The existence of a valid permit shall not be construed as authorizing construction if construction is not commenced within 18 months after the date of permit issuance, if construction is discontinued for a period of 18 months or more, or if construction is not completed within a reasonable time as determined by the department. Up to an 18-month extension may be granted by the department upon a showing of good cause in a written request by the facility owner or operator. The department may suspend, modify or revoke the permit or registration pursuant to Part 621 of this Title if construction has not commenced within 18 months of issuance of such permit or registration, or construction has been discontinued for a period of more than 18 months at any point after issuance of such permit or registration.