"It's an increasingly negative picture in that market," said Stan Humphries, vice president of data and analytics for Zillow in Seattle.

Zillow said home values for the area fell 18.3 percent in the fourth quarter compared with a year ago, reaching a median value of $503,397. That was bigger than the overall U.S. decline of 11.6 percent in the quarter.

The Bay Area "is definitely in the pack of metropolitan areas that are declining worse than the national average," Humphries said. "Those areas are our usual suspects - metro areas in California, Florida, Phoenix, Las Vegas and Detroit."

What those areas have in common is boom-time speculation mania that artificially fattened prices, which are now deflating.

Unlike monthly median sales prices, which are skewed by the mixture of homes sold - more low-cost homes means a lower median sales price - the Zillow figure attempts to calculate the change in value for every single home, not just those that changed hands.

Zillow's ZIP code breakdown of valuation changes around the Bay Area showed that only 12 of 228 ZIP codes inched out valuation gains in 2008 compared with the previous year. Those rare "appreciating" ZIPs are in affluent areas such as Atherton, Palo Alto, Menlo Park, parts of San Francisco, Berkeley and Bodega Bay.

About 60 other ZIP codes, also in affluent areas of Marin, the East Bay and the Peninsula, saw their valuations decline by single digits.

The other 146 ZIP codes all had double-digit depreciation. Foreclosure-ridden areas such as eastern Contra Costa County, parts of San Jose and Vallejo experienced declines of more than 35 percent.

Zillow's report showed that 27 percent of Bay Area homeowners now have negative equity, meaning they owe more than their home is worth. That's a grim indicator, because being underwater, as the condition is also known, increases the likelihood of foreclosure.

Nationally, about 1 in 6 (17.6 percent) of all homeowners were underwater at year-end, Zillow said, up from 1 in 7 in September.

"Negative equity is a definite driver for foreclosure rates," Humphries said. "That plus a triggering event like death, divorce or loss of a job - the options for navigating that distressing event will be limited."

Increasingly, homeowners with significant negative equity are choosing to surrender their house to foreclosure even without a "trigger" event like losing a job, Humphries and other experts say.

"People feel some futility in making those payments and are walking away from homes," he said.

Looking at sales data, Zillow found that 50 percent of the homes sold in the Bay Area went for less than the original purchase price.

Nationally, Zillow said that U.S. homeowners lost a total of $3.3 trillion in home value during 2008. Value fell faster as the year progressed - $1.4 trillion was wiped out during the fourth quarter alone, more than the $1.3 trillion lost during all of 2007, Zillow said. A total of $6.1 trillion in U.S. home value has evaporated since the house market's peak in 2006, according to the Zillow.

E-mail Carolyn Said at csaid@sfchronicle.com.

This article appeared on page C - 1 of the San Francisco Chronicle