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How much is your house worth? Zillow knows

Start-up provides valuations, purchase prices, comparable data, tools for adjusting values based on improvements.

A Seattle start-up is launching a Web site this week designed to help people get estimates on home valuations and gather all sorts of detailed information in their quest to sell or buy property.

Zillow.com, a beta version of which will hit the Web on Wednesday, is a sort of Kelley Blue Book for homes. It lets users type in an address and see specs on the home--its valuation, the date and purchase price of the last sale, and data on comparable homes. The service is free.

The site offers valuations on more than 40 million homes in the United States, with some data on an additional 20 million homes, Zillow.com said. There are an estimated 85 million single-family homes in the country, said Spencer Raskoff, Zillow.com's chief financial officer and vice president of marketing.

The ad-supported site also offers historical value changes for homes in chart form; historical value changes for homes as compared with the surrounding ZIP code, city, state or country; all comparable home sales in an area; and satellite, aerial and parcel views of homes.

Users can also create confidential personal worksheets on individual homes and use a tool that lets them refine the value of a home based on changes or additions to that home, such as a kitchen remodel. The tool calculates the remodel value for the area based on local remodel data and depreciation.

The site does not require users to enter any personal information and promises that no one will contact the user.

There are numerous real-estate Web sites now, including HomePriceRecords.com, HomePages.com and Trulia.com. However, Raskoff said Zillow is the first to focus on home valuation data and link it to all the different information.

"Listings are out there--sites that help people find what homes are for sale," he said. "But home valuation information is every bit as important as listing information, and yet it's unique, differentiated and hard to find."

Zillow.com, the name of which comes from the word "pillow" and the notion of zillions of pieces of data, was founded by Expedia founder Rich Barton and former Expedia senior vice president Lloyd Frink. It has raised $32 million in funding from Benchmark Capital, Technology Crossover Venture and other private investors.

The Web site was inundated with traffic and inaccessible on Wednesday, but it was expected to be reachable later in the day, a Zillow representative told CNET News.com.