No permit for the erection, razing, change, alteration or removal of buildings shall be valid or effective after six months from the date of issuance thereof, and it shall thereafter be void, unless the work authorized by such permit shall have been substantially commenced within six months from the date of issuance and proceeded with, with due diligence. If, however, the applicant has been delayed in proceeding with the work for which the permit was granted by reason of any reasonable cause not due to his own negligence, the permit may be renewed without additional cost to the applicant.