These regulations shall apply to every subdivision (as defined herein) occurring inside the corporate limits of the City of Monticello, Illinois or occurring in the unincorporated areas within 1-1/2 miles from the city’s corporate limits, as those limits exist at the time the subdivision occurs; provided, however, that if the majority of the subdivision, as shown on the preliminary plat thereof, is situated further than 1-1/2 miles from the corporate limits, the city shall apply to the subdivision the subdivision standards of the County of Piatt.