BEND, Ore. (CNN/Money) – Erika and her husband are in a common predicament. They'd like to start a family but can't imagine how they could swing it in San Francisco, where commutes are long, childcare is expensive and the median home price is more than $500,000.

So they're considering the unthinkable: "We've talked about asking my parents to move up to San Francisco and buy a house with us," said Erika, who asked that we not use her last name, lest her parents get their hopes up or her employer find out she's thinking about getting pregnant.

Nationally, "multigenerational" households represent just a fraction of the population. According to the 2000 Census, 4 percent of all U.S. households have three or more generations under one roof.

But in some parts of the country, these living arrangements seem to be growing in popularity.

Twenty-somethings are taking their time moving out of the house, and young families are turning to their parents to help out with children and share the mortgage.

"Among the oldest households, the share of people living with their family has gone down, but among people in their 50s and 60s it has gone up," said Michael Carliner, economist for the National Association of Home Builders, noting that childcare seems to be a significant factor, particularly if the middle generation is divorced.

According to a poll by homebuilder Del Webb, in fact, one-quarter of baby boomers say they expect their children or grandchildren to live with them at some point during their retirement.

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"In the past two years this has become more than just anecdotal," said Ron Phipps, a real estate agent near Providence, R.H. who first noticed an increase in multigenerational buyers about five years ago.

Phipps, who sells real estate in an area where median home prices have increased dramatically over the past three years, sees all kinds of arrangements. Families, he said, are buying three-story town houses with their parents or adult siblings. They're shopping around for houses with two master suites or looking to build apartments -- often called "granny flats" -- over the garage, in the backyard or within the house.

Traditionally, multifamily housing has been more common among recent immigrants and their children. In Hawaii, where there is a large Asian population, more than 8 percent of households are multigenerational, and in California, where there is a large Hispanic population, close to 6 percent of households fall under this arrangement.

Contrast this to North Dakota, where only 1 percent of households include several generations.

But culture is only one influence.

"Our own sense is that a lot of this (trend) is driven mostly by economics," said Nicolas Retsinas, director of Harvard University's Joint Center for Housing Studies. He believes the percentage of multigenerational households has likely increased over the past few years right along with home prices. "In certain markets where affordable housing is scarce this is much more common."

In the course of three years, Anthony Durbin, 34, has lived in two different houses with his parents. "I kind of needed their good credit and they kind of needed me to make the mortgage payment," said Durbin, who lives in Costa Mesa, Calif. Culture had some influence; Durban's mom is Korean and his dad is Irish and Italian.

"I have friends who are in the same situation," added Durbin. "Home prices in many areas have doubled and I doubt most people's income has."

Homebuilders in many parts of the country have taken note. So-called accessory dwelling units, such as granny flats, are a common feature in some new housing developments.

In Baldwin Park, near downtown Orlando, new townhomes come with the option of a first-floor studio apartment with its own entrance, a feature popular with families who want private space for parents or adult children. "I would say about a quarter of the townhomes I've sold are multigenerational," said Patrick de la Roza, a real estate agent who works for the developer, Rey Homes.

Janice Strauss, owner of Ashley Custom Homes in Baltimore, has been getting requests for houses that can accommodate more family members. "Right now we're working on a Colonial with 5,000 square feet for the main house and 1,200 square feet for the 'mother-in-law apartment.'"

Granny flats aren't without their problems. In many cities and states, zoning laws prohibit accessory units. "When you bring in a two-family house there is always the question 'Are you going to rent it?'" said Strauss. She's seen cases where owners have to sign an affidavit promising there are no renters and never will be, even after the house is sold.

Also, such set-ups don't always save money. "It can be pretty pricey because essentially you are building two houses," she said. "A lot of people just can't afford to do this."

Nancy Sharifi and her husband discovered this reality when they started looking to upgrade their Orlando home a few years ago. "To stay in a good neighborhood and move to a house double the size, that created a major challenge," said Sharifi, whose in-laws moved in eight years ago after the Sharifi's second child was born.

Sharifi, who is the senior deputy director of Fannie Mae's Central Florida office, was so inspired by the number of people she'd met in her situation that she set out to develop a new mortgage to address multifamily households.

In September, Fannie Mae launched a pilot in Orlando called "Seniors and Family Together." The mortgage, which defines seniors as people 55 and older, considers up to 30 percent of seniors income when qualifying borrowers for a mortgage but doesn't require that seniors actually be listed on the note. "I hope the program will eventually go national," said Sharifi, noting that families need to have income at or below 130 percent of their area's median income to qualify.

Meanwhile, Sharifi and her extended family have moved into a house they built with the idea of giving each generation its own space. Despite having to live in close quarters initially, Sharifi has not regrets about the decision to cohabitate with her in-laws.

"This arrangement has been very convenient for me because we didn't have to worry about childcare," she said. "Now that (my in-laws) are older we can take care of all of their needs."