Increased intensity of use. When the intensity of use of any building, structure, or premises shall be increased through the addition of dwelling units, gross floor area, seating capacity, or other units of measurement specified herein for required parking or loading facilities, parking and loading facilities as required herein shall be provided for such increase in intensity of use. However, no building or structure lawfully erected or use lawfully established prior to the effective date of this chapter shall be required to provide such additional parking or loading facilities unless and until the aggregate increase in units of measurement shall equal not less than 15% of the units of measurement existing upon the effective date of this chapter, in which event parking or loading facilities as required herein shall be provided for the total increase; provided, however, that in the case of the expansion or alteration of residential buildings, required parking or loading facilities shall be provided on the basis of the total required units of measurement for the entire capacity of the buildings.