What's the Maximum Size for an ADU?

By Buz Artiano, BuildX | November 2025

One of the first questions we usually get when people are thinking about building an ADU is about size. They want to know how big they can actually build their ADU. The answer is important because it will decide things like whether or not you can fit two bedrooms for aging parents, whether the space will be big enough for a comfortable rental unit, or whether you can just simply build something that will increase your property value.

At BuildX, we have helped many Massachusetts homeowners understand the state's rules about the size of ADUs. We are very familiar with all the rules because every week we work with building departments across the state of Massachusetts from the South Shore up to Cape Cod. So we know how the calculations for amount of total floor space can make or break a project, and we also know exactly how to help get the most out of your ADU's square footage and at the same time stay within all the regulations.

The good news is that most homeowners can build a bigger ADU than they think. The 900 square foot number is talked about a lot, but there are clever ways to maximize space that a lot of builders simply don't think of. We can show you exactly how size limits work and how to get the most from your ADU.

How Does Massachusetts Calculate Your Maximum ADU Size?

Under Massachusetts law, your ADU can be the smaller of two choices. Either 900 square feet or 50% of the main house's gross floor area. The state is very specific about this: they use a calculation called GFA (gross floor area) that many homeowners and even some building inspectors don't quite understand.

GFA is measured from the inside of your exterior walls, not the outside. This actually works in your favor because you are measuring a slightly smaller area. On a small structure, measuring from the inside can gain you six inches all the way around.

Here is where it gets interesting. Your existing basement counts toward your main home's GFA as long as the ceiling height is 6'8" or higher, even if the basement is completely unfinished. A 1,200 square foot house with a full basement could have a GFA of 2,400 square feet, which would let you build the full 900 square foot ADU instead of being limited to 600 square feet. If you want all the details on the state requirements, see the Massachusetts ADU regulations.

What Spaces Don't Count Toward Your ADU's Square Footage?

The state law does not include all spaces in the GFA calculation, which means you can build a bigger ADU with more space than the GFA limit says. Understanding these exemptions allows you to get the most out of your investment.

Exempt spaces that do not count toward your ADU's GFA:

• Sunrooms
• Mudrooms
• Garages
• Decks and porches
• Breezeways

By being strategic about how rooms are designated and used, you can build an ADU that feels much bigger than its square footage suggests. A 900 square foot ADU with an attached sunroom, mudroom entry, and covered porch provides much more living space and doesn't count towards your allowed square footage.

Why Does Crawl Space Height Matter for ADU Size?

The same 6'8" basement rule that helps you calculate your main house's GFA as higher can actually work against you on your ADU. If your ADU has a basement or crawl space with ceilings at 6'8" or higher, that space counts toward your ADU's GFA.

But we have come up with a work around. We build ADU crawl spaces at 6'7" or lower. Which gives you access space for mechanicals and storage without counting towards your allowed square footage. One inch makes the difference between a 900 square foot ADU and one that is technically much smaller.

We have worked through these calculations tons of times. The benefit of counting your main home's basement toward GFA outweighs the downside of it counting toward the ADU side. Because of the basement law most clients with 1,400 square foot homes and full basements can now build a two bedroom ADU that would have been impossible if they had been limited to a 700 square foot limit.

Can You Build Larger Than 900 Square Feet in Some Towns?

Yes, there are several towns in Massachusetts that have passed local bylaws which let you build ADUs that are bigger than 900 square feet which is the state's limit. This is through a special permit process.

Towns with expanded ADU size limits:

The normal 900 square foot or 50% calculation still applies as a right. If you want to build an ADU that is bigger than 900 square feet, you will need to get the town planning board to approve it. And we always check your town's specific bylaws before we start any design work.

What Can You Fit at Different ADU Sizes?

The square footage of your ADU will decide what your layout options are. Based on our experience building ADUs in Massachusetts, here is what you can realistically fit in each of the different size ranges.

Around 500 square feet: This will be for a studio or one bedroom. It is an open floor plan with the living, dining, and kitchen area combined and one full bathroom. This works well for a single occupant or a couple who do not need any space for a guest.

600 to 700 square feet: This will be a comfortable one bedroom ADU with a separate enclosed bedroom space. There is plenty of space for a full living room apart from the bedroom. In some designs that are 700 square feet, you can fit two small bedrooms, but the living area would be an open kitchen/living room without any separate dining space.

800 to 900 square feet: With this square footage two bedrooms are possible. There's enough space for one and a half baths too. It will have an open kitchen and living area with room for a small dining table. This size works well for aging parents, adult children, or it makes a great rental option.

How Do We Verify Your Maximum ADU Size?

GFA calculations have to be accurate because building inspectors will check all the measurements during the permit process. About 25% of our time that we spend with building departments is helping to understand what the law actually says. The calculation seems like it is very straightforward, but some inspectors still interpret it in different ways.

We use Matterport 3D scanning to create these detailed floor plans of your existing home. The scanner gets exact measurements and gives us a GFA calculation we can attach to your building permit application. This usually ends any argument about what size ADU you are allowed to build.

The scanning process takes about an hour and produces the exact paperwork the building department needs. We have found this approach saves weeks of back and forth with inspectors who might question our measurements and calculations.

Is Maximizing Square Footage Always the Right Goal?

We specialize in helping people figure out how to build the largest ADU their property allows. But bigger is not always better for every situation.

When a smaller ADU might make more sense: If you have a smaller lot, a compact ADU design may be easier to permit and build. If your budget is limited, a well-designed 600 square foot ADU often makes more sense financially than trying to build one that is 900 square feet. And if the occupant's needs are simple, a studio or one bedroom might be all they really need, without there being unused space to heat and maintain.

The cost difference between 800 and 900 square feet is not much because the bathroom, kitchen, electrical, and mechanical systems are already figured in. But the cost difference between a 600 and 900 square foot ADU is quite a bit more. We help you find the size that matches your needs, your budget, and your property's specific situation.

Your ADU Size Starts With Your Property's GFA

Every ADU project begins with understanding the numbers. Your main home's gross floor area, your basement's ceiling height, and your town's local bylaws together determine exactly how large your ADU can be.

Read our 2026 ADU Buyer's Guide to start gathering the information you will need for your project.