The Internal Revenue Service Building is located in the center of the Federal Triangle complex in Washington, D.C. It is bounded on the north by Pennsylvania Avenue, and the west by 12th Street. The IRS Building is rectangular in form with a circular section on its northwest corner facing 12th Street and an L-shaped arm from the northwest corner on 10th Street. The main building has 4 interior courts which divide the great mass of the building and provide the inner offices with daylight and ventilation. The building is 7 stories in height, although the architectural treatment of the exterior walls gives the illusion of 5 stories. Th 6th floor is located behind the main cornice and the 7th floor is set back behind the balustrade. The general architectural style for the Federal Triangle was the 18th century French Renaissance style, which in turn derived its inspriation from the Italian Renaissance architecture. The IRS Building is one of the more simple interpretations of this style in the Federal Triangle.
The Constitution Avenue facade contains the main entrance to the IRS Building and received the most elaborate decorative treatment of all the building's elevations. The remaining facades have the same general characteristics as the Constitution Avenue elevation, which has a rusticated limestone base of 14 courses extending from the pink granite base course or stair at ground level through the second floor to form the first horizontal division. Above the first floor windows, the rusticated design resembles a jack arch with the three central keystones extending vertically through two courses. The Constitution Avenue elevation is 27 bays wide. All except the first and last three bays of the base project approximately 4 feet from the building plane.
The upper floors are dominated by a 3-story colonnade which extends above the projecting base. Above the 3rd floor belt course, a balustrade motif defines the beginning of the principal stories. On the colonnade, a balustrade separates the column bases. The wall surface is smooth faced limestone ashlar. Rusticated limestone, more finely detailed than the base of the wall, serves as quoining on either end of the elevation, and faces the wall surface behind the colonnade. Pilasters shadow the pairs of columns at either end of the colonnade. The facade is capped by a Corinthian entablature, which begins at the 6th floor level. This is composed of 3 horizontal divisions. Another balustrade with paneled piers sits atop the facade at the 7th floor level.
The 10th Street elevation is the longest of the building, extending over 600' from Constitution to Pennsylvania Avenue. Two pedimented prostyle porticos of 3 bays each mark each end of the elevation. The south portico has four marble columns, the north portico has 4 limestone columns. Two end pilasters, identical to those on Constitution Avenue, mark the sides of each of the porticos on the facade wall. Centered in the 10th Street elevation is a major entrance with a single wide stair and three flat arched openings. Above this central entrance, beginning at the 3rd floor level, is a screen of 18 limestone, unfluted pilasters of the Doric order. A street level arcade with 3 arched openings approximately 1-1/2 stories high is centered between the projecting central bays and the north portico. Balustrades matching those at the 7th floor level extend across the bottoms of the openings.
On the Pennsylvania Avenue facade, the 5 bays nearest the corner of 10th Street and Pennsylvania Avenue are idential to the unpilastered bays on 10th Street. The remaining six bays project several feet and have a screen of eight limestone pilasters similar to those on 10th Street.
The 14 bays on the south eand of the 12th Street facade repeats the basic wall design established on the Constitution Avenue facade. Small one story entrances are located on the second bay in from each end. Because the circular elevation on the northwest corner of the 12th Street facade was to form part of the Great Circle with the New Post Office (Ariel Rios) Building across the street, its decorative details mirror those of the other Federal building. The circular elevation has a 3-sided pavilion at its corner, directly opposite the south pavilion of the New Post Office. The decorative details on both are identical.
The interior spaces of the IRS Building are organized around four courtyards with office bays located on either side of a central corridor. With this arrangement, all offices are provided with operable windows, facing the streets or the courts. The most elaborate space in the building is the 2 story lobby in the center of the south side of the building. The original design of this space remains intact. Most of the office corridors retain their original terrazzo floors with brass dividing strips and marble borders, marble baseboards and plinth blocks. All the special offices in the IRS Building, the Tax Court Hearing Rooms, the Commissioner's office and the 7th floor Court Room have undergone remodeling. General office space is typical and has also undergone remodeling.