America’s most expensive ZIP codes (© Coldwell Banker Previews International)
This $20 million mansion in Atherton, Calif., shares the pricey ZIP code with tony neighbors. © Coldwell Banker Previews International
more on Forbes.com
In the Long Island ZIP code 11962, better known as Sagaponack, N.Y., the most expensive property for sale is listed at $30 million. That’s for a partially developed 3.8-acre lot of prime beachfront land with a six-bedroom house, a quaint cottage and “room for tennis and pool,” according to the Corcoran Group listing.
The property comes with billionaire neighbors such as industrialist Ira Rennert, whose massive Fair Field estate is assessed at $200 million, and hedge fund boss David Tepper, who recently tore down his $43.5 million house to build a bigger one.
The thick concentration of some of America’s richest people helped make the swanky Hamptons village the third most expensive ZIP code in the country for home sales this year, with a median asking price of $3.6 million. It comes in behind two ZIP codes that regularly grace the top spots of our list: Alpine, N.J., 07620, at No. 1, and Atherton, Calif., 94027, at No. 2.
Slide show:  10 most expensive ZIP codes
Alpine is an exclusive New York City suburb where the median home price is $4.3 million, street addresses are regularly scrambled on Google and the residents include celebrities such as Stevie Wonder and Sean “P. Diddy” Combs.  In Atherton, a tony town in the San Francisco Bay area, $4.3 million is the median home price thanks to tech billionaires such as Eric Schmidt and Meg Whitman.
What's your home worth?
Behind the numbers
Forbes compiled this list with the help of Altos Research, a Mountain View, Calif., company that tracks housing data in real time. It pulled pricing information for more than 20,000 ZIP codes across the U.S. for June 2 to Sept. 2, zeroing in on the 500 most expensive.
Altos calculated the median asking price for both single-family houses and condominiums, weighting the price based on the mix of local property types.  The list did not include co-ops. Altos limited the search to ZIP codes where 20 or more residences were listed for sale, including short sales and bank-owned foreclosures on the market.  To smooth out any wrinkles caused by a week’s unusual activity such as expensive home coming to market in an area where luxury properties are rare, Altos used a rolling average for the 90-day period.
Since the list is based on asking prices rather than tax assessments, it may be unfair to assume that the list is completely representative of the communities featured. For example, there could be pockets of longtime residents in modest homes in areas that have become swankier in recent years.
Rather, Forbes’ list is a snapshot of each market’s current activity. “We look at listing prices because, if you were to go into one of these markets with the intention of buying a home, this is what you would see in the market,” says Michael Simonsen, CEO of Altos Research. In some cases a ZIP may appear more than once on the list.  This happens when two or more towns share the same ZIP code.
Article continues below
Tour Iowa's Most Expensive Homes
Date:10/4/2011Duration: 003:002Video By: KCCI Des Moines
Get a peek inside some of the most expensive homes for sale in Iowa.
More Videos
Median home prices in the 500 ZIP codes were down 2% overall from the 2010 list, which is the mildest depreciation in years. In 2010 prices were off 5% year-over year and 7% in 2009.  Inventory levels have remained about the same since 2009, but real-estate appraisers like Jonathan Miller, chief executive of New York’s Miller Samuel, say there’s been an uptick in listings in the luxury end of major U.S. markets this year.  “It’s not that we’re seeing prices rise, it’s that we’re seeing more activity,” Miller told Forbes recently.