The hottest rentals around aren’t necessarily those gleaming high-rises with doormen and valets, Olympic-size pools, climbing walls, and happy hour. Instead, single-family homes are seeing the biggest spike in popularity.

The number of house rentals jumped 31% from 2006 to 2017, according to a recent RENTCafé report. Meanwhile, apartment building rentals grew only 14% over the same decade.

The rental listings site analyzed national U.S. Census data to come up with its findings.

“We found it interesting that there was such a huge increase on the single-family rental side, especially given the large number of new apartments completed in the last few years,” says Nadia Balint, a real estate writer for RENTCafé. Many of “these people who are renting single-family homes cannot afford to buy homes.”

There was an influx of single-family homes hitting the rental market after the financial crisis, as these types of houses were scooped up by investors when prices hit rock bottom. The abodes were often fixed up and then rented out to tenants who had lost their own homes to foreclosures or short sales.

Often these ex-homeowners couldn’t afford to buy again, or they didn’t have good enough credit or were just reluctant to make the commitment again.

Even now that the economy has recovered, rising prices and mortgage rates are making it difficult for many renters to make the transition into buying. Rental homes tend to be leased out to younger adults with roommates, families with children, and baby boomers seeking to downsize.

“The availability of starter homes and more affordable homes is pretty low right now,” Balint says. “That’s probably why renters are looking at single-family rentals when they get to a point…