The Fire Marshal shall have power to modify any of the provisions of the Standard Fire Prevention Code hereby adopted upon application in writing by the owner or lessee, or his duly authorized agent, when there are practical difficulties to 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 Fire Marshal thereon shall be submitted to the City Secretary and a signed copy shall be furnished the applicant.