While the exterior skin of the Guaranty expresses a new form for the steel skyscraper, its plan indicates those hard realities of function necessary to construct such a building and to sell it. The building is essentially a U-shaped plan stacked upon a rectangular solid. The interstitial spaces between wings of the “U” create opportunities to introduce skylights to the lobby below, and to cover the ceilings with stained glass. The plan contained a single vertical circulation core with four elevators, a mail slot, and staircase. No fire-stair was provided or necessary. The internal portion of the “U” faces south so as to collect light for the interior recesses of the building- light being a necessary commodity to attract good tenants. Sullivan spared nothing to accomplish this end for: “In order to increase the amount of light to the interior, the stairwell and the light slit facing the inner courtyard were lined with white glazed terra-cotta that was more costly than normal tiles.” [4]
The first and second floors are united both spatially and visually through additional staircases and the intention of retail occupation. Mechanical systems were relegated to the basement, including the motors for the elevators, boilers, and electrical “dynamos.” Entrances were provided on both Church and Pearl Streets. A concierge desk offered services to tenants and guests including mail delivery. Above the “base” of the building were a series of office floors of identical plan were placed. These floors featured private lavatories in reconfigurable office spaces. The halls were defined by wood and glass partition walls, intended to give the interior a bright and “club” like feeling. The elevators and staircases were enclosed not by walls, but metal cages permitting southern light to penetrate through the circulatory systems and into the hallways.
The only exception to the rise of offices was the seventh floor with lavatories and a barbershop, and the top floor with a US Weather Service Bureau office and spaces for building attendants.