Connection to a public water supply system is required for every lot in a proposed subdivision located within the corporate limits. Whenever a subdivider can provide conclusive evidence to the Board that a public water supply system is not reasonably available, the subdivider may design the subdivision to be served by individual water supply systems; provided that all lots not served by a public water supply system shall have a minimum width of 100 feet, measured at the building line, and a minimum area of 20,000 square feet.