The Chief of the Fire Division, with the approval of the Mayor, shall have power to modify any provision of the Fire Prevention Code upon application in writing by the owner or lessee, or his duly authorized agent, when there are practical difficulties in the way of carrying out the strict letter of the Code, provided that the spirit of the Code shall be observed, public safety secured and substantial justice done. The particulars of such modification when granted or allowed and the decision of the Chief of the Fire Division thereon shall be entered upon the records of the Division and a signed copy shall be furnished the applicant.