“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.