What's the Maximum Size ADU You Can Build? The 50% Rule Explained

By Buz Artiano, BuildX | February 2026

One of the first questions we hear from Massachusetts homeowners is about what size they can build their ADU. They want to know exactly how big it can be before they start looking at floor plans and designs. We have built dozens of ADUs here in Massachusetts, so we totally get why this is one of the first things that people ask. It's because size determines everything! It's how you know whether you can fit one or two bedrooms, whether your parents or family member will have enough space, or if you'll be able to rent it out for a decent profit.

The problem is that the answer is a lot of math that most people have never come across. The Massachusetts ADU law uses something called "gross floor area", and making sure you understand the math here can mean the difference between a very 600-square-foot unit and a spacious, comfortable 900-square-foot home. We have spent a lot of time educating building inspectors on what the law actually says, so we know exactly how confusing this particular subject can be.

At BuildX, we specialize in ADU construction in Massachusetts. We use 3D scanning technology to create detailed floor plans and gross floor area calculations, which we attach directly to building permits. This ensures there are no disputes or issues with building departments and it helps make sure homeowners get the largest size for their property.

What Is the 50% Rule and How Does It Work?

The Massachusetts ADU law makes it pretty simple: your ADU can be 50% of your principal dwelling's gross floor area, up to a maximum of 900 square feet. If 50% of your home equals 700 square feet, that is as big as you can build. If half of your home is 1,100 square feet, you can still only build up to 900, because 900 square feet is the maximum.

For example, a 1,400-square-foot home means you can build a 700-square-foot ADU. A 2,000-square-foot home allows a 900-square-foot ADU (even though half of that would be 1,000). To make sure you have the right size for your ADU, it's important to calculate your existing home's gross floor area correctly.

How Do You Calculate Gross Floor Area?

Gross floor area is measured from the inside face of the outside walls, not the exterior. The difference matters on smaller homes because you pick up approximately six inches all the way around when measuring inside dimensions. The law is also very specific about basements: gross floor area includes basements with a height of 6 feet 8 inches or higher from the concrete floor to the ceiling or to the bottom of the floor joists.

Think about a 1,200-square-foot ranch. Without a basement, you could only build a 600-square-foot ADU. But if you have a full basement with a high enough ceiling, even if it's completely unfinished, your gross floor area becomes approximately 2,400 square feet. Now you qualify for the full 900-square-foot ADU.

Does Your Basement Count Toward Size Calculations?

Yes, and this is where the calculation becomes a double-edged sword. Your existing home's basement counts toward GFA if it meets the 6 foot 8 inch height requirement, which benefits you by increasing your allowable ADU size. However, when you build your ADU, most towns will count a full basement toward the ADU's gross floor area as well.

What Spaces Are Exempt from Square Footage Limits?

There are certain spaces that do not count toward your ADU's gross floor area, which lets you build a larger and more functional unit while staying within legal limits. Exempt spaces include sunrooms, mudrooms, garages, decks, porches, and breezeways. Even heated mudrooms are exempt.

For loft or attic spaces, the ceiling has to be 6 foot 6 inches high in order to count towards the gross floor area. In a space with a sloped ceiling, only the area where you can stand upright counts toward GFA. By understanding these restrictions and being strategic about how you utilize the rooms, you can create a very spacious ADU while technically staying within the size limits.

Can Any Massachusetts Towns Exceed the 900-Square-Foot Limit?

Yes absolutely. Several Massachusetts towns have already passed local bylaws that let you build a larger ADU through a special permit processes. Plympton actually allows up to 1,200 square feet. Norwell has now approved 1,300 square feet. Bourne lets you build up to 1,500 square feet, and Cohasset offers an ADU-L (large) category where you can request larger sizes through the planning board.

These laws still have to be approved by the Massachusetts Attorney General's office after passing town meeting votes. This tells us that more cities and towns will do the same, especially as demand for two-bedroom ADUs with lots of space for aging parents continues to grow.

Is 900 Square Feet Enough for Your Needs?

For most uses, yes. A well-designed studio or one-bedroom ADU is usually between 500 and 750 square feet. A one-bedroom with a more comfortable layout ends up being 700 to 750 square feet for the most part. Two bedrooms can fit within 800 to 900 square feet, it just requires some creative design work.

We should be honest: 900 square feet can feel a bit cramped for a two-bedroom, two-bath layout that some families need. For instance when housing elderly parents with different sleep schedules and personal needs. If your situation requires more than 900 square feet, a special permit in a participating town or alternative housing arrangement may be the best way to go.

One thing you should definitely consider is that the cost difference between an 800-square-foot and 900-square-foot ADU is really not much. Once we have installed the kitchen, bathrooms, HVAC system, and utility connections, adding an extra 100 square feet really only means more siding, roofing, and probably an extra window. The most expensive infrastructure costs are there regardless of whether your ADU is 700 or 900 square feet.

What If Your Building Inspector Questions Your Calculations?

Building inspectors can sometimes interpret GFA rules differently. It turns out that we spend about 25% of our time with building departments educating them on what the law actually says. The law is very clear: gross floor area is measured from the inside face of the outside walls and includes basements of code-compliant height. Simple.

The most effective approach is providing documentation upfront. We use Matterport 3D scanning technology to create detailed floor plans with very precise gross floor area measurements. These are what produce those virtual tours you see in real estate listings, along with engineering-grade floor plans. We attach this documentation directly to building permit applications. We have found that this usually ends any calculation disputes before they start.

Is Maximizing ADU Size Always the Right Goal?

We specialize in helping homeowners build the largest ADU their property allows, so you might expect us to always push for maximum square footage. But the truth is, bigger is not always better. Some families do better with a smaller, more efficient design that costs less to build and maintain. If your property has significant site work challenges, a compact 600-square-foot ADU might make more financial sense than stretching to 900. We are always honest about when a smaller design serves you better.

Ready to Find Out What Size ADU Your Property Allows?

Understanding the 50% rule and gross floor area calculation is how you are going to know what is possible for your project. Whether you have a compact design with a full basement or an open ranch without one, your specific measurements will determine what your options are.

If you want to explore ADU possibilities for your Massachusetts property, we offer site assessments that include precise GFA calculations using 3D scanning technology. You will know exactly what size ADU you qualify for before investing time in design concepts.