At the time of initial occupancy, major alteration, or enlargement of a site, or of completion of construction of a structure or of a major alteration or enlargement of a structure, there shall be provided off-street loading facilities for trucks in accordance with the schedule of off-street loading berth requirements prescribed in § 36-25.8. For the purposes of this section the terms "major alteration" or "enlargement" shall mean a change of use or an addition which would increase the number of loading berths required by not less than ten (10) percent of the total number required. The number of loading berths provided for a major alteration or enlargement of a site or structure shall be in addition to the number existing prior to the alteration or enlargement, unless the pre-existing number is greater than the number prescribed in § 36-25.8, in which instance the number in excess of the prescribed minimum shall be counted in calculating the number provided to serve the major alteration or enlargement.