Comcast Center is a skyscraper in Center City, Philadelphia, Pennsylvania, United States. The 58-story, 975 feet (297 m) tower is the tallest building in Philadelphia and the fifteenth tallest building in the United States. Originally called One Pennsylvania Plaza when the building was first announced in 2001, the Comcast Center went through two redesigns before construction began in 2005. Designed by architect Robert A. M. Stern for Liberty Property Trust, the Comcast Center was delayed as the developers tried to get the Center designated a Keystone Opportunity Improvement Zone. The designation would exempt tenants from most taxes for fifteen years as a way to encourage development in disadvantaged areas. Giving the Comcast Center the designation was supported by many state and city officials who hoped to keep corporations within Philadelphia, but was strongly opposed by other building owners who felt the building would have an unfair advantage in attracting tenants. Even though the bill was not approved in the Pennsylvania General Assembly in 2004, Liberty Property Trust went forward with construction.