When it came time for Tom Moser and his wife to consider having his 83-year-old father move in with them, a house with a special multigenerational floor plan solved their problems - and may have saved his dad's life.

For the same $335,000 he might have paid for a traditional home in the Tucson suburbs, Moser, 61, was able to pick a "Next Gen" model from Lennar Corp, one of the biggest U.S. homebuilders. It had an 800-square-foot house-within-a-house for his dad that had a separate entrance and its own patio, plus a bedroom, sitting area, and bathroom.

One morning, Moser noticed his dad didn't look right, and at the emergency room they caught a heart problem early enough to save him. "If he had been in his own home, he would have sat in the chair with his clicker, and I would have found him days later," Moser says.

More than 50 million Americans already live in multigenerational situations, according to Pew Research, and the number is expected to grow as baby boomers age. For an increasing number of families, clearing out an extra room for Grandma (or for "boomerang" kids and grandchildren) isn't good enough. Homebuilders are responding with new models that have separate wings, attached apartments, dual master suites or finished basements.

"The great thing is that it is independent living," says Jeff Roos, who originated the Next Gen design for Lennar as Western regional president, and has overseen sales of 1,700 multigenerational units in the last two years. Mega-builder Toll Brothers Inc. has similar models in 20 states, which account for about 2 percent of sales. Prices range from $250,000 and up depending on the area and features, says Tim Gehman, director of design for Toll Architecture.

There are also a myriad of small builders in the game that homebuyers can find through real estate agents or local searches. Gertz Fine Homes, which builds between 12-30 houses a year near Portland, Oregon, says about 30 percent of sales are now multigenerational models, which can cost around $600,000.

Define What You Need

Privacy seems to be the prime concern of most people about to enter multigenerational living situations -- otherwise any four- or five-bedroom house might do.

"The key is to balance proximity with privacy," says John Graham, co-author of All in the Family: A Practical Guide to Successful Multigenerational Living. He says the most important features people look for are: private entrance, separate kitchenette (often without a full stove because of zoning laws) and no stairs.

One layout that works for many people is called a "dual master" -- where at least two bedrooms have their own private bathrooms; sometimes one is on the ground floor.

This is the type of house that Susan Couveau, 61, is looking for near Portland, Oregon from Gertz to accommodate her 38-year-old disabled son and her 78-year old mother, who now has an upstairs spare room but may not be able to do stairs much longer.

Ultimate Privacy

The gold standard has become an internal apartment -- what might also be called a mother-in-law apartment, maids' quarters or casita -- that is usually on the ground floor.

Moser saw this layout in a house he visited and the next day did an internet search for "multigenerational housing." He found a Lennar development in his area, and within days put down a deposit on a Next Gen home.

The plan was so appealing that Moser's sister bought the plot next door and built a Next Gen home so her 90-year-old in-laws could move in. For a few months, a grown daughter and her children also lived there -- four generations under one roof.

Moser's 83-year-old mother-in-law didn't want to be left out, so she bought the plot on the other side and built a traditional home for herself.

Communities filled with multigenerational clusters like this are springing up all over, says Keller Williams real estate agent Kevin Kieffer, based in Danville, California. The sales now account for 5 to 10 percent of his business, boosted by international clients moving from Asia and India. His last multigen sale was a 4,200-square-foot home for $1.2 million.

In areas where there is not a lot of new construction, the search can be harder, says Brandon McNamee, a Coldwell Banker real estate agent in the St. Louis, Missouri, area. He had a family of seven people, ranging from age 3 to 81, ask him to find them a house where they could all live together. "It took us months," McNamee says.

McNamee found them a 3,500-square-foot ranch for $520,000 with a master suite on each side of the house and a finished basement with two bedrooms, full bath and a wet bar.

"A lot of families might say, that's too close together, I couldn't handle it. But it worked for us," says Moser. "I think more and more communities will have these choices. Multigen will be the norm."

Photo: REUTERS/LENNAR Corporation