WASHINGTON -- The White House is valued at more than $308 million, making it the most expensive residence in the United States, a real estate research firm said Thursday.

It is still about $23 million less than its value a year ago, Zillow.com said in an indication that the White House has not been spared from the US home mortgage meltdown, the epicenter of the global financial crisis.

Using a formula that determines a home's current estimated worth based on public data and recent sales, Zillow valued the residence of the US president at $308,058,000.

It "would make this by far the most expensive residence in the United States," the Seattle-based firm said in a statement ahead of president-elect Barack Obama's January 20 inauguration.

Obama takes over the sprawling presidential residence when incumbent George W. Bush officially vacates it on that day.

Zillow, which estimates values for 84 million homes or more than 90 percent of the US residential market, said it decided to put a figure for the White House just for fun.

"We thought it would be fitting to provide a (estimate) value for the most famous US home of all as president-elect Obama and his family prepare to move in," said Stan Humphries, Zillow's vice president of data and analytics.

"Obviously the White House will never be for sale, but given the immense amount of public data on this home, we thought it would be fun to crunch the numbers and say, 'what if.'"

The White House includes 55,000 square feet (5,109 square meters) of indoor space, 132 total rooms, 35 bathrooms, 16 bedrooms, three kitchens and 18 acres (7.3 hectares) of premium downtown Washington land.

Aside from studying the home's physical characteristics to determine its value, Zillow considered the most expensive home sales in Washington in 2008, as well as other historic homes in the DC area recently for sale, and calculated the premium historic homes typically fetch.

Zillow's statisticians deemed the White House "the most historic home in America and applied a maximum historical premium to their models to determine" its current estimated value.

The White House's estimated value has decreased by 7.2 percent, or more than $23 million, in the past year, the company said.

However, the home's value has nearly doubled from the $167.86 million Zillow estimated it was worth in January 2001 when Bush was sworn in as president.