Tom and Kristin Moser's new house — nearly 3,000 square feet in a development outside Tucson — has all the modern amenities, including solar panels and an open kitchen. But their house also has a feature that the builders are betting will be a hit, like the dog showers and craft rooms that beckoned during the boom. Tucked inside is a one-bedroom apartment with its own garage and a discrete entrance around the side.
The Mosers wanted the built-in apartment not to bring in a renter to help pay the mortgage, but rather as a home for Mr. Moser's 82-year-old widowed father.
"More than weekly visits and phone calls, he really needs to be around family," Mr. Moser, an investment manager, said of his father, Lee. "It's the way he was raised. I think as a society it's a way we have to step back into."
Built by Lennar, one of the country's largest homebuilders, it is the most extreme example of the sort of options, like 400-square-foot "bonus" rooms, that many of the big builders are now offering to accommodate the changing shape of the American family: boomer couples with boomerang children and aging parents, an increasingly multiethnic population with a tradition of housing three generations under one roof, and even singles who may need to double up with siblings or friends in this fraught economic climate.
Lennar started marketing its new designs last fall with particular gusto: "Next Gen — The Home Within a Home" is a title and tag line intended to wrap the notion of multigenerational living in a futuristic gloss. But it is more than just marketing; the blueprints themselves are changing.
In fact, architectural historians, statisticians and builders themselves are pointing out that the new household — and the house that can hold it — is much like the old household, the one that was cast aside after World War II by the building boom that focused on small, tidy dwellings for mom, dad and their two children.
Population statistics help tell the tale. A Pew study reports that 41 percent of adults between 25 and 29 are now living, or have lived recently, with their parents. Over all, more than 50 million Americans are in multigenerational households, a 10 percent increase from 2007. It is a back-to-the-future moment.
"You have to go back to the 1940s to see those kinds of numbers," said Stephen Melman, director of economic services at the National Association of Home Builders. "What the recession has done has really hit household formation hard, so instead of forming households we are having some contractions: the college student moving back in or someone's brother-in-law loses a job. It's an opportunity for the builders."
Scott Thomas, national director of product development for PulteGroup, the largest homebuilder in the United States, said his company now offers layouts with larger "flex rooms" and an over-the-garage apartment it calls the Grand Retreat. Ryland and KB Homes have been offering similar alternatives, and have seen their popularity increase as multigenerational households become more common.
"For whatever reason," Mr. Thomas said, "whether it's the return of something that was part of our lifestyle in the past, or simply related to the economy, multigenerational living is definitely taking place." Thirty percent of Pulte customers are asking for such features, the company said.
The developers are catering to people like Tina O'Donnell, 34, and her husband, Joe Rigby, 48, who bought a house in a Pulte development in San Diego last year. They moved in with his mother, Cheryl Rigby, 65, and three daughters. Mrs. Rigby retired early to help with the children; in return, her son and daughter-in-law paid for her health insurance until Medicare kicked in.
"She's very important to us," said Ms. O'Donnell, who works with her husband at a home-security company. "And it was important that she felt like it was her home."
The $680,000 model they bought came with four bedrooms upstairs, and "flex space" on the first floor, which they turned into a bedroom for Mrs. Rigby. It has a walk-in closet and its own bathroom, and it can be entered through the garage as well as the kitchen.
Wid Chapman, an architect and co-author of "Unassisted Living: Ageless Homes for Later Life," said the 2010 census showed that the shift to the "nonlinear family" is part of an evolution that will be accelerated now that mainstream builders are responding to it.
"These so-called atypical households will be deliberately created and marketed in geographic locations that might have been the epicenter of the suburban classic nuclear family in the past," he said.
But how do you make a home that is flexible enough for those who have been used to separate nests?
The "granny flat" or "mother-in-law apartment" has been around for decades. But municipalities do not always love accessory dwelling units, or A.D.U.'s. Builders who have tried to market them have become entangled in delays by zoning boards in many communities that frown on anything other than single-family homes.
"We still have zoning that was put in place in the 1950s, when farmlands turned into suburbs overnight, with houses designed for mom, dad and 2.3 kids," said Michael Litchfield, the author of "In-Laws, Outlaws, and Granny Flats: Your Guide to Turning One House into Two Homes."
Mr. Litchfield warns of the "dead hand of single-family zoning" that inhibits the formation of households like those headed by a single mother, who needs to rent out space to make ends meet, or by boomers who want to build an accessory dwelling unit for an aging parent. Only California, he added, has a state law that allows homeowners to build such a unit "by right," though some cities have altered their zoning codes to encourage their creation.
To circumvent zoning that is leery of duplexes, Lennar's Next Gen houses run on a single electric meter, have only microwave convection ovens in the apartment, and from the outside look like other houses.
"One address, one hookup, one electric meter," said Alan Jones, Lennar's Arizona division president. It was Mr. Jones who took a Las Vegas architect's concept and ran with it. In the last year, Lennar has developed more than 40 Next Gen plans in 100 communities in 10 states. In Arizona alone, over 100 have been sold so far. By next year, the company will be rolling out Next Gen houses in all their markets, Mr. Jones said.
"Once I've gotten to the right people, to the mayors and vice mayors of municipalities, they've been supportive," he said. "We're in a situation where the world is changing. We need a home for the way people are living today."
For their part, when Tom and Kristin Moser moved into their new house last week, they formed a thoroughly modern commune, joining Mr. Moser's sister, brother-in-law and his parents, who were already ensconced in a Next Gen house right next to theirs. Ms. Moser's mother is in a traditional house on the other side.
"I think it's a natural way to live," Mr. Moser said. "Communal living fosters love, and commitment, and all the good values we want. Think of a rubber band being extended. That's the way most of us have been living. That is going to snap back when people say, 'This isn't working.' "
This story originally appeared in The New York Times.