In 1914 the Art Commission (now the Public Design Commission) moved its offices from the second floor of City Hall to the third floor, where it resides to this day. Originally a half-story attic space, the third floor was rebuilt as a full story after a fire substantially damaged it in 1858. It then served as an apartment for the building's custodian and his family, with four bedrooms, a kitchen, dining room, parlor, and bathroom.
The Commission's new space was renovated by Grovsenor Atterbury to meet the needs of a public review agency and officially reopened on December 28, 1914. The floor included a storeroom, offices, library, and board room, all connected by a circular hallway that surrounds the rotunda with windows that provide views of the "floating" staircase below and doors that lead to the east and west attic spaces. Atterbury also added a marble stairwell from the second floor rotunda to the northwest corner of the third floor. During the 2010-2015 City Hall rehabilitation project a limited use elevator and new egress stairwell were added to meet building codes.
The board room was originally decorated with antiques donated by the Commission's president, Robert W. DeForest, most notably the fireplace mantel from the Arnold Tavern in Morristown, New Jersey (built 1735-1750). Along with the mantle, many other pieces from the late 18th century to early 19th century still adorn the offices today, including a cherrywood secretary, hand-painted grandfather clock, and tavern table that is still used for the Commission's monthly meetings. A selection of items, including an 18th-century settee and a large Dutch kas, were restored and loaned to Gracie Mansion.
A number of other antiques and objects have been collected by the Commission over the years, including a French neoclassical bronze clock, two 19th-century giltwood convex mirrors, a circa 1820 Dutch blanket chest, two wrought iron American colonial chandeliers, a wooden boat model, and a 1920s settee (featured in the first Ghostbusters movie), as well as various artwork models that were submitted for review, such as the studies for the Maine Monument (1901-1913) by Attilio Piccirilli and the Henry Hudson Memorial (c. 1935-1938) by Karl H. Gruppe.
The Commission's library includes thousands of publications, most of which were collected during the City Beautiful movement between the late 1890s and early 1900s. In addition, selections from the City Hall Portrait Collection line the walls, including Edward Ludlow Mooney's painting of the secretary to the Sultan of Zanzibar, Ahmad bin Na'aman. The Commission acts as caretaker of this collection, as well as City Hall's other historic artworks, antiques, and decorative objects.