The county planning board may prepare and adopt regulations governing the subdivision of land, within the unincorporated area, including all plats or proposed streets prepared for record, and all division of land or lots into two or more parts, except the division of land resulting in tracts or lots, the smallest of which is an area of five acres or more, or ten acres or more in any county having a population of at least two hundred seven thousand. The regulations may provide for the proper location and width of streets, building lines, open spaces, minimum width and area of lots, access to major streets, street grading and paving, drainage or utility easements, sewers, water and other utilities, which are necessary to avoid the congestion of population, and to protect the public health, safety or the general welfare of the inhabitants in the unincorporated portions of the county. The regulations shall be adopted by the board only after public hearing has been held thereon at any one place in the county, public notice of which shall be given in the same manner as provided for hearings in section 64.231. The regulations shall become effective after their approval and adoption by order of the county commission, and may after their adoption be amended by the county commission after public hearing and recommendation by the board as provided by section 64.231. In lieu of the immediate completion or installation of improvements required under the regulations adopted, the county auditor shall accept bond for the county in an amount and with surety and conditions prescribed by the county commission. The bond shall provide and secure to the county the actual construction of such improvements and utilities within the period prescribed by the regulations.