For the purpose of this chapter, "floor area," in the case of offices, merchandising or service types of uses, shall mean the gross floor area used or intended to be used for service to the people as customers, patrons, clients, or patients, or as tenants, including areas occupied by fixtures and equipment used for display or sale of merchandise. It shall not mean floors or parts of floors used principally for nonpublic purposes such as storage, incidental repair, processing or packaging of merchandise for show windows, or for offices incidental to the management or maintenance of stores or buildings when these areas are in separate rooms from the main use of the building. Floors or parts of floors used principally for toilet or rest rooms or for utilities or for fitting rooms, dressing rooms, and alteration rooms, shall also be excluded from the definition of "floor area" for the purpose of this chapter.