Whitemarsh Hall, a massive estate that was located on 300 acres of land in Wyndmoor, Pennsylvania and owned by banking executive Edward T. Stotesbury. At 100,000 square feet, it was likely the third largest house ever built in America (tied with Arden, the E. H. Harriman estate), bested only by Biltmore House and Oheka Castle. Today regarded as one of the greatest losses in American architectural history, it was completed in 1921 and demolished in 1980.
Source: garylawrance.blogspot.com