Whenever a plan for major highways has been prepared, the county commission, upon recommendation of the planning board, is hereby authorized and empowered to establish, regulate and limit and to change and amend, by order, building or setback lines on the major highways, and to prohibit any new building being located within the building or setback lines outside the corporate limits of any city, village or incorporated town. When a plan for proposed highways or other public improvements has been prepared, the county commission in the same manner is authorized to prohibit any new building being located within the proposed site or right-of-way when the center line of the proposed highway or the limits of the proposed sites have been carefully determined and are accurately delineated on maps approved by the planning board according to the procedures set out in section 64.231 and adopted by the county commission. The county commission shall provide for the method by which this section shall be administered and enforced and may provide for a board of adjustment with powers to modify or vary the regulations, in specific cases, in order that unwarranted hardship, which constitutes an unreasonable deprivation of use as distinguished from the mere grant of a privilege, may be avoided. If there is a county board of zoning adjustment, as herein provided, the board shall be appointed to serve as the board of adjustment for the building line regulations. If there is no county board of zoning adjustment, the personnel, length of terms, method of appointment and organization of the board of adjustment for the building line regulations shall be the same as provided in section 64.281 for the board of zoning adjustment. The regulations of this section shall not be adopted, changed or amended until a public hearing has been held thereon by the planning board, public notice of which shall be given in the same manner as provided for the hearing in section 64.231.