How long is a building permit valid?

2024 International Residential Code, R105.5 Expiration. a.) Every permit issued shall become invalid unless the work authorized by such permit is commenced with 180 days after its issuance or after commencement of work if more than 180 days pass between inspections. The building official is authorized to grant, in writing, one or more extensions of time, for periods not more than 180 days each. The extension shall be requested in writing and justifiable cause demonstrated. b.) Every building permit shall expire two (2) years from the date of issue or upon the issuance of a certificate of occupancy unless: 1). The Building Official determines that substantial progress has been made in the work authorized by the permit; and the permit holder is granted an exception or extension after submitting a written request to the Building Official.