What Doesn't Count Toward Your ADU's Square Footage in Massachusetts? (Garages, Porches, Breezeways, and Basements)

By Buz Artiano, BuildX | July 2026

Watch: How Much Does It Really Cost to Build an ADU in Massachusetts, The BuildX Way

The thing that sometimes stops families from going ahead with an ADU is the 900 square foot number. When you hear the state limit on size, you picture a small studio, and may decide there isn't any way to fit a real second home, a place for a parent, or a unit worth building into that space. If you have looked at that allowance and thought the ADU will simply be too small to live in, you're asking the right question. You are just measuring the wrong thing.

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 do the design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We evaluate septic constraints, zoning issues, utility connections and the requirement to get a permit from the city. We've been building under the state's by-right ADU law since it took effect in 2025, and one thing has stayed the same: the 900-square-foot number is measured in a certain way and almost nobody outside the permit process knows the rules.

Here's what changes the math. Your 900 square feet is 900 square feet of finished living space. A garage, a porch, a deck, a breezeway, and an under-height basement don't count against it. Once you know what is counted, the 900-square-foot ADU doesn't seem like a studio and seems more like a full home with two-car garage and a sunroom. What matters isn't the limit. It's how you measure and where you put the space that doesn't count.

What Doesn't Count Toward Your ADU's 900 Square Feet?

Let's begin with the rule that changes everything. The 900-square-foot cap only applies to gross floor area, which is the full-height living space inside the unit, measured at 6 feet 8 inches or taller. Anything that's attached to the ADU that's not that kind of space doesn't apply toward the 900 square feet. In Massachusetts the exempt list is always the same.

• Attached and under garages don't count. You can add a one-car or two-car garage without using even one foot of your allowance.
• Exterior decks don't count, so an outdoor living space off the back doesn't add anything to your square footage on paper.
• Covered porches and farmer's porches don't count, and that means you can build a real front entry and a covered outdoor room.
• Breezeways don't count in most cases, so the connector between a garage and the living space is usually free space.

If you put all those together, the 900-square-foot unit changes completely. One of the plans we build is a 900-square-foot, two-bedroom ADU with an attached two-car garage and a sunroom off the back. On paper it is 900 square feet but in real life the family has a garage, a sunroom, two bedrooms, and full living space. That is a very different home than the studio most people picture when they hear the cap.

These exemptions come out of the state's by-right ADU law, and how each one is applied is a regulatory question, not our opinion. You can review the state's ADU requirements at mass.gov, and we confirm the specifics with your town on every permit, because interpretation still varies locally.

Here is the short version of what lands inside the count and what sits outside it, based on how we measure gross floor area on our projects.

Why Does Your Assessor's Card Make Your Home Look Too Small to Qualify?

How large an ADU you can build is tied to your home's gross floor area, and this is where most homeowners undercount their own house. The instinct is to pull the square footage straight off the assessor's card. Do not do that. The assessor's number often leaves out space that legally counts toward your home's floor area, and the biggest omission is almost always the basement.

Here is the rule we use on every measurement. If your existing home has a basement that is more than 6 feet 8 inches from the concrete floor to the bottom of the floor joists, that space counts as part of your primary house's gross floor area, finished or unfinished. So a 1,000-square-foot ranch sitting on a full-height basement is not really a 1,000-square-foot house for this calculation. Measured from the concrete walls, it carries roughly 1,900 square feet of gross floor area. That larger number is what can move a modest ranch from a small allowance up to a full 900-square-foot ADU. The exact size the state allows is set by that same ADU law, which you can read at mass.gov, and your town applies it to your address.

There is a limit to the rule, and it matters. If your home sits on a slab, this does not apply. If you have a crawl space, this does not apply. The basement has to be full height. So before you assume you only qualify for a small unit, go down and measure that basement from the concrete to the joists. That one measurement can change your entire project.

The threshold works in both directions. On a recent Westfield unit, the ADU's basement measured six and a half feet, just under the line, so it did not count as living space. Under 6 feet 8 inches, it is exempt. Over it, it counts. The tape measure decides. We measure this ourselves on every project rather than trusting any card, because towns still argue about how to read the number, and winning that argument is part of the job.

How Big Can a 900-Square-Foot ADU Actually Live?

Now put the exemptions and the measurement together, and you don't have to worry anymore. You were afraid that 900 square feet is too small for a parent, an adult child, or a rental home. But that 900 square feet of living space is just for the inside. You can still add a two-car garage, a covered porch for a real entry, a deck for outdoor space, and a breezeway to connect everything together, and none of it counts towards your 900 sq ft.

With 900 sq ft we usually lay out two bedrooms, a full bathroom, a real kitchen, and a living area, with extra room for accessibility modifications later on. Whoever moves in gets a private home with tall ceilings, not a converted garage. If you use it for a rental, it's a legitimate two-bedroom unit. The cap on square footage doesn't determine how the home feels when it's finished. What it told you is that your 900 square feet is just the living space, and everything that makes a house feel like a real house doesn't count against you.

That's why the sizing really has more to do with the design than actual size. Once you know what's exempt, you're not so worried about 900 square feet being enough, instead you start planning out where to put the garage, the porch, and the deck so the place feels as big as possible.

Does Every Town Apply the Exemptions the Same Way?

The seminars and the free internet plans won't tell you that the exemptions and the measurement rules come from the state's by-right ADU law, but towns still interpret gross floor area differently, and the law is brand new, it's only been around for a year and a half. We've been in site plan review meetings where all they talked about was gross floor area, even though gross floor area isn't even part of the site plan review. We've had one town grant an exemption for an interior stairway in a two-story plan, which was entirely up to them, it doesn't mean you'll get the same exemption. Breezeways are exempt in most cases, but again, it's on a case by case basis, it's not guaranteed.

So to be honest, you shouldn't base your whole plan on an exemption unless you've checked and confirmed with your town that you will get that exemption. Building code, health code, and the rules for setbacks and height can still be applied by the city even if the ADU is allowed by right. There are real exemptions but they are approved by human beings, not a computer, so the answer can change from one building department to the next. So we manage all of that for our clients, and it's worth getting in writing before you commit to a plan.

Find Out How Large Your ADU Can Actually Be

The limit isn't just the 900 sq feet you know of. Your 900 square feet is only interior living space, and the garage, porch, deck, and breezeway around it are not counted in it. The only number that applies now is your home's real gross floor area, so we'll measure that first.