It’s not uncommon for adult children to welcome aging parents into their homes, making space for the seniors in a first-floor bedroom, a refinished terrace level or an in-law suite. But today, those older adults are getting some competition for the extra elbow room from “boomerangs” - adult children who come back to roost after college before heading out on their own.

An April edition of the AARP Bulletin cited a recent Pew Research Center analysis of U.S. Census Bureau data that counted about 51 million Americans - almost 17 percent of the population - living in a house with at least two adult generations. The same report tracked a 10.5 percent increase in these “multigenerational” households from 2007 to 2009.

So it’s no wonder that home buyers are making more requests for flexible spaces that can give everyone under one roof the privacy and independence they need - whether they are spaces for grandparents or returning children. Builders and remodelers have responded to the need for multigenerational living by creating separate quarters in finished basements, bonus rooms, attics or, in some cases, detached suites within a short walk of the main living area.

“We definitely saw this as a trend,” said builder Tony Perry, head of the Greenstone Group that is building homes at the Village of Towne Lake in Woodstock. “With the [economic] downturn, there are now a lot of under-30s who are back at home.”

Perry’s response to the trend was to create what he calls “a house in a house.” At the Village development, priced from the $300,000s, floor plans with 2,200 to 3,500 square feet include two configurations that allow either aging parents or boomerang children to have separate spaces while still under the same roof as the homeowners. Perry found the room by transforming a third or fourth bedroom into the private area outfitted with a full bath, small living area, dining space, closets and a kitchenette with a microwave, sink, refrigerator. There’s even room for a cooktop, oven and dishwasher, if the owners desire.

“This space not only works for kids; it gives every American what they want: the ability to live in their own homes,” said Perry. “It could also be a space for a caregiver.”

Steve Romeyn, managing partner of Windsong Properties, has been creating multigenerational spaces in communities around the metro area for 10 years. Inspired by designs he saw in the Southwest, Romeyn builds homes that feature a “casita,” a little house connected to the main living area by a covered breezeway. At Windsong’s Bel-Aire in Powder Springs, Windsong Manor in Paulding County and Heron Pond in Cherokee, floor plans with a casita range from $250,000 to $300,000. The area is finished with a full bath, closet, kitchenette, living space and a separate entrance.

“The casita was a big hit from the first one we did,” said Romeyn. “It has so many uses. It allows privacy for the homeowner as well as the person in the casita, who could be a returning child who is independent and doesn’t want to be underfoot. But we also have a number of owners who have parents with them, so the casita becomes the place where Grandma lives.”

Other area builders are tapping into the need of their buyers to have expandable, flexible space that can accommodate adult residents under the same roof. Traton Homes at Great Oaks Estates incorporates a guest room with a full bath on the main level that appeals to buyers with aging parents. At the Estates at Davis Ridge, buyers have the option of turning a third-floor bonus room into a retreat with a full bath and sitting area.

The Providence Group, with communities in Cobb, DeKalb, Forsyth, Fulton and Gwinnett counties, offers several home designs with finished retreats over the garage as well as third-floor suites with a full bath and walk-in closet. Both configurations appeal to buyers with adult children who want privacy away from main living space of the house.

But it’s not just buyers of new homes who are considering how to fit everyone into the layout. Mary Ellen Vanaken of Keller Williams Realty said more of her clients are searching for resales with room to house several adults.

“So many want their parents living with them,” she said. “But they also have kids who have come home from college, and they’re happy to have the kids there to save money rather than renting an apartment or house.”

Vanaken has found solutions in homes with walk-out terrace levels and unfinished attics. But it takes a lot of looking to come up with the right design.

“Right now, it’s rather hard to find that kind of space because there isn’t a lot of it on the market because it’s popular,” she said. “Some spaces don’t work because older people usually can’t do stairs. Having a space that can be redone or added is nice, but it’s also a lot of work…. Most of the time, people want ready-to-move-in homes.”

Walter Lewis, president of Neighbors Home Remodeling, agreed that retrofitting existing spaces to accommodate additional adults in the household isn’t always an easy assignment. His company has worked on various multigenerational projects that included additions as well as converting existing spaces by taking out walls.

“Creating space in an attics means you need to handle the issue of how to get stairs up to it, and that can be easy or sometimes a nightmare,” he said. “Then there’s the issue of the insulation. The whole envelope has to be converted into a conditioned space that usually requires its own heating and air conditioning system because attic space in the South can get very hot.”

Transforming a basement into an in-law or boomerang space is an easier task, as long as access is not an issue.

“Usually, basements are not as hard, especially if it’s a walk-in with doors to the outside,” said Lewis “Typically, basements will have some walls framed out, so we can just come in, do the insulation, add dry wall and finish the floors and ceilings. It’s also not too hard to put a bathroom in a basement, either - even if there’s no plumbing there.”

Lewis’s best bet for adding more room is to build an addition that will have all the features homeowners need.

“Additions are my favorites because most of the work is outside of the living space, so there is minimal impact to owners while construction is going on,” he said.

Whether extra living spaces are added on, retrofitted or built into a new design, they are fast becoming the norm for many buyers, said Perry.

“The world is changing, and this segment of the market has definitely changed with it,” he said. “It’s important to have options.”