An approved conditional use shall be completed within two years following the date of approval. However, the Zoning Officer may grant an extension of time if the landowner, or his agent, requests such an extension, and if good cause for the extension is shown to the Zoning Officer's satisfaction. There are no other exceptions to this rule. If, at the end of the two-year period, the conditional use is not completed, and if no extension has been granted, the approval of the conditional use shall be null and void unless renewed by Council.