 Once construction of the power substation was complete in October 2003 , work proceeded on building the office tower . An unusual approach was used in constructing the building ; erecting the steel frame before adding the concrete core . This approach allowed the construction schedule to be shortened by a few months . Construction was completed in 2006 at a cost of $ 700 million . Though Silverstein received $ 861 million from insurance on the old building , he owed more than $ 400 million on its mortgage . Costs to rebuild were covered by $ 475 million in Liberty Bonds , which provide tax @-@ exempt financing to help stimulate rebuilding in Lower Manhattan and insurance money that remained after other expenses . 
