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. The department may grant one extension for a period not to exceed 18 months upon a satisfactory showing that an extension is justified. A permit shall become subject to revocation or modification if construction is not commenced and completed as provided in this section. This provision does not apply to the time period between the completion and start of construction, of the approved phases, of a phased construction project; each phase must commence construction within 18 months of its projected and approved commencement date.