Abstract: This study focuses on the possibility of On-Site Stormwater Detention (OSD) underneath a residential car porch. The space provided in the car porch area can be made good of it by introducing an OSD beneath it to temporary store stormwater from roof during raining in hoping of reducing the surface runoff. The OSD is subjected to 15-minute, 10- year Average Recurrence Interval (ARI) design rainfall. This process in urban hydrology is presented by using Storm Water Management Model (SWMM). Efficiency of the OSD is also investigated by different number of orifice outlets. Modelling efforts report that one orifice outlet is preferable, and it marks approximate 95% discharge reduction at the outfall.