Complex sentence: The 20,000-square-foot waterfront mansion had originally been built for A.C. Sloane, originally named Keewaydin, had been designed by Stanford White and built in 1902.
Split into simpler sentences: The 20,000-square-foot waterfront mansion had originally been built for A.C. Sloane and originally named Keewaydin. Reportedly the home had been designed by Stanford White and was built in 1902.