The Popularity of Accessory Dwelling Units

Do you worry about an elder parent living on his or her own? Do you have a child who has graduated from college but doesn’t have the financial means to rent an apartment in your community? Are you looking to add a rental unit to your property?  The solution in cases like these might be an accessory dwelling unit (ADU).

You might know ADUs as “accessory apartments” or “in-law apartments.”  Particularly in a region like Greater Boston, where affordable housing is increasingly scarce, the appeal of ADUs is obvious. Over the past few years, communities that Custom serves have loosened restrictions on ADUs, making them a viable option for more of our clients. But in February 2025 a new state-wide law went into effect making it even easier to build ADUs.

Could an ADU be right for you? Read on to learn more.

This ADU was built as an in-law apartment attached to a home in Lexington. The main living area includes a compact, but full kitchen, and a living room.

Staging a Comeback in Response to Housing Costs and Scarcity

“Accessory apartments were fairly commonplace before World War II, when extended families were more likely to live together,” according to a November 2017 article in The Boston Globe. “But they fell out of favor — or were zoned out of existence — during our postwar obsession with single-family houses and sprawling lawns.”

Now that the laws are changing, homeowners are seeing what an opportunity an ADU could provide. A common scenario where an ADU might come into play is when homeowners are looking to take care of an elderly parent. In these instances, the parent might be selling his or her home and putting the money toward construction of the unit. Having the parent living close by can give the homeowners peace of mind while also offering all parties a measure of privacy.

Or, take a child who is ready to move out but can’t afford rent in a community like Arlington or Lexington. For example, she might have graduated from college and has just started working but now has to pay off $80,000 of student loans. Living rent-free in an ADU can help her pay down her debt while enjoying the independence that most 22-year-olds need.

In other cases, the homeowners simply want extra space for an au pair or guests. Or, they might be empty nesters who want to have an ADU to rent out. This can provide them with income that lets them stay in their home longer. It also creates the possibility of moving into the new unit later in life and renting out the main house.

The same Lexington in-law apartment is featured above and below. The bathroom includes safety handrails in the shower and next to the toilet. Below: From the outside the addition blends into the original home as if it was part of the original construction. As a bonus, we were able to build a deck off of the homeowner’s second floor primary suite above the apartment.

Approving Broader Use

In response to growing concerns over the cost and availability of housing, Massachusetts recently adopted new laws that make it far easier to build an ADU than ever before. In fact, today, no special or discretionary approval is required from the local zoning authority.

There are four basic options if you are considering an accessory dwelling unit:

  • Building an apartment inside your house such as converting a basement or attic space
  • Building an addition attached to your house
  • Building an addition detached from your house
  • Building an ADU over an existing or new garage

Homeowners do still need to comply with some requirements including:

  • Maintaining a separate entrance, either directly from the outside or through an entry hall or corridor shared with the principal dwelling sufficient to meet the requirements of the state building code for safe egress;
  • The ADU must be either no larger than half the gross floor area of the principal dwelling or 900 square feet, whichever is smaller;
  • Meeting local municipal restrictions, including, but not limited to, additional size restrictions and/or restrictions or prohibitions on short-term rental if the city or town decides to impose these. However, municipalities cannot require owner occupancy or a family relationship for the ADU or the primary residence.

Here are links to additional information for some of our local communities:

Arlington

Belmont

Winchester – Note that Winchester had to amend its local bylaw to comply with the new state law. Previously the town had an age and/or disability restriction as well as be owner-occupied.

Lexington

Of course, we at Custom would be happy to help you figure out what makes sense for your situation. To start the conversation, call us at 781-648-2835.