The percentage of Americans living with friends, family or roommates is up sharply this century, according to Zillow, an online real estate database.

The analysis focuses on what it calls "doubled-up households ... in which at least two working-age (23-65), unmarried or un-partnered adults live together," according to Zillow's news release.

Think roommates, people staying with friends, and children living with parents, not people in romantic relationships or ailing retirees cared for by their aging kids.

Nationwide, the percentage of doubled-up households rose from 25.4 percent in 2000 to 32 percent in 2012, led by metropolitan areas where housing costs have risen faster than income, primarily in California and Florida.

The Top 5 in terms of percentage, as of 2012:

  • Los Angeles, 47.9 percent.

  • San Jose, 39.4 percent.

  • New York, 42.4 percent.

  • San Francisco, 39.2 percent.

  • Miami-Fort Lauderdale, 44.5 percent.

Out of 32 U.S. cities, only two – Kansas City and Minneapolis-St. Paul – fell short of having doubled-up households account for a quarter of all households, and they were close, with 24.2 percent each.

Among Northeast cities, Philadelphia has seen the highest rate of increase, from 27.9 percent of households to 35.2 (No. 13 nationally), a rise of more than 26 percent.

One implication is that the rebound in the housing market has a lot of room to grow, according to Zillow.

"There's potential for housing demand to outpace population growth, because of the pent-up demand in all those guestrooms and basements," Emily Heffter writes on the Zillow blog. "If salaries improve or housing costs fall, some of those doubled-up Americans will likely want a home of their own."

Zillow projects how many more households would be created if percentages returned to 2000 levels. Los Angeles and New York topped 300,000, trailed by Miami-Fort Lauderdale with 233,000. Philadelphia was No. 9 with 115,000, behind Riverside, Calif.; Washington; Chicago; and Dallas-Fort Worth.

Since the data is two years old, it's possible some of this reversal has already occurred.

Though the research fails break down causes or demographics, the so-called "boomerang generation" was clearly a major factor.

"Between 2005 and 2014, the fraction of young adults aged 18 to 31 who live with their parents rose 15 percent," according to a recent Federal Reserve report. "Not only is this rate of change unprecedented, but the fraction of young adults residing with parents has reached a historic high of 36 percent."

Rising student loan debt accounts for about 30 percent of the increase, the report concludes. That debt, of course, is related to wages and housing expenses.

View chart for 32 cities: The Rise in Doubled-Up Households, 2000-2012.

Contact staff writer Peter Mucha at 215-854-4342 or Follow @petemucha on Twitter.