Climate change and the increase of impermeable surfaces due to urbanization have led to an increase in the frequency of flooding events. The occurrence of water shortage problems is attributed to an increased population and water pollution. Traditional methods of draining rainwater from rooftops, based on the Rainfall-Discharge (R-D) model, are challenged. By storing some of the rainfall that falls on building rooftops, flooding of nearby sewer systems can be mitigated, and the reduction of peak runoff can then calculated using the Rainfall-Storage-Discharge (R-S-D) model. By utilizing stored rainfall in or near a building, flooding can be further mitigated while conserving water, and the reduction of peak runoff and the amount of used rainwater can also be calculated using the Rainfall-Storage-Utilization-Discharge (R-S-U-D) model. The R-S-U-D model uses site-specific rainfall characteristics; thus, we take Seoul, Korea as a study case. The reduced peak flow and increased design return period for a combination of tank volume and utilization rate can be calculated and presented by a curve, which can be used in the design and operation of sewer systems. Results from the R-S-U-D model can help in designing methods to reduce the flooding risk in existing sewer systems without increasing their capacity, thus reducing expenses. Water conservation is an additional benefit.