As the nation grew, so did its government. Congress enlarged the Capitol with new wings flanking the original building. Each wing was three stories high, 140 feet wide, and 235 feet long, built of brick faced with Massachusetts marble. Construction took 17 years (1851-1868).

Philadelphia architect Thomas U. Walter designed the wings. The engineer Montgomery C. Meigs (and, later, William B. Franklin) oversaw their construction. These additions provided spacious new chambers for the House of Representatives and Senate, with scores of new offices and committee rooms, plus grand lobbies, corridors, and staircases. For the convenience of lawmakers (who often lived in crowded boardinghouses), the wings also included bathing rooms, barbershops, and restaurants. Steam-powered fans drove a central heating and ventilation system, and every room was lighted by gas. In 1852, Walter also designed an innovative cast-iron room for the Library of Congress.