Home values in the United States extended their fall in the first quarter, with more than one in five homeowners now owing more on their mortgages than their homes are worth, real estate website Zillow.com said on Wednesday.

U.S. home values posted a year-over-year decline of 14.2 percent to a Zillow Home Value Index of $182,378, resulting in a total 21.8 percent drop since the market peaked in 2006, according to Zillow's first-quarter Real Estate Market Reports, which encompass 161 metropolitan areas and cover the value changes in all homes, not just homes that have recently sold.

U.S. homes lost $704 billion in value during the first quarter and have depreciated $3.8 trillion in the past 12 months, according to analysis of the reports.

Declining home values left 21.9 percent of all American homeowners with negative equity by the end of the first quarter, Zillow said.

By comparison, 17.6 percent of all homeowners owed more on their mortgage than their property was worth in the fourth quarter of 2008, and 14.3 percent were underwater in the third quarter of last year, the reports showed.

Nine consecutive quarters of declines have left eight regions -- including the Modesto, California, Stockton, California, and Fort Myers, Florida regions -- with median value declines of more than 50 percent since those markets peaked.

In 85 of the 161 markets covered in the report, the annualized change over the past five years is negative or flat, the reports showed.

But in an early sign of improvement, 17 metropolitan areas across the country -- notably several hard-hit markets in California, including Los Angeles, San Diego and Modesto -- have seen two or more consecutive quarters of smaller year-over-year declines in home values, the reports showed.

Meanwhile, potential sellers appear to be holding back until evidence of an improved housing market. In a separate survey of homeowner sentiment, nearly one-third, or 31 percent, of homeowners said they would be at least somewhat likely to put their homes on the market in the next 12 months if they saw signs of a recovering real estate market, the reports showed.

Slowing declines in select markets are a bright spot or, at least, what passes for one given current market conditions, Dr. Stan Humphries, Zillow vice president of data and analytics, said in a statement.

Unfortunately, given the magnitude of the current rates of decline, we're still many months away from a bottom even as depreciation slows, he said. Moreover, the additional information we have this quarter on 'shadow inventory,' with one-third of homeowners indicating they would like to put their home on the market if conditions improve, confirms our earlier fears that a bottom in home values could be quite protracted.

By our calculations, this could translate into as many as 20 million homes that could seep into the market as prices stabilize, maintaining a constant stream of supply that far outpaces demand, thus keeping prices flat. I'm doubtful that we'll see the bottom until 2010, and thereafter it's increasingly clear that we're likely to have a long bottom before we see meaningful recovery in home values, Humphries said.

Of all transactions is the past 12 months, 20.4 percent were foreclosures, up slightly from 19.9 percent in the fourth quarter, while 11.9 percent of homes sold were short sales, also up slightly from 10.9 percent in the fourth quarter, the reports showed.

California Florida