FORT LAUDERDALE, Fla. – Jenny Kerns can join her grandkids for dinner almost any night just by going downstairs. Her daughter and son-in-law monitor her health without infringing on her privacy.

Those are just some of the benefits that Kerns and her family have found since moving four months ago into what is essentially two homes under one roof — a multigenerational property.

It’s a growing market that home builders are targeting by designing homes with separate living quarters: bedrooms and bathrooms, and sometimes kitchenettes and entrances.

Once known as mother-in-law suites, the accommodations have come back in vogue as the economy sputters. Aging parents and adult children with young kids are moving in to baby boomers’ homes to help cope with dwindling investments, foreclosures and uncertain jobs.

“This is something that’s becoming a necessity,” Miami analyst Lewis Goodkin said of the new housing option. “We’ve had more economic pressure than we’ve had in a long time. And it’s not a temporary thing. I think this is going to be with us for a while.”

Pulte Homes and GL Homes say they’ve had success selling the concept at developments in Florida’s Palm Beach County. Lennar Corp. started building these homes in Phoenix and Las Vegas last year and will do so in the Miami area this summer.

Multigenerational residences are larger and more expensive than traditional homes. In some cases, analysts say, the grandparents or adult children help pay the mortgage and housing costs.

The number of these households nationwide increased by 30 percent from 2000 to 2010, far more than the 11 percent growth in total households, according to figures from the U.S. Census Bureau. A household is considered multigenerational if three generations live there.

Chuck and Jennifer Thomas sold one house in Jupiter, Fla., last year to buy another that could accommodate Kerns, Jennifer’s 70-year-old mother.

In December, the Thomases and their three children found a two-story home, built by Pulte’s DiVosta division, that offers a floor plan above the garage known as a “grand retreat.”

The room is about 700 square feet and includes a private entrance, bedroom, full bathroom and a kitchenette.

Before, Kerns was in an assisted-living facility and not doing well, said Chuck Thomas, 39, a lawyer.

“I’d say there’s a marked difference in her condition,” he said. “The doctors are thrilled with her progress, and a lot has to do with her mental attitude and being around us.”

The grand retreat option costs about $75,000 extra without upgrades and is included in homes that range from the $600,000s to the $800,000s, said Scott Mairn, Pulte’s vice president of sales for South Florida.

Nearly 60 percent of buyers take the grand retreat floor plan when it’s an option, Mairn said.

In a development near Boynton Beach, Fla., GL Homes has sold 47 homes featuring the new floor plan, compared with 40 sales of a model that has a standard secondary bedroom, DePlaza said. The extended-family floor plan starts at about $407,000, as opposed to roughly $387,000.

The multigenerational market provides a boost in sales at a time when many builders are trying to recover from the housing bust.

Some industry experts wonder how long the trend will last. Ivan Choi of Virginia-based Matt Martin Real Estate Management said the demand for these properties could wane once the economy improves.

“When people have money and staying power,” Choi said, “they tend to be pretty independent at that point.”