The building is a three story rectangular building with the basement set partially below grade. The basement is clad in granite that rises to the level of the belt course of the first floor window sills. Above that, the first and second floors are clad with light brown Pompeiian brick and articulated with matching brick Corinthian pilasters containing terra cotta capitals. Windows are rectangular fixed light units on the first floor, and round-headed fixed light units on the second floor. The window frames are terra cotta and provide visual contrast with the brick on the facades. Above the second floor is the terra cotta Ionic order entablature, which contains an architrave, a floriated, bracketed frieze set on top of egg & dart mouldings, and a decorative cornice embellished with lions' heads. Above the entablature is a terra cotta balustrade, set on a terra cotta plinth.
The massing of the building is rectangular, roughly 144' by 275', oriented so that the short dimension faces South Park Row, to the north. The front (north) elevation contains a slight projecting center bay, at the entrance. In 1992, the large projecting Ionic Order covered porch was dismantled and store on site due to structural instability. As part of the 2002 New Addition/New Federal Building, this porch was restored back to its original location. Directly above the main entrance is a terra cotta Palladian Window, with recessed circular niches and festooned garlands across the head.
The east and west facades contain projecting central sections; each is three bays in width. On the east side, the secondary entrance to the building is set in a rusticated granite entranceway with an arched door. The arched window on the second floor, directly above, is also flanked by coffered niches, set in the brick. On the west side, a portion of the building is currently enclosed within an interior glass atrium, which was the result of the 2002 New Addition/New Federal Building.
On the interior of the building, the primary public space is the Main Hall, positioned directly in the center, and designed so that each major room is directly connected. The Main Hall is open at the second floor level (which contains a balcony that connects all the public spaces on the second floor) and is capped with a square laylight above. On the west side, a large wooden stair with a landing provides the primary means of getting between floors.
Flanking the Main Hall, in the northwest and northeast corners of the first floor (respectively) are the Fiction Room and Reference Room. These rooms are largely unaltered and contain plastered ceilings and walls, with decorative coffered areas in the ceilings to break up the mass and perceived size of the rooms. These rooms also contain original decorative wood bookshelves, set on wood bases, and capped with decorative wood cornice trim. Directly to the south of the Reference Room is the Small Reference Room, which contains a mezzanine and built-in wood shelves on both levels. To the south of the Main Hall is the Nonfiction Room, which was modified in the 1920s and contains metal shelves and a combination metal shelving/mezzanine structure, with integrated partitions for offices. The Fiction Room and Reference Roob have been converted into Courtrooms and Offices.
The primary room on the second floor is the Plavcan Gallery, located in the center of the north side, connecting with the upper part of the Main Hall. The Plavcan Gallery is a large room that contains a vaulted, coffered ceiling with laylights set in the coffers. On the south side of the second floor, two of the offices contain laylights in the ceilings.
The basement has been completely remodeled in 2002, and now features offices and storage space.