Hydrologic impact assessment is necessary for a planned development area in designing detention storage for urban drainage systems to minimize the effect of urbanization. It also has strong implication on pollutant loads from urban areas. For such assessment, single or two hydrologic models can be paired for pre- and post-development conditions. Typical pairings are the use of synthetic hydrograph methods for both conditions or synthetic hydrograph for pre-development and urban hydrology model for post-development condition. The former has the difficulty of accounting for drainage structure for post-development condition and the latter can run into erratic evaluation as the error can be introduced due to different model conceptualizations and parameterizations. In order to overcome the aforementioned shortcomings, the use of Storm Water Management Model (SWMM) for both pre- and post-development conditions is proposed in this study. The SWMM was applied to four planned development areas in Korea. The comparison of the results with previous assessments done for the same sites showed that the new approach can resolve the irrationalities that can occur with the combination of two different models such as smaller peak flow and longer time to peak for post-development condition. It is thought that the proposed method improves the accountability of the hydrologic impact assessment on planned development areas.