What Is an ADU? Complete Massachusetts Guide

By Buz Artiano, BuildX | October 2025

Every week, we hear people in Massachusetts ask the question "Can I really build a small home for my parents in my backyard?" The answer used to be no, but everything changed dramatically in 2024 when Governor Healey signed the Affordable Homes Act.

At BuildX, we've built dozens of ADUs since Plymouth passed their own ADU bylaw in late 2023. We built the very first ADU under the new law, and shortly after that we launched a full scale ADU building operation. We've had more than 300 conversations with families, and we didn't expect to learn that ADUs aren't about investment properties or rental income at all. It turns out that most ADU builds are about keeping families together.

What Does Massachusetts Law Actually Say?

The Affordable Homes Act defines an ADU as "a self-contained housing unit, with sleeping, cooking, and sanitary facilities on the same lot as the principal dwelling, subject to dimensional and parking requirements."

Basically if you own a single-family home in Massachusetts, you have the legal right to build an additional complete small home on your property. It just has to be subordinate to the main house and obey zoning setback regulations.

The law was signed by Governor Healey in August 2024 and became effective statewide on February 2, 2025.

What Are the Size Limits for ADUs?

Massachusetts makes it real easy:

• Maximum of 900 square feet
• Or 50% of the main house's gross floor area
• Whichever number is smaller applies to your project
• Height cannot exceed your main house

One thing to remember is that the state law counts basement space toward your square footage. Which means that most ADU projects can't have a full basement: they use a slab or crawl space foundation instead.

What Types of ADUs Can You Build?

ADUs can take several forms depending on your property and goals:

• Attached ADU: Connected to your main house, but has its own entrance. This is a common setup for when aging parents want to be close to family, but keep their independence.
• Detached ADU: A standalone structure in your backyard. Most people we work with like the idea of a detached ADU because they give you separation but still keep family close.
• Garage Conversion: Transforming an existing garage into living space. This is a cheaper option for families who don't really need their garage.
• Basement ADU: Converting your lower level into a separate unit. You have to have two egresses (which can be windows) and a separate entrance, which is usually through a walkout basement.
• Interior ADU: Using existing space inside your home. Doing it this way works well if you already have an older in-law layout with a kitchen and bathroom that you can just expand or upgrade.

Who Actually Lives in ADUs?

After more than 300 conversations with Massachusetts families, a pattern has developed.

• Aging Parents (40%): Mom or dad lives anywhere from ten minutes to many hours away. Maybe one is getting sick or older, or one has even passed away. They probably can't drive at night and they're still in the old family home but can't afford the taxes anymore. The family decides to sell that house and use the money to build an ADU so they can keep everyone close.
• Adult Children (35%): Adult children coming home to save up some money or just get through big life changes. The ADU gives them independence and keeps the family connected.
• Family Members with Special Needs (20%): This is actually how we got started with ADUs. Our first project in Plymouth was for a family friend whose son is nonverbal autistic and was getting too old for his current living situation. Mom called us because she was losing sleep at night worrying about putting her child in a group home.
• Rental Income (5%): There was a lot of talk and worry about ADUs contributing to crowded rental housing, but surprisingly rentals only represent about 5% of ADU use. It's just not profitable with the high cost of construction and interest rates these days.

Can You Rent Your ADU?

Yes, but with one major restriction. Massachusetts law allows you to rent your ADU to anyone: they don't have to be related to you. You can also rent your main house to anyone. Neither one needs ownership in the property.

The only thing you are not allowed to do is use an ADU as a short-term rental of less than 30 days. So that means you can't turn it into an Airbnb-style vacation rental.

What Are the Parking Requirements?

The new law removed a major obstacle for many homeowners:

• No extra parking required if you're within half a mile of public transportation
• If you're not, you only need one additional parking space at the most
• Existing driveways usually suffice
• Tandem parking (one car behind another) is allowed

What Kitchen Options Work for ADUs?

The kitchen decision is one of the biggest choices you'll make:

• Full Kitchen: Full-size appliances, dishwasher, and plenty of counter space. These are best for long-term living and anyone who cooks regularly.
• Efficiency Kitchen: Compact appliances with a 24-inch range and apartment-size refrigerator. Saves space but you can still cook.
• Kitchenette: Mini-fridge, microwave, and hot plate. Works for studios or temporary stays where meals come from the main house.

Massachusetts requires "cooking facilities" but doesn't define size, so you can match the kitchen to actual needs.

Is an ADU Right for Your Family?

Over 550 ADUs were permitted across Massachusetts through the first half of 2025, and it keeps going up. The families building them all have one thing in common: they're keeping family close while maintaining some independence and privacy.