Kathleen Mahon thought she'd live in her Huntingdon Valley home forever. Then came the soaring taxes, the realization that she no longer needed a 3,000-square foot home on two acres, and the opportunity to share a home with her stepdaughter and grandkids.

"My husband Joe's daughter, Lynda Schmidt, was divorcing and struggling to make ends meet while keeping three children in Catholic school and working two jobs," recalled Mahon, 54, president of J&K Mahon Builders in Huntingdon Valley. "Looking to the future, the upkeep on our home would become more difficult and most likely unaffordable."

A year and a half ago, when the Mahons' taxes hit $12,000, the timing was right.

So her company built a house in the Montgomery County town that suited Schmidt's family and included a 700-square-foot in-law suite for herself and Joe.

The two families share a garage entrance leading to a mudroom and first-floor office and a family room and storage area in the basement. The Mahons' private space on the second floor includes a living room, full kitchen, bedroom and full bath. Preparing for the future, they installed an elevator.

A record 60.6 million people, or 19 percent of the U.S. population, live with multiple generations under one roof, according to the most recent data analyzed by the Pew Research Center. For many families, sharing space saves money while allowing children to help care for aging parents and grandparents to help care for grandchildren.

The Philadelphia area has especially taken to the idea. According to the online real estate database Zillow, 1.2 percent of all the homes nationally listed for sale in 2017 included the term in-law, while in this region, the number was 2.7 percent, said Skylar Olsen, Zillow senior economist.

"That's more than twice as prevalent in the Philly metro area than in the U.S. as a whole," she said.

The approximately half-dozen units Mahon's company has built in the Northeast, Newton and Holland have been for people in their 50s and 60s. "We haven't seen much where it was an elderly parent yet but people planning for the future," she said.

Most in-law suites function as small apartments with a bedroom, bathroom, full kitchen or kitchenette, living space, and private entrance. They can be attached to a main house or sit on the same property in a refurbished garage or carriage house. On average, they range in size from a few hundred to 1,000 square feet, with costs from $5,000 for minor changes (moving some doors or walls) to $200,000 to build or refurbish a free-standing structure.

Four key considerations for planning an in-law suite are privacy, zoning regulations, accessibility, and flexibility, said Michele Thackrah, principal of Archer & Buchanan Architecture in West Chester.

"Creating a sense of privacy is always a priority," she said. "Ideally, you want to promote a sense of independence for all family members, both the in-laws and those that own the property."

Regulations vary widely by municipality; zoning and code restrictions dictate what is allowed. For example, some municipalities won't permit a second kitchen, and others may limit the amount of impervious surfaces on a property.

"Do that homework early in the process to figure out what's allowed," Thackrah said.

Space and budget are important considerations. Especially in city dwellings, is there room for a lift or elevator? Does the dwelling need to be wheelchair accessible? Can rooms such as a bathroom or kitchen be shared?

Before committing to intergenerational living, families should consider the changes the new lifestyle will bring.

"It took a little time to get used to downsizing, getting rid of a lot of my things,"  Mahon admitted. "Holidays were difficult for me at first because I was always the house that had Christmas and Thanksgiving, but now we share that."

It's also important to think long term about how the space could be used over time.

"Your in-laws presumably won't be there forever," Thackrah said, "so do you need that space to function as a place for the adult kids to come back to, or if the township allows it, can it be a rental property?"

In 2015, Jane Anderson and her husband, Christopher Jones, gutted and rehabbed the trinity behind their Queen Village home to include a 350-square-foot in-law suite for Anderson's parents, who visit from Minnesota. Because her parents come only a few times a year for a couple weeks each time, the Andersons also rent out the space as an Airbnb, hoping to recoup some of their $50,000 investment.

Alongside the first-floor garage is a private outside entryway and utility closet. The second floor features a bathroom with a stall shower, living room, washer/dryer and kitchen, and the bedroom occupies the full third floor. They added two exterior doors — one to the street and one for a potential second story balcony — tore out the original chimney, installed all new plumbing, electric, drywall, flooring and HVAC, and rebuilt and rerouted a staircase.

"It's turned out to be very popular, so my parents have to book their time quite a few months in advance," Anderson said, joking.

The biggest expenses when building an in-law suite are typically kitchens, baths, and new construction. Renovating existing space can save money.

"Finding inventive ways to share space can minimize the budget," Thackrah said. Perhaps an unused dining room can become an in-law suite next to a shared kitchen and a powder room can be expanded to add a shower.

When Jane and Michael Stolper first suggested that Jane's parents move from Massachusetts into an in-law suite in their Berwyn home, Harriet and Sandford Chodosh declined.

"We had lived there for 55 years," recalled Harriet Chodosh, now 88. "And I was concerned about damaging my relationship with Jane."

But realizing that they were getting older and had no family nearby, the couple reconsidered and moved in 2005. A garage separates the main house from the 1,600-square-foot suite, which has a living room, dining room, kitchen, his and hers bathrooms, study, laundry room, and its own front entrance.

"We have huge windows that make it sunny and bright and very cheerful," Chodosh said.

After her husband died in 2010, she was glad to be living with family, she said. "Fortunately, I'm in decent health and I can still be quite active, but I'm really glad this opportunity arose. It was a real blessing."