If the deal closes when the 26-story, 35-unit high-rise is completed in the spring of 2017, the two-story, 8,900-square-foot penthouse will be the most expensive residence ever sold in the city - more than $5 million more than the penthouse at Scannapieco's 1706 Rittenhouse Square Street, which sold for $12.5 million in April 2010.