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House For Rent
Bruce Leighty—Getty Images

So the good news is the real estate market is really, officially on the mend. The bad news is that means more renters are getting squeezed, especially those who might have lost a home to foreclosure.

According to a new annual report from Harvard's Joint Center for Housing Studies, more than 11 million spend more than half of their income on rent, and more than 21 million families pay 30% of their income just to keep a roof over their heads.

Homeownership has been falling at an “unprecedented” rate for the past decade. Last year, fewer than two-thirds of Americans lived in homes they owned. The problem is that it's getting increasingly hard for today's growing number of renters to find places to live they can actually afford.

Read More: Home Sales Are at a 9-Year High

"The large backlog of foreclosures is still a serious drag on homeownership,” the group's managing director, Chris Herbert, said in a statement. People who went through a foreclosure and those who otherwise took a financial hit during the recession are still having a hard time securing mortgages. Other factors keeping people from buying homes are high student loan debts and low incomes, the report found.

This all adds up to a lot more renters, but most new apartments being built today are at the high end of the market — not so helpful given that more than a third of renters have a household income of $25,000 or less and need more affordable housing.