Parking facilities meeting these Regulations shall be provided off the street right-of-way for all new buildings erected, for all existing buildings which are enlarged to an extent exceeding 20% of the floor area existing on (effective date of these Regulations) and for all premises where the use is changed subsequent to said date. Parking facilities shall be sufficient to accommodate the motor vehicles of all occupants, employees, customers, and others normally visiting the premises at any one time. For those buildings where on-street parking is provided, 25% of the designated on-street parking spaces within 500 feet of the property lot may be included in the minimum required parking spaces for that building.