LAND SUBDIVISION AND PLAT ADMINISTRATION. The Zoning Department is charged with regulating the division of land in the unincorporated areas of the county. It is the responsibility of the department to ensure that the Illinois Plat Act Law is adhered to. The county has also adopted rules and requirements that need to be met to make legal split (a new property created from a larger piece). For example, each Zoning District has a designated lot size and minimum acreage requirement.