The number of U.S. homes worth less than their outstanding mortgage jumped in the fourth quarter as prices fell and lenders seized fewer properties from delinquent borrowers, according to Zillow Inc.

About 15.7 million homeowners had negative equity, also known as being underwater, at the end of the year, up from 13.9 million in the previous three months, the Seattle-based real estate information company said in a report today. The total represented 27 percent of mortgaged single-family homes, the highest in Zillow data dating to the first quarter of 2009.

Home prices are declining as foreclosed properties sell at discounts and unemployment at 9 percent limits buyer demand. Values will fall as much as 5 percent this year, putting more homeowners underwater, before finding a floor as the economy improves, said Stan Humphries, Zillow’s chief economist.

“These seem like fairly grim numbers,” Humphries said in a telephone interview. “We’re still expecting a bottom in home values later this year. And this, if anything, makes me a bit more confident because I’m seeing very large corrections now, which means the market can start to repair itself.”

The median value for a U.S. single-family home was $175,200 in the fourth quarter, down 2.6 percent from the end of September and 5.9 percent from a year earlier, according to Zillow. Values have fallen 27 percent from the June 2006 peak.

Foreclosure Moratorium

Foreclosures slowed in the fourth quarter as lenders including Bank of America Corp. and Ally Financial Inc. halted some home seizures after accusations they used improper documentation and processes. Attorneys general in all 50 states are investigating the practices.

Lenders also are taking a longer amount of time to take over properties as they are overwhelmed with record defaults. Homes in the foreclosure process in December were an average 507 days delinquent on their payments, 25 percent longer than a year earlier, Lender Processing Services Inc. reported this week.

Las Vegas led the nation in homes with negative equity, at about 81.5 percent of all properties with mortgages, Zillow said. It was followed by 69.9 percent in Phoenix; 67.9 percent in Reno, Nevada; 61.7 percent in Orlando, Florida; and 58.5 percent in Modesto, California.

The median home value in Las Vegas was $125,156 in December, the lowest since February 2000 and down 59 percent from the 2006 peak, Zillow said.

Atlanta had the fastest increase in negative equity, with the percentage of underwater homes rising to 54 percent from 37.6 percent the previous quarter. Atlanta’s median home value was $128,064 in December, equivalent to November 1999 and 29 percent below the area’s 2006 peak, according to Zillow.

The total value of U.S. single-family homes tumbled about $798 billion in the fourth quarter, according to Zillow. For the year, values fell by more than $2 trillion to $22.3 trillion.

Zillow’s estimates are based on sales data and public information about individual homes that aren’t on the market, such as their size and amenities, Humphries said. They exclude homes that are resold after foreclosure, when prices are usually at least 20 percent less than similar properties, he said.