Connection to the public sanitary sewer 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 public sanitary sewers are not reasonably available, the subdivider may design the subdivision to be served by individual subsurface treatment facilities; provided that all lots not served by a public sanitary sewer shall have a minimum width of 100 feet, measured at the building line and a minimum area of 30,000 square feet.