Despite a healing housing and mortgage market, three out of 10 Americans remain unlikely to qualify for a mortgage, according to a Zillow Mortgage Marketplace analysis.

The research analyzed 13 million loan quotes and more than 225,000 purchase loan requests on Zillow Mortgage Marketplace in September 2013, and compared it to a similar study conducted in September 2010.

Borrowers with credit scores under 620 who requested purchase loan quotes for 30-year fixed, conventional loans were unlikely to receive even one loan quote[1] in September 2013, unchanged from three years ago, even if they offered a relatively high down payment of 15 to 25 percent. Nearly 3 out of 10 Americans, or 28.4 percent, have a credit score of 620 or lower, according to data provided by myFICO.com.

Meanwhile, the bar has risen for borrowers to get the lowest available mortgage rates. The best mortgage rates are typically reserved for those with credit scores of 740 or higher, compared with 720 in 2010. According to myFICO.com, 40.3 percent of Americans currently fall into this category. In 2010, 47 percent of Americans had credit scores over 720 and were able to get the best rates. Borrowers with credit scores above 740 did not receive significantly better mortgage rates.

In the 2013 study, borrowers with credit scores of 740 or above got an average low annual percentage rate (APR) of 4.42 percent for conventional 30-year fixed mortgages. Borrowers with mid-range credit scores between 620 and 739 received APRs, on average, between 5.09 and 4.47 percent, with the APR rising as the credit score drops. Those with credit scores below 620 received too few loan quotes to calculate the average low APR.

"Despite all-time high levels of affordability in the housing market, tightened lending standards mean that nearly one-third of Americans are unlikely to be able to achieve the American Dream of homeownership because they can't qualify for a mortgage due to a low credit score," said Erin Lantz, director of mortgages at Zillow. "Your credit score is the single most important factor in determining your mortgage interest rate and monthly payment. To avoid any surprises when buying a home, check your credit score and report at least six months before you intend to buy to see if there are any costly inaccuracies, pay down high-balance lines of credit and make sure your bills are always paid on time."

Zillow Mortgage Marketplace received nearly 17 million loan requests in the past year from borrowers, who can anonymously request mortgage quotes from hundreds of lenders across the country. Lenders then submit loan quotes customized to each borrower's financial situation. This analysis looked at the number of quotes and the average lowest APR for 30-year fixed, conventional mortgages that were quoted to borrowers who intended to purchase a home with down payments between 15 and 25 percent, along with credit score information from myFICO.com.  The analysis took into account more than 13 million quotes submitted between Sept. 1, 2013 and Sept. 15, 2013.