The regulations imposed and the districts created under authority of this act¹ may be amended from time to time by the board of county commissioners by order after the order establishing the same has gone into effect, but no such amendment shall be made without a hearing before the county planning commission. In case of written protest against any proposed amendment, signed and acknowledged by the owners of twenty percent (20%) of the frontage within one thousand (1,000) feet to the right or left of the frontage proposed to be changed, or by the owners of twenty percent (20%) 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 (1 1/2) miles of the limits of a municipality, by the governing body of the zoned municipality, filed with the county planning commission, such amendment may not be passed except by the favorable vote of all members of the board of county commissioners.