Many existing detention basins have the potential to be retrofitted to improve water quality control performance. A single family residential development with two (2) existing dry detention basins was selected to perform drainage evaluations and final design studies as part of implementation of a sub-surface wetland (SSW) system. Composite base plans were developed to perform the site assessment and the hydrologic and hydraulic studies. An overall model of the entire project area was prepared utilizing the US Army Corps of Engineers (USACOE) HEC-1 Model with the Natural Resources Conservation Service (NRCS) Methodology to facilitate calculation of peak flows and hydrographs, routing through the detention basins, and combination of hydrographs. The model was calibrated and verified based on previous measured storm events. The study also included evaluations of forebays, flow splitters, cost estimates, and identification of permits. The detailed hydrologic and hydraulic evaluations revealed that the potential negative hydrologic/hydraulic impacts and the site constraints necessitated the need to investigate alternative locations and designs for the SSW. This study demonstrated the importance of the need to perform a detailed hydrologic and hydraulic analysis of anticipated retrofits. Incorporation of retrofits into existing basins, will enhance water quality-treatment, but may have negative hydrologic/hydraulic impacts, such as increasing existing peak stormwater flows downstream or flood water surface elevations upstream of the basin because of the lost of existing flood storage. Siting SSW's outside existing stormwater management facilities may be necessary to minimize hydrologic and hydraulic impacts. A retrofit of an existing stormwater management facility must balance providing enhanced water quality control against costs and secondary adverse impacts.