The Obamas' favorite summer retreat is getting more expensive.
Sales and rental prices on Cape Cod in Massachusetts have surged in recent years, with the rising stock market and housing recovery. Prices and demand are especially strong at the top end of the market, brokers say, with ocean-front estates and lavish family compounds quickly getting snapped up for millions.
Brokers said many wealthy Cape Codders are trading up. Other buyers see the Cape as a strong investment, with bigger properties still selling at a 20 percent to 50 percent discount to the Hamptons in New York.
CNBC has learned that billionaire William Koch—the sailor, wine-collector and brother of famed conservatives David and Charles—is putting his historic compound on the private island of Oyster Harbors up for sale. Robert Paul Properties, an affiliate of Luxury Portfolio International, is about to list the property for $15 million.
Koch is selling after he purchased a nearby estate previously owned by Rachel "Bunny" Melon for $19.5 million, according to reports.
The property Koch is selling was built in 1929, and has been expanded and renovated to include a state-of-the-art wine cellar, chef's kitchen, beach house and guest quarters. Since Koch is a longtime sailor—his boat "America" won the America's Cup in 1992—the property also features a 106-foot dock and a "ship's room modeled on the captain's quarters of a Napoleonic-era frigate," or boat of its time.
The Cape has always been the less-expensive, more laid-back cousin to Newport, R.I. and the Hamptons. But the Cape is quickly gaining in price and luxury, to the consternation of many longtime residents. Two of the most expensive listings this summer are priced at around $20 million.
(Read More: Mansion Shortage Could Slow Sales in Rich Towns)