1. The regulations imposed under authority of sections 64.211 to 64.295 may be amended, supplemented or changed from time to time by the county commission after the order establishing the same has gone into effect, but no such amendment shall be made without a report and recommendation from the planning board after public hearing before the board, public notice of which shall be given in the same manner as provided for the hearing in section 64.231.

2. The districts created under authority of sections 64.211 to 64.295 may be amended, supplemented or changed from time to time by the county commission after the order establishing the same has gone into effect, but no such amendment shall be made without a report and recommendation from the planning board after a public hearing before the board. The hearing shall be held in any one place in the county designated by the planning or zoning board regardless of the location of the land affected by the amendment. Public notice of the hearing shall be given in at least one publication in a newspaper of general circulation in the county at least fifteen days before the date of the hearing.

3. In case of written protest against any proposed amendment, signed and acknowledged by the owners of thirty percent of the frontage within one thousand feet to the right or left of the frontage proposed to be changed, or by the owners of thirty percent of the frontage directly opposite, or directly in the rear of the frontage proposed to be altered, or in cases where the land affected lies within one and one-half miles of the limits of a municipality, by the city council or zoning board of any such municipality, filed with the county clerk, such amendment may not be passed except by the favorable vote of two-thirds of all the members of the county commission.