In some cases, it may be Grandma moving in with the family. Other times, it may be a recent college graduate returning to the nest.

For all sorts of reasons — financial, medical, personal — a rising number of Americans are moving into extended family households.

Spotting a niche in the growing trend, Lennar Corp. has launched a new concept tailor-made for multigenerational family living.

It’s basically a house within a house: a smaller living unit next to the main home designed to provide independence but also access to the rest of the family household.

“People are really loving the whole concept,” said Carlos Gonzalez, president of the southeast Florida division of Lennar, a Miami-based home-building giant. “We adapted to the market from a design standpoint.”

In Miami-Dade County, Lennar is selling various versions of multigenerational homes in three new developments in Doral, Kendall and Homestead.

Louis Moreno of Kendall and his wife, Danilza Velez, signed a contract for a large NextGen home in The Vineyards development in Homestead last October — even before the models had been built.

“We loved it,” said Moreno, a 45-year-old engineer.

Moreno said his mother-in-law will be able to use the new suite when she visits, as will his family members who frequently come to town from Puerto Rico. “This will provide them with more comfortable space and more privacy,” he said. He also plans to use it as a game room and entertainment area.

The two-story Zinfandel home Moreno picked has three bedrooms and 2 1/2 bathrooms in the main home with a family room and two-car garage. In addition, it has an ample 789-square-foot suite with two bedrooms, a bathroom and a kitchenette. The suite has its own garage, a separate front entrance and an internal door connecting to the main home.

The Zinfandel, which has 2,249 square feet of air-conditioned space in the main house, starts at $283,990 in the Homestead community at 128 SE 28th Ter., but a similar home in Kendall would run about $100,000 more, primarily because of higher land costs, Fernandez said. (In Doral, there is a NextGen home priced at $677,990.)

Some multigenerational models have suites as small as 489 square feet, but all have a separate entrance, a bedroom, a bathroom and some sort of kitchen space.

The idea takes various shapes. One option at the Kendall Square development at 16950 SW 90th St. is a Granny unit above a detached garage.

“Independence is the key word,” said Frank Fernandez, director of sales and marketing for the southeast Florida division.

Depending on local zoning rules, some homes can have full kitchens, others are restricted to kitchenettes with a microwave but no stove. Similarly, some municipalities permit the space to be used as a rental, others prohibit it.

The choice is proving popular. Fernandez said in The Vineyards development in Homestead, 10 of the 14 homes sold to date are NextGen. At Kendall Square, 35 of 107 sales are multigenerational, and at the Isles at Grand Bay development at 11301 NW 74th Street in Doral, five of 48 houses are.

Adapting homes for special needs, such as wheelchairs and safety railings, is done at cost, Fernandez said: “That is company policy.”

As one of the nation’s largest home builders, Lennar has been rebounding strongly from the housing crash. Last week, the builder, whose shares trade on the New York Stock Exchange, posted better than expected earnings for the fourth quarter and fiscal year ended Nov. 30, 2012.