What Parts of a Massachusetts ADU Don't Count Toward Size Limits?

By Buz Artiano, BuildX | February 2026

When homeowners learn about the 50% or 900 sq ft rule for ADU sizes, they almost always want to know if that's really all the space they get. After all the ADUs we have built throughout Massachusetts, we understand the frustration when it comes to lack of size. If you're gonna invest the time and the money you want to build something that is not only functional, but comfortable. The good news is that Massachusetts law exempts several room types from that square footage calculation, and we know how to use those exemptions to dramatically change what your ADU looks like and increase the size.

At BuildX, we specialize in ADUs throughout Massachusetts, and we have helped lots of people get the most usable space out of their ADU by adding exempt areas into their designs. We understand how the right approach turns a seemingly very cramped space into a spacious, livable home.

Which Room Types Are Exempt from Square Footage Limits?

According to Massachusetts ADU regulations, the following spaces are exempt from your ADU's gross floor area calculation:

Sunrooms: These can be fully insulated with windows and doors. The main thing is just that heat cannot be installed at the time of final inspection. A sunroom can be up to 16 by 25 feet or even bigger without adding a single square foot to your ADU's official size.

Mudrooms: Even heated mudrooms don't count toward your square footage. This creates a practical entry space for boots, coats, and storage without taking away from your allowable square footage.

Garages: Attached or detached garages, including underneath garage setups, are completely exempt. This actually opens up some great design possibilities we will talk about in a minute.

Decks and Porches: Outdoor living spaces, including farmer's porches and wraparound decks, add usable area without taking away from your allowed square footage.

Breezeways: Covered walkways connecting structures also don't count, which can be extremely useful when connecting an ADU to a main house or a detached garage.

By combining all these exempt spaces, you can build what we call "the mother of all ADUs": a home with a two-car garage, front farmer's porch, sunroom, and deck with tons of square footage.

How Does Ceiling Height Affect What Counts?

The Massachusetts building code defines habitable space using a specific ceiling height requirement: 6 feet 8 inches (see state ADU guidelines). This number alone decides whether your basement counts toward your square footage or not.

At 6'7" or below: The space is considered a crawl space and is exempt. You can use this height strategically to create storage or utility areas that do not count.

At 6'8" or above: The space is now considered a full basement and counts toward your ADU's square footage. For the sake of being precise, this is measured from the concrete floor to the bottom of the floor joists above.

The code language is very specific: gross floor area is "measured from the inside face of the outside walls" and "includes basements of code compliant height." There is no room for interpretation here. If you want an oversized crawl space that does not count, stay at 6'7". Hit 6'8" and you are adding that entire basement to your square footage calculation, giving you a lot less to work with.

What Is the Garage-Under Strategy?

One of the best ways to add usable space to an ADU is by calling the lower level a "garage under" rather than a basement. If you do it this way, it exempts the entire lower level from the square footage calculation.

The way it works is pretty slick. If we install a garage door on the lower level, that space is now technically a garage rather than a basement, regardless of how finished it is inside. We have done builds where the garage-under level includes full insulation, sheetrock, windows, doors, electrical outlets, lighting, and even heating just like the rest of the house. All of it is exempt.

At one of our Plymouth projects, we built a two-car garage under an ADU. The lower level has washer and dryer and also works as a fully finished additional room, but none of that square footage counts toward the ADU's legally allowed size. The homeowners got almost an extra 500 square feet of usable space by going this route.

Adding the garage door simply allows you to designate the space as a garage. It's not necessarily about vehicle use. We position it where it makes sense for you, a lot of times toward a walkout grade. Doing it this way just gives you the exemption. What you do with the space afterward is entirely your business.

Do Sloped Ceilings Reduce Counted Square Footage?

Yes. In areas with sloped or vaulted ceilings, only the area where the ceiling reaches 6'6" or higher counts toward your square footage. Everything below 6'6" is excluded.

This is especially important for ADUs built into attic spaces or with loft type designs. The sloped areas that run along the eaves become bonus space that doesn't affect your square footage limit. You can use these spaces for built-in storage, seating nooks, or any other purpose you would like.

How Does Your Main House Basement Affect Your ADU Size?

The basement height rule can actually help you in multiple ways. For your main house, a full basement works in your favor by adding to your gross floor area and therefore allowing you to build a larger ADU under the 50% rule established by Massachusetts ADU law.

Let's break it down. If you have a 1,200 square foot ranch with no basement, you can only build a 600 square foot ADU. But if that same house has a full basement at 6'8" or higher, even if it's completely unfinished, your GFA becomes 2,400 square feet. Now you can build a 1,200 square foot ADU, twice the size.

The other side to that is that adding a full sized basement to your ADU counts against your allowable size. Which is exactly why we recommend a 6'7" crawl space for ADUs, but also using the garage-under strategy when there is a need for even more finished lower-level space.

What Can Go Wrong with These Exemptions?

Not every one of these strategies works on every property, so we always verify whatever approach you want to take before we commit to a design.

Town interpretation varies. The gross floor area calculation is standard in Massachusetts law, but how it's enforced can be very different depending on your town. Most towns use the 6'7" threshold we have described, including Cohasset and many South Shore communities. However, some building inspectors interpret things a bit differently. We always test out our strategies with local building departments before we finalize plans and designs.

Site constraints limit options. A garage-under works best on sloped sites with walkout potential. Sunrooms require extra lot space. Large porches may not fit within setback limitations. Things like grade conditions, lot coverage limits, and setback rules all have an effect on which exemptions will work for your property.

Verbal confirmations are not the same as permits. Some of these strategies are fairly new, but we continue to validate them with different building departments. We have successfully done garage-under exemptions in multiple towns, but some other exemptions like breezeways could still be in the verbal confirmation stage depending on your town. When something hasn't been successfully proven in a certain town, we let clients know how likely we are to qualify before committing to that type of design.

The best approach is to start with a site evaluation that identifies which exemptions apply to your property before spending money on plans.

Can Creative Room Designations Expand Your Usable Space?

Absolutely. This is where experienced ADU builders really earn their value. You can build a large, comfortable, functional ADU by being specific about what certain rooms are called and how they are used.

If you have an 850 square foot living space with a two-car garage under, a 16 by 25 foot sunroom, a covered front porch, a rear deck, and a mudroom entry: on paper, your ADU is 850 square feet. In reality, your ADU has over 2,000 square feet of usable finished space. By doing this you are not violating the spirit of the law, you are simply using the exemptions that have legally been put into place.

The sunroom has one exception: there can be no heat installed for the final inspection. What happens after the certificate of occupancy is issued is your business. The space is already insulated, finished, and ready for use.

If you want to get an idea of how much usable space is possible on your property, a site evaluation is the first step. We can walk your lot, look at grade conditions, go over setbacks, and identify which of these exemptions will actually work for your specific situation.

Building Bigger Within the Rules

The 50% rule does not have to put strict limitations on your design plans. Sunrooms, mudrooms, garages, decks, porches, breezeways, and strategically sized crawl spaces are all free space and do not count toward your final square footage. The garage-under strategy is especially nice because it adds so much finished space without adding a single square foot to your official ADU size.

Whether you want a simple two-car garage beneath your living space or a more extravagant design using multiple exempt areas to add to your square footage, make sure you understand these rules before you finalize your floor plan.