What Is an ADU, and Where Did It Come From?

Written By Paul Vernucci

A plain-language guide to accessory dwelling units: what they are, the century of zoning that made them rare, and the 2025 law that brought them back to Massachusetts.

If you have heard the term "ADU" and nodded along without being totally sure what it means, you are in good company. Accessory dwelling units are suddenly everywhere in Massachusetts, and a law that took effect in February 2025 is the reason. So let's start at the beginning: what an ADU actually is, how these little homes went from ordinary to nearly illegal, and why your town has to allow one now.

What an ADU actually is

An accessory dwelling unit is a second, smaller home on the same lot as a main house. The state's own definition is refreshingly plain: "a small residential living space located on the same lot as another home." It has everything a home needs to stand on its own, so it is a real dwelling, not just a spare room: its own kitchen, its own bathroom, its own entrance, and its own space to sleep.

You may already know ADUs by their older, friendlier names:

  • In-law apartment or in-law suite

  • Granny flat

  • Carriage house or garage apartment

  • Basement or attic apartment

  • Backyard cottage

They come in three basic shapes:

  • Detached: a standalone cottage, often in the backyard or built from a converted garage.

  • Attached: an addition connected to the main house, with its own private entrance.

  • Interior conversion: a basement, an attic, or part of the existing house turned into a separate unit.

Whatever you call it, and whatever shape it takes, the idea is the same. One more home, quietly, on land that already has one.

A quick history: they used to be normal

Here is the part that surprises people. ADUs are not a new invention. For most of American history, a second unit on a lot was completely ordinary.

Walk through an older New England neighborhood and you can still see it. Carriage houses behind the main home, apartments above detached garages, a small unit carved into a big Victorian for a boarder or a widowed parent. Before the 1920s, towns and cities were full of these arrangements, because families needed the flexibility and nobody had written a rule against it.

Then zoning arrived. New York City adopted the first comprehensive zoning code in 1916, and in 1926 the U.S. Supreme Court blessed the idea of dividing towns into single-use districts. After World War II, single-family zoning spread across the country as suburbs boomed. Town after town rewrote its rules to allow exactly one house per lot, and federal lending programs favored the single-family model. The accessory unit that had housed a grandparent, a boarder, or a young couple starting out was quietly zoned out of existence.

For a few decades, building one was often illegal in the very neighborhoods where they had always been common.

The nickname "granny flat" tells you what kept the idea alive. By the 1980s, as the population aged, advocates for older adults pointed out that a small unit near family was one of the best ways to let a parent age close by while keeping some independence. That practical, human need never went away.

The real turn came with the housing shortage. As prices climbed through the 2010s, states started looking at all the homes their own rules had made impossible. California led the way, easing its ADU rules between 2016 and 2020, and ADU construction there exploded almost overnight. Oregon, Washington, Vermont, Maine, and others followed. The pattern was consistent: when a state made ADUs legal again by right, homeowners built them.

Massachusetts joins in: the 2025 law

Massachusetts made its move through the Affordable Homes Act, signed in August 2024. The ADU section took effect on February 2, 2025, and it changed the rules in every single-family neighborhood in the state.

Here is what it means in plain terms:

  • By right. In single-family zones, a town cannot make you win a special permit or discretionary approval to use or rent an ADU. It is a protected use, not a favor you have to ask for.

  • Size. An ADU can be up to half the gross floor area of your main house, or 900 square feet, whichever is smaller.

  • No owner-occupancy rule. Your town cannot force you to live in either the main house or the ADU, and it cannot require that the person in the ADU be a relative.

  • Parking. A town can require no more than one parking space for an ADU, and none at all if your property is within a half mile of a commuter rail station, subway station, ferry terminal, or bus station.

  • A separate entrance. The unit needs its own way in, either directly outside or through a shared hall.

That is a genuine shift. A decade ago, many of these projects would have meant a zoning board hearing and a coin-flip vote. Today the use itself is protected across the state.

The catch worth knowing

"By right" does not mean "no rules." Your town can still apply reasonable, evenhanded standards: setbacks, height limits, the building code, and health requirements like septic capacity. Towns can also set limits on short-term rentals. And the details still vary a lot from one town to the next, because each community adopted its own local ADU bylaw on top of the state floor.

So there is a real gap between what the state now allows and what actually gets approved in your specific town, on your specific lot. That gap, the difference between the law on paper and the permit in your hand, is where most homeowners get stuck. It usually comes down to a handful of questions: How does your town's building, health, and planning process actually work? Can your lot and your septic system handle it? What will it realistically cost, and how long will it take?

Why this matters for you

Most people come to an ADU for a very personal reason. A parent who should not be living alone anymore. An adult child who cannot afford a first apartment nearby. A little rental income to steady the budget. A quiet office or studio in the backyard. The 2025 law finally makes those options possible in neighborhoods where they were off the table for years.

The possibility is the easy part now. The process is the hard part. Knowing which questions your town will ask, and in what order, is the difference between a smooth project and an expensive false start.

That is exactly the work we do at Together Home. We are an independent guide for Massachusetts homeowners, working for you, not for a contractor, an architect, or a lender. If you are wondering whether an ADU makes sense on your property, the fastest way to find out is a free 30-minute consultation, and our $995 feasibility study gives you an honest, insider read before you spend real money on design or construction.

Thinking about an ADU? Book a free consultation and let's talk through your property.

Paul Vernucci