classified differently, as a normal addition or a new single-family home, the stricter new-construction energy code will still apply.

This is one reason why the classification of your permit for your project matters. A builder who knows what they are doing will make sure your project permit is for an ADU so it qualifies for the right energy code. If your builder does not mention the energy code difference during the design phase, that is a question you should be asking before you sign a contract.

It is also worth mentioning that energy code regulations in Massachusetts can change. The state has changed these requirements many times, and changes in the future are possible. Any estimates or decision about the system that you make today should be based on the code that is in force at the time your building permit is issued. Your builder and HERS rater should be watching for those changes so you do not have to.

Is Mixed-Fuel Always the Right Choice for Your ADU?

The energy code change gives you more options, but mixed-fuel isn't always the best choice for every project. If your property already has a robust electrical service that works well and you want the long-term efficiency of a heat pump system, an all-electric ADU would still be a good choice. Heat pumps provide both heating and cooling in a single system, which makes maintenance simple and there is no need for a separate fuel delivery. In some towns, all-electric construction may also provide for additional incentives or rebates.

The right answer depends on what you already have on your property, which fuel you prefer, what your budget is, and what your property can handle. A builder who just gives you one option without explaining the tradeoffs is not giving you the full picture.

What Does This Mean for Your ADU Project Right Now?

The Massachusetts energy code change for ADUs allows choices for heating, cooking, and hot water that were off the table before. For many families, this means lower construction costs and simpler electrical requirements. The key is working with a builder who understands the current code, works with a HERS rater from the first, and can explain exactly how the energy requirements affect the budget for your project.