SELLING UP: Cornelia Guest is moving: She’s put her historic family manse, Templeton in Old Westbury, N.Y., on the market for $20 million. The house, on 15 acres, has 28 rooms, including 11 bedrooms, six full baths, a barn, greenhouses, tennis court, swimming pool and formal gardens. “It’s just a magical home,” said Guest, who plans to find another house nearby and now divides her time between New York and Los Angeles for her acting career. “I grew up there, but now I’m looking forward to creating a place of my own.” Templeton occupies a major place in New York social history as the home of Guest’s parents, C.Z. and Winston, who hosted everyone from the Duke and Duchess of Windsor to Andy Warhol and Truman Capote to Gloria Guinness. C.Z. Guest was often photographed there, most notably in the iconic shots of her taken by Slim Aarons.