The regulations imposed and the districts created by this chapter may be amended from time to time by the Council, but such amendments shall not be made without public hearing before the Council and after a report has been made upon the amendment by the Commission. At least seven days’ notice of the time and place of such hearing shall be posted as required by Code of Iowa. In case the Commission does not approve the change or, in the case of a protest filed with the Council against a change in district boundaries signed by the owners of 20 percent or more either of the area of the lots included in such proposed change, or of those immediately adjacent thereto and within the 200 feet of the boundaries thereof, such amendment shall not be passed except by the favorable vote of three-fourths of all members of the Council. Amendments need only refer to the legally described land involved and need not refer to the Code section to be valid. Such referral to only the legal description will not invalidate the map or this chapter other than to change the zoning of the land described.