What Do You Do When Your Town Says Your ADU Has to Be Smaller Than 900 Square Feet?

By Buz Artiano, BuildX | August 2026

Watch: How Peg and Jim Duggan Are Staying in the Home They've Loved for 42 Years

You walked into town hall with a plan for a 900-square-foot ADU and walked out with a much smaller plan. Nobody really explained the math to you, they just told you what your property was allowed to support. So the space you were counting on for a parent, for an adult child, or for yourselves has now shrunk. Now you have to decide whether it's even worth it to build if the space is going to be so small.

My name is Buz Artiano, founder of BuildX. We've built dozens of ADUs all over Massachusetts, but most of our jobs have been on the South Shore and in Plymouth County. We do everything, the design, permitting, and construction. That's important here because we are very familiar with size limits, Title 5 septic capacity, lot coverage limits, setbacks, and by-right permitting. Those things are what we take care of first because they decide what can be designed and built on your property.

Peg and Jim Duggan are our clients in Rockland. We're building a 900-square-foot ADU on their property for the two of them to live in, but the original plan wasn't 900 sq ft. Jim was told he could only have, in his words, "X amount of space," which was much smaller than the 900 sq ft. We brought in someone who mapped their house out, and we were able to finally get the town to accept the full 900 square feet. Jim calls it "a long, long fight." It was 12 months before he got the call telling him everything had gone through.

We always love to tell stories of people that are happy and everything worked out, but we're using this story specifically because the methods and the details matter more than the ending. A town's opening figure is a statement about your existing house, but that can always be double checked.

Is Your Town's First Number Actually Final?

No. A size determination is a reading of the rules applied to one property, and readings get reviewed. The Duggans are proof of that. Rockland told them the unit had to be smaller than they planned. The project we are building is 900 square feet.

Review is possible because the allowed square footage is based on a specific mathematical formula, it has nothing to do with opinion. Under the Affordable Homes Act, an accessory dwelling unit qualifies as a protected use when it's no larger than half the gross floor area of the main house or 900 square feet, whichever is smaller. It also has to meet local requirements, which are allowed to include additional size restrictions. But you can read all the details on the state law at mass.gov.

So that could go two different ways. If the smaller figure came from the calculation of your house's square footage, it can always be double checked. If it came from a size restriction your city or town adopted, it is the rule and you just have to work around it. Figure out which one you are dealing with before you make major changes to your plan.

Where a Number Smaller Than 900 Square Feet Comes From

The state has two options that apply depending on your situation. They can either allow you 900 square feet or half the gross floor area of the main home. On a large house, the 900 is usually used. On a smaller main house, the half calculation is used and can be less than 900. That's because of the recorded size of the home you already own.

There's a separate set of local rules in addition to that. Massachusetts allows cities and towns to impose their own additional requirements in these areas:

• Site plan review, a municipal check on where the unit sits on the lot
• Title 5 compliance, the state septic code that governs the wastewater flow your system is approved to handle
• Dimensional setbacks and bulk and height limits, the local rules on where a building can stand and how large it can be

Those are legitimate requirements, and they are not the same thing as discretionary approval. A city can't require a special permit or zoning approval for the use or rental of an ADU. The state is very clear about this, you can get all the details at mass.gov.

So a number below 900 comes from the size of the main home, an additional local size restriction, or a dimensional limit on where a structure can fit on your lot. Those are three different problems with three different answers, and only the first one is settled by measuring the main house.

What Changed the Town's Answer in Rockland?

Jim's story is short and simple. He was told that his ADU could only be a certain size. We brought in a person who mapped the house and then the town let him have 900 square feet. The Duggans didn't have to be involved in the middle part of that process, which is exactly why they hired us. Their story just tells us the order of things, it's not a guarantee of anything. But for them the process was they got a number, then a measured record of the existing house, then a different number.

We settle the maximum size issue before we start designing, and this is why. The half calculation runs off the gross floor area of the main home, so the ADU you're allowed to build is determined by the size of a house that was built a very long time ago before ADUs were even thought about. When the number they are using for the calculation goes back to an old record rather than how the home is today, the maximum size calculation for your ADU can be off. So make sure you get all this double checked before you design a floor plan around a limit that isn't even correct. We would rather spend extra hours up front confirming all this info than spend those hours drawing the same ADU twice.

We can't promise you that your town will make the same decision that Rockland did, because that wouldn't be accurate. Every municipality keeps its own file on your property and applies its own local rules on top of the state law. The Duggans' project just lets us know that the question is answerable, and you can get it answered before you commit to a smaller ADU.

What Does the Fight Cost You in Time?

About one year. Jim said that's how long it was between the start of the process and the call telling him it had all gone through, so that's what you want to plan for when you make your decision. A size dispute costs time, not money.

You are the one who decides what that time will actually cost you. Maybe your parents are struggling to manage alone at home right now. There could be a rate lock with an expiration date on it in the picture. Or maybe you wanted to break ground in a certain season. That 12 months is one project's record, but it's not a guarantee, and every town is different. Weigh the decision against the wait time, not the conversation you have to have with the town, because that part only takes an afternoon.

The upside is that this is one of the few issues on an ADU project that resolves early. A size determination can be settled long before construction, or a deposit on cabinets, and before your family reorganizes their life to accommodate a certain move-in date.

Who Should Be Carrying the Fight With Your Town?

Peg puts the answer plainly. "We didn't have to go through this whole process. We just handed it over to them, and they did it." She also said there was "alleviation of stress on us, too." Jim was glad that everything was being taken care of since the minute they signed, on most anything the town pushed back on. Their contact through everything was Michael Fleck on our team. Peg says he picked up the phone every time they called, and got whatever answer they needed.

Peg's advice to anyone in her same position is "Get somebody who knows what they're doing and can help you with this. Because trying to do it alone wouldn't be a good thing." A homeowner walking into a size dispute alone is having a very difficult technical conversation with a department that does the same thing almost daily, it's much better to have someone with experience have that conversation on your behalf.

Whoever is having that talk will need access to what the town already has on file for your property, a measured record of the existing house, and the patience to work a long calendar without handing any of the issues over to you. Ask any builder you interview what happens when a town pushes back. If they tell you that you will be the one calling the building department, now you know what you'll be dealing with.

When Is the Smaller Number the Right Answer?

We build ADUs, and a larger unit is a larger project for us, but that doesn't change the fact that sometimes the town is right.

The number the town gives you is not negotiable when the half calculation is double checked and really is below 900, or when your city or town has its own size restriction, and when setbacks, buildable area, or septic capacity decide how big it can't be before the zoning department ever even gets a vote. Building code, health code, and local dimensional requirements all still apply. By-right status only governs the use of the building. It doesn't override the other laws.

Sometimes the smaller unit is just the better decision for other reasons. If 700 square feet holds the layout your family needs, then getting the permit and building in the next couple months beats 900 square feet being built over a year from now. The reason to check the number is not to win an argument with your town. It's so you can decide what your plan should be once you know the numbers for sure.

What to Do Before You Redraw Your ADU Plan

The decision in front of you is not whether to fight against your town or not. It's whether the number you were handed came from your house or from your town's record, because that determines what your next step will be. Peg and Jim Duggan got the full 900 square feet in Rockland after their house was mapped, and it took 12 months of somebody else's patience to get there.

Find out which number you are actually dealing with before you shorten the plan around it. That answer takes a records check and a measurement, not another year.