Billionaire William Koch, a former winner of yachting’s America’s Cup and brother to conservative activists Charles and David Koch, has listed his Osterville estate for sale for $15 million, Banker & Tradesman has learned.

Koch recently purchased two other large estates nearby, including a 26-acre waterfront parcel owned by philanthropist Bunny Mellon, which sold for $19.5 million in January.

Koch plans to revamp those parcels into a personal compound and in the meantime is looking to sell his current home, said Robert Kinlin, co-broker owner of Robert Paul Properties, the listing agent.

The property at 177 Seapuit River Rd. features an 8,000-square-foot main house, a guest house, a pool, sauna and steam room and a deep-water dock, Kinlin said. 

A longtime Cape resident, Koch is a prominent figure in opposition to the controversial Cape Cod wind energy plant.