In accordance with section R105.5 every permit issued shall become invalid unless the work authorized by such permit is commenced within 180 days after its issuance, or if the work authorized by such permit is suspended or abandoned for a period of 180 days after the time the work is commenced. The building official is authorized to grant, in writing, one or more extensions of time, for periods of not more than 180 days each. The extensions shall be requested in writing and justifiable cause shall be demonstrated. Exception: The building official shall be permitted to specify an expiration date of not less than 30 days, nor more than 180 days for commencement of work under permits issued to abate unsafe conditions pursuant to Section R115 of this code. Work performed under such permits shall be completed as expeditiously as possible.