The Federal Building is a five-story building, including a full-height basement, located on I Street between 8th and 9th Streets. The building is 145 feet wide by 267 feet long, rectangular in form to the second floor, and E-shaped above that level. The middle leg of the "E" is shorter than the east and west wings, which were extended northward to the alley in 1938. The building rises 78 feet to a hipped clay tile roof.
The building combines elements of the Beaux Arts tradition with those of the Mediterranean Revival style. The simple massing and surface treatment of the building is offset by the richness of ornament and pattern exhibited in the metal grilles and the cast terra cotta surface ornament at doorways, friezes, cornices, keystones and grilles. The base of the building to the first floor is clad in a smooth ashlar granite. The upper floors of the three principal facades are deeply rusticated at the first floor, with shallow rustication above. The surface material of the three principal facades is granitex, an architectural terra cotta with a granite-like finish. The north or rear elevation along the alley is surfaced with a gray colored brick which matches the color of the granitex material. The distinguishing feature of the primary elevation along I Street is a recessed bay punctuated by a series of fourteen three-story fluted doric columns.
The building is organized around an internal corridor running east-west along the southern portion of the building and extending into the three legs of the "E" at the upper levels. Tenant spaces are grouped around the corridors and receive light from perimeter windows. The main corridor at the ground level, by contrast, receives no natural light. This spectacular public space features a coffered ceiling, finely detailed marble, patterned terrazzo floors and an ornamental metal and glass bay system. Main entrances to this space are located at the east and west ends of the south elevation. Stairs and elevators are also located at the far ends of the main east-west corridors.