Every grading permit shall expire by limitation and become null and void if the work authorized by such permit has not been commenced within six months or is not completed within one year from the date of issue. The City Engineer may, if the permit holder presents satisfactory evidence that unusual difficulties have prevented work being started or completed within the specified time limits, grant reasonable extensions of time, not to exceed three month extensions, provided the application for extension of time is made before the date of expiration of the permit.
(`94 Code, § 151.39)