Despite the end of the Great Recession, there’s been a surge in multigenerational households — but even more rooms that could be rented out.

Tens of thousands of homes have 3.6 million unoccupied rooms that could be rented out to a family member or stranger, according to an analysis of U.S. Census Bureau data in the 100 largest U.S. housing markets released Wednesday by real-estate website Trulia. “For many older Americans, renting a room provides an economic boost that may help them stay in a home longer.” Annual income could range from $14,000 to $24,000 per year, Trulia found. “Prices are high, inventory is low, and new housing growth is stagnating,” it said. “But what if these two generations got together to solve their mutual housing-related problems?

The good news: Many already are. One-in-nine baby boomer parents said their adult children returned home within the last year, according to a a separate study released in May by financial services firm Fidelity Investments and Stanford Center on Longevity, which surveyed 9,000 employees. The adult children save money on rent and household goods, but more than half (53%) of the parents said they had less leisure time and were less happy after the return of their “boomerang kids.” Trulia’s solution: pay to live at home when you are an adult and use the savings for a down payment on your first home.

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Over the last four decades, the number and share of Americans living in multigenerational family households has actually, according to a 2016 study by the Pew Research Center, a nonprofit think tank in Washington, D.C. In 2014, a record 60.6 million people, or 19% of the U.S. population, lived in a multigenerational household, up from 42.4 million (17%) in 2009 and 27.5 million (12%) in 1980. Multigenerational families — households with two or more adult generations, or one that includes grandparents and grandchildren — is growing among nearly all racial groups and age groups, says D’Vera Cohn, a senior writer and editor at Pew.

There are some demographic reasons. The Asian and Hispanic populations are growing more rapidly than the white population, Cohn’s analysis of the latest U.S. Census Bureau data found, “and those groups are more likely than whites to live in multigenerational family households.” What’s more, foreign-born Americans are more likely than the U.S.-born people to live with multiple generations of family, and Asians and Hispanics are more likely than whites to be immigrants. Some 28% of Asians lived in multigenerational households versus 25% of Hispanics and African-Americans and 15% of Caucasians.

More young U.S. adults live with their folks, Cohn says, and that doesn’t include those still studying. Among Americans aged 25- to 29 in 2014, 31% were part of multigenerational households. The share and number of 18- to 34-year-old adults living with parents surpassed other living arrangements in 2014 for the first time in more than 130 years. Education levels make a big difference: Young adults without college degrees are now are more likely to live with parents than to be married or cohabiting in their own homes, but those with college degrees are more likely to be living with a spouse or partner in their own homes.

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As child care costs have surged 50% over the last decade, grandparents can also provide a valuable role as care givers. In fact, 60% of grandparents provided some care for their grandchildren during a 10-year period, and 70% of those who did provided care for two years or more, a 2012 study of over 13,000 grandparents aged 50 and older by the University of Chicago found. “Importantly grandparents with less income and less education, or who are from minority groups, are more likely to take on care for their grandchildren,” Linda Waite, a professor in urban sociology at the University of Chicago, wrote in the report.

Another major change to family life: The share of children living in a two-parent household is at the lowest point in half a century, a separate 2015 Pew report found. Some 69% of children lived in two-parent households in 2014 versus 73% in 2000. People are marrying later, cohabiting rather than marrying, becoming single parents or forming blended families and — as the latest report shows — living with their grandparents. Philip Cohen, a sociology professor at the University of Maryland, sees this trend as a sign that the definition of families is more fluid in America, and that they are more culturally diverse rather than a decline in the traditional family.