The Cannon House Office Building, completed in 1908, is the oldest congressional office building as well as a significant example of the Beaux Arts style of architecture. It occupies a site south of the United States Capitol bounded by Independence Avenue, First Street, New Jersey Avenue, and C Street S.E.
The first congressional office buildings were constructed immediately after the turn of the 20th century to relieve overcrowding in the United States Capitol. Previously, members who wanted office space had to rent quarters or borrow space in committee rooms. In March 1901 Congress authorized Architect of the Capitol Edward Clark to draw plans for fireproof office buildings adjacent to the Capitol grounds. In March 1903 the acquisition of sites and construction of the buildings were authorized. In April 1904 the prominent New York City architectural firm of Carrère and Hastings was retained. Thomas Hastings took charge of the House Office Building project, while John Carrère oversaw the construction of an almost identical office building (now named the Russell Senate Office Building) for the United States Senate. Their Beaux Arts designs were restrained complements to the Capitol.
Architecturally, their elevations are divided into a rusticated base and a colonnade with an entablature and balustrade. The colonnades with thirty-four Doric columns that face the Capitol are echoed by pilasters on the sides of the buildings. Both buildings are faced with marble and limestone; the Russell Building's base and terrace are gray granite. Modern for their time, they included such facilities as forced-air ventilation systems, steam heat, individual lavatories with hot and cold running water and ice water, telephones, and electricity. Both are connected to the Capitol by underground passages. Originally there were 397 offices and fourteen committee rooms in the Cannon Building; the 1932 remodeling resulted in 85 two- or three-room suites, 10 single rooms, and 23 committee rooms.
Of special architectural interest is the rotunda. Eighteen Corinthian columns support an entablature and a coffered dome, whose glazed oculus floods the rotunda with natural light. Twin marble staircases lead from the rotunda to an imposing Caucus Room, which features Corinthian pilasters, a full entablature, and a richly detailed ceiling. The Cannon Building was occupied during the 60th Congress in December 1907. By 1913, however, the House had outgrown the available office space, and fifty-one rooms were added to the original structure by raising the roof and constructing a fifth floor. In 1962 the building was named for former Speaker of the United States House of Representatives Joseph Gurney Cannon.
http://www.aoc.gov/cc/cobs/chob.htm , a public-domain publication of the Architect of the Capitol