The Arlington County Board has approved a developer’s plans to build an enormous office building next to the Rosslyn Metrorail station in a deal that will net the county a new station entrance and the free use of the old Newseum site.

Monday Properties Services’ proposed 600,000-square-foot, 470-foot-tall office building at 1812 N. Moore St. will tie with the JBG Cos.’ planned office building across the street for the title of Rosslyn’s tallest building.