No decision of the Zoning or Planning Administrator, Planning Commission, Board of Zoning Appeals or City Council approving a site plan, permit, variance, or other zoning approval shall be valid for a period longer than two years, unless a building permit is obtained within such period and the erection or alteration of a building is proceeding under the terms of the decision, or the use is established within such period by actual operation pursuant to the applicable conditions and requirements of the approval, unless the Zoning or Planning Administrator grants an extension not to exceed one year.