If, after a period of ten years, a work of improvement for which the benefit district was formed is not commenced, the city council may by resolution extend the benefit district for a period not to exceed five years, if it finds that less than seventy-five percent of the undeveloped land area in the district at the time it was formed has not been developed and charged the benefit fee applicable to the district. The city council may continue to extend the benefit district in each instance for a period of five years, if it makes such finding upon substantial evidence at the time of the extension. In no event shall the amount of charges applicable to real property already approved for development within the district be increased as a result of an extension of the benefit district. (Ord. 1303 NCS §1 (part), 1978.)