The depth of sewers in residential complexes was determined to prevent the separated sewers from misconnection between storm sewer and sanitary sewer, and from the submersion of the basement by minimizing the phenomenon of backwater when it rains. In residential complexes, main causes of the submersion were the misconnection of sewers, rising of the backwater level at outfall in sewer system, poor maintenance of sewers, and lacking in their cross section. Minimum depth of sewers should be over 1.2~1.5m. According to the economic analysis, the depth of 1.5m~3.0m was appropriate for minimizing the submersion of basements and for making the disposal of domestic wastewater more easily.