The mortgage crisis isn't over yet. Roughly a quarter of all mortgages reportedly remain underwater, and many borrowers still face tight lending criteria that make it hard to secure a home loan. In addition, borrowers could now face an "adverse market" surcharge on their interest rate -- one that could cost them whopping sums over the life of their mortgage.

This surcharge isn't exactly new, but it's probably news to most borrowers. Back in 2007, Fannie Mae introduced an adverse market charge of 0.25% of the amount borrowed, effectively increasing each borrower's rate on their mortgage loan by that quarter of a percentage point. The fee applied to properties deemed to be in declining markets. Now, Fannie Mae has hiked that fee again, and plans to apply it even to borrowers with the best credit scores.

Here comes the pain
Per the new rules, those with credit scores of 740 or more will pay an extra 0.25% on any loan that covers more than 75% of a home's value. Those with scores between 720 and 739 will pay 0.5%. On a $150,000 mortgage, an extra 0.5% amounts to $750 per year -- and that's for those with good credit scores.

Would-be homebuyers with scores below 620 face fees of as much as 3.25%. On a $150,000 loan, that amounts to nearly $5,000 per year!

The charges will make getting or affording a mortgage even harder, even though the National Association of Realtors recently urged lenders to loosen their standards a little. In 2010, major lenders Wells Fargo (NYSE: WFC), Bank of America (NYSE: BAC), JPMorgan Chase (NYSE: JPM), and Citigroup (NYSE: C) saw their total mortgage loans originated drop 12% from 2009 levels to $916 billion.

What to do
If you're planning to get a mortgage in the near future, you might want to move up your timetable if you can. Fannie Mae's new charges go into effect on April 1, but some lenders will start applying them sooner. Thus, it may pay more than ever to shop around. If your loan is insured by the Federal Housing Administration (FHA), it won't be affected by the adverse market surcharges.

It has also become more important than ever to keep your credit score as high as possible. According to a Bloomberg report, "Mortgage lenders including Wells Fargo and Bank of America [two of the largest lenders in the United States] have raised the minimum credit score on FHA-insured loans that they will buy to 640 from 620." Whatever kind of mortgage you're looking at, you might even want to delay getting it until you boost your score.

Planning to buy, sell, or refinance your home? Arm yourself with knowledge in The Fool's Home Center.

Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article. The Fool owns shares of Bank of America, JPMorgan Chase, and Wells Fargo. Through a separate account in its Rising Star portfolios, the Fool also has a short position on Bank of America. Try any of our investing newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is Fools writing for Fools.