A proposed change of zoning classification of any individual parcel or parcels may be initiated by City Council, the Planning Commission, or by petition of one or more owners of property within the area proposed to be changed or affected by the change in such classification. Such change must be adopted by ordinance by City Council following referral from the Planning Commission. All proposed changes or petitions shall be in writing and shall specify the nature and extent of the change desired, as well as justification for the change. In every instance such amendments shall be consistent with good zoning practice. Considerations of public necessity, convenience, and general welfare shall be relevant to any rezoning request considered to the extent such factors are pertinent to the subject matter of the amendment.