Can I Build Both an In-Law Suite AND an ADU on the Same Property in Massachusetts?

By Buz Artiano, BuildX | June 2026

Watch: ADUs, Nursing Homes & Inheritance: What Families Get Wrong | Atty. Brian Barreira

You've been researching ADUs for weeks and maybe you already have a parent living with you, or are thinking about building an addition for them. But the question is if you build something now, do you still have the option to build a separate ADU later? Most homeowners think the answer but most homeowners are wrong.

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. Understanding how local in-law bylaws work with the state's by-right ADU regulations is part of what we do on every project. It is one of the most valuable conversations we have with families at the start of the build.

This article talks about the two-build strategy that most homeowners don't know exists: what to call your first addition as an in-law suite to keep your future ADU rights, which towns make this possible, and what the real version of this approach is like on a completed project.

Can You Really Have Both an In-Law Suite and an ADU on the Same Lot?

Yes. The way we put it: you can have an in-law and an ADU. That is a fact most families never hear because most builders never bring it up. The key is understanding that Massachusetts zoning treats in-law suites and ADUs as separate categories under separate bylaws. An in-law suite permitted under your town's in-law bylaw does not count as your one ADU allowed under the state's Affordable Homes Act. That means a homeowner who builds an in-law suite first can still exercise their right to build a detached ADU later, as long as the property comes up to the standard requirements for lot size, setbacks, and the capacity of the septic.

This isn't a loophole, it's how zoning classifications work in Massachusetts. The state's by-right ADU law, part of the Affordable Homes Act, lets you build one ADU for each single-family lot. A structure that gets a permit as an in-law suite under a town in-law bylaw is a separate classification completely. You can review the state's ADU requirements at mass.gov.

What Is the Dual-Build Strategy and How Does It Work?

The dual-build strategy is simple but it does need to be planned from the start. When a family comes to us and wants an attached addition for an aging parent, we look at two things before we decide how to classify the project. One is the town's in-law bylaw and the other is whether or not the size of the lot is big enough for a detached structure.

If the town has a favorable in-law bylaw that doesn't have restrictions like three-year look-back requirements or rules about how many people can live in the ADU, we suggest calling the attached addition an in-law suite instead of an ADU. The in-law classification does two things. First, it gives the family the attached living space they need right now. Second, it keeps the family's right to build a detached ADU on the same property in the future.

The other option is to classify the first build as an ADU. That works for families who don't have any interest in a second unit. But for families who want to keep their options open, like for rental income, a second family member, or long-term property value, using the in-law classification first is the smart thing to do.

We learned about this from dealing with lots of town codes and saw a pattern. In towns where the in-law bylaw is easy to understand and has no excessive rules, no long look-back periods and no sunset rules, then as a first step it's better to classify it as an in-law suite. The family will have everything they need structurally while keeping the ADU idea for future use. That is a decision most builders discuss because they are not thinking about what happens five or ten years after the first build is done. We are.

What Does This Look Like on a Real Property?

We just finished a project in Pembroke that was built just this way. A mother and daughter wanted to live on the same property. The daughter owns the main house, and the mother needed her own living space, connected but independent.

We built an attached in-law suite connected to the main house by a 12 to 14 foot breezeway. The mother lives in the in-law suite, and the daughter lives in the main house. They can go between the two without going outside. The breezeway gives separation and privacy but keeps the family physically close.

In this case, the property had enough square footage to build a 900-square-foot detached ADU as well. The family chose the attached in-law suite first because it filled their immediate need. But because we classified it under the town's in-law bylaw instead of the state's ADU law, they were able to keep the right to build a detached ADU on the property later on.

Here is something that matters: Pembroke's in-law bylaw was favorable. It didn't have the restrictive conditions that some towns have put in so that made it the right classification for this project. Not every town has a simple in-law bylaw, and that is exactly why this decision should be made before you get your permit, not after.

Does the Breezeway Count Toward My ADU Square Footage?

This is a question that the families ask often, and the answer is important for planning. When an ADU is considered an in-law suite by municipal bylaw, the breezeway and all the area that's connected counts toward the area that is allowable in the in-law's allowable space. That is actually an advantage. The breezeway space then becomes a usable, counted space under the in-law classification.

Under the state's ADU law, the size limit is generally 900 square feet or 50 percent of the main home's gross floor area, whichever one is smaller for detached units. But that limit applies just to the ADU classification, not the in-law classification. So by building the attached unit as an in-law first, the family avoids having the breezeway go toward the ADU's allowable square footage in the future.

This kind of thing is why the classification decision has to be made early. If a family builds an attached ADU with a breezeway, they may have already used their ADU allowance. But if they build an in-law suite with a breezeway, that square footage will be under a totally different category and they can still build an ADU later on.

Does This Strategy Work in Every Massachusetts Town?

No. This plan depends on the quality of your town's in-law bylaw. Some towns have favorable in-law bylaws with simple to understand rules. But others put in restrictive conditions that make the in-law classification not practical or appealing.

The restrictions to look out for are a three-year look-back provision that needs proof of family relationship over a long period of time, a sunset clause that cancels the in-law designation when the person that qualifies moves, and occupancy restrictions on who can live in the unit. Towns that have these sorts of restrictions might cause families to go with the ADU classification instead, even for an attached addition.

The first step is always to check your town's in-law bylaw and compare it against the state's ADU law. In our experience, towns on the South Shore and in Plymouth County can be very different. Some have bylaws that make this plan easy. Others make it difficult. That research should be done before you decide on a design, because the classification affects everything from permits to future property rights.

Is the Dual-Build Strategy Right for Every Family?

No, and we won't recommend it to every family that comes to talk to us. If you don't have an interest in ever building a second unit on your property, this plan just adds complications with any benefit. Classifying your addition as an ADU might be easier in your town, and easier is usually better when you don't need the flexibility.

This strategy is best for families who want to keep their options open for the future. For instance, maybe you are building for a parent now but think your adult child could need housing in five years. Maybe you want to have a possible income from a rental in the future or you're just protecting the resale value of the property by keeping your right to add a second unit.

If none of these things apply to you, the standard ADU classification works fine. We bring this choice up because most builders don't, and families deserve to know it's a possibility before they choose a permit path they can't undo.

Your Lot May Have More Potential Than You Think

The difference between building one addition and keeping the option to build two all depends on which classification you choose before you get your first permit. Most families never learn about this option. Now you have the information so you can ask the right questions such as does your town have a favorable in-law bylaw, and does your lot have enough space for a detached unit in the future. You can then decide if the dual-build plan is worth working on for your family's long-term goals.