As an incident to the operation of any parking facility, the authority may devote a portion of its property to uses such as retail stores, bus terminal, gasoline service station, helicopter landing area, or any other commercial use, when in its judgment it is convenient or necessary to conduct or permit such use in order to utilize properly the property as a parking facility. Any such incidental use shall be secondary to the primary use as a parking facility, and the portion of the land devoted to the incidental use shall not exceed 25 percent of the surface area of the property. If a building is erected on the property for the purpose of parking motor vehicles, the incidental use of the building shall not occupy more than 25 percent of the floor area.