Rents have been soaring in urban areas, but that's not the only thing driving up housing affordability, according to CNBC's Diana Olick.

Big cities have always been expensive, but in a troubling new twist — suburban areas are becoming unaffordable faster than their urban neighbors, according to a new report by Trulia.

For home buyers, a tight supply supply of homes on the market is driving prices through the roof. Combine that with high down payments and its hard for new buyers to find homes.

"Saving for a down payment can be difficult for prospective first-time home buyers given the absence of substantial wage growth in recent years combined with the burden of student loan debt many are struggling under," said Daren Blomquist, senior vice president at RealtyTrac. "Even just a 3 percent down payment requires 14 percent of annual wages on average across the 513 counties we analyzed, and in 67 counties a 3 percent down payment requires more than one-fifth of annual wages."

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