The Interior Department Building is a 7 story building of steel frame construction, clad in granite and limestone. It consists of a north-south core which bisects six equally spaced transverse wings. The central north-south section includes the major entrances and circulation spaces. The nearly identical east-west wings each span 382'. These wings contain the majority of office and specialized space. The building's strong massing is expressed at the longer east and west elevations where the facade is broken into, alternately, six forward and five deeply recessed planes. The stripped classic style of the building features a tripartite vertically stacked arrangement with a stylobate (1st and 2nd stories), a colonnaded mid-section (3rd through 5th stories), and a set back (6th and 7th stories). Secondary elements include the cornice (mechanical) story and the 8th story penthouse. The building has a terrace, or flat, roof. The United States Department of the Interior building occupies two city blocks bounded on the south by C Street, on the north by E Street, on the east by 18th Street, and on the west by 19th Street. Six wings run east and west from 18th to 19th Street and are connected by a central spine running north and south. This building massing creates ten U-shaped courts, allowing every office an exterior exposure. East-west wings are three bays wide and courts are four bays wide making the east and west facades each 18 bays wide. North and south facades are 27 bays wide with the five center bays occupied by the main entrances. The site, sloping down from E Street to C Street, allows a second floor entrance on the north facade and a first floor entrance on the south facade. The first floor is surrounded by a pink granite stylobate separated several feet from the sidewalk by a planting strip. The limestone facade, set back from the granite stylobate, rises eight floors. Smooth limestone walls at the first and second floors create a base on the street facades upon which sit three story limestone pilasters. The regularly spaced pilasters separate flat recessed panels with windows and support a plain architrave that hides the mechanical floor. Corners of the facades are smooth limestone walls without pilasters. The limestone facade steps in above the architrave on all of the street facades and rises two more stories. A penthouse rises two more stories above the central spine sitting back four bays from both ends of the building. A column and beam structural steel frame supports the building. The columns, positioned along the exterior walls, are roughly in line with the limestone pilasters on the street facades and interior columns flanking the ample central corridor. The structural steel frame, encased in concrete for fireproofing, supports eight inch thick concrete and terra-cotta hollow core floor slabs spanning between beams. Plastered terra cotta tile non-load bearing partitions are metal studs with lath and plaster. Exterior facades are ashlar limestone with brick back-up and bronze cramps. The interior plan consists of double-loaded corridors with each of the 2200 rooms set along the outside walls with windows. The corridor plan repeats the building part, with a long north-south, or main, corridor and six, evenly spaced axial corridors. The bulk of the building is devoted to office use, with special function rooms concentrated at the basement, first, and second floors and in the penthouse. Executive offices are located on the sixth floor and, to a lesser extent, on the 5th floor. By virtue of the sloping site, the two entrance lobbies are at different levels. To reconcile this disparity of floor levels, a grand staircase links the first and second floor Main Corridors. Circulation patterns were carefully considered with the result that both the main and axial corridors were exceptionally wide. The majority of interior spaces are, for the most part, given over to simple offices of modular design. A number of important, specialized spaces occur within the building: among them, the Library, Auditorium, Museum, Indian Arts and Crafts Shop, and the large Executive Suite for the Secretary of the Interior. Particularly noteworthy are the commodious circulation spaces which lend an aspect of logic and order to a building of some 2200 rooms. While architectural ornamentation was held in check, the building was generously filled with murals and sculpture.
The building which today houses the Interior Department was intended from the earliest planning to serve as offices for that Department. Harold Ickes, Secretary of the Interior from 1933 to 1946, had requested and secured approval for the erection of the building. Growth of the Interior Department was rapid during the period between the two World Wars, and the need for space was significant. Well-known architect, Waddy B. Wood, designed the building which was project number 4 of the WPA. Construction began in August of 1935 and was completed in December of 1936. The Interior Department Building was Wood's largest, and last, commission before his retirement in 1940.
The design for this building was influenced in large measure by Secretary of Interior Harold Ickes. The WPA was initially placed under Interior's jurisdiction, therefore Ickes worked closely with Wood and exerted a great deal of influence. Ickes personally selected Wood. He was insistent that the building not mimic the classical revival style of so many Federal buildings in D.C., and suggested that all offices be outside rooms. He especially enjoyed central air conditioning and wanted it to be included; the Interior Building was one of the earliest Federal buildings to be air conditioned. It was also the first Federal building to have a central vacuum cleaning system, and one of the first to incorporate a parking garage in the building. Ickes appointed various committees to oversee construction thereby limiting the role of the Treasury Department as supervising architect.
Among the most significant aspects were the spacious central corridors, open courtyards, movable steel office partitions, acoustically treated ceilings, an entire floor reserved for mechanical equipment, and fireproof design. Of special concern to Ickes was providing spaces intended to add an air of luxury and identity. These included an Art Gallery, Auditorium, Broadcast Studio, Employee's Lounge, Gymnasium, Library, Museum, and Indian Arts and Crafts Shop.
So identified was Ickes with the Interior Department Building that contemporary newspaper accounts refer to the building as "Ickes new home." In fact, the official portrait of Harold Ickes depicts him in his old Interior Building office studying the plans for the new Interior Building.