No required open space in any off-street parking or off-street loading facility shall be encroached upon by buildings, storage or any other use; nor shall the required number of parking spaces or loading berths be reduced, except upon formal approval of the Planning Board and then only after proof that by reason of diminution in the floor area, seating capacity, or change in other factors controlling the regulations of the number of parking spaces or loading berths, the proposed reduction is reasonable and consistent with the public convenience and safety.