Permanent stormwater quality structures in developed urban areas require the use of smaller footprints and underground structures. This project was undertaken to see if an underground treatment structure composed of offthe- shelf precast concrete sections would have adequate performance and require minimum maintenance. The project consisted of a literature review of small footprint and underground stormwater treatment devices, their cost, performance, and maintenance considerations; a physical model study to test the principles to be employed and construction of a full-scale prototype for proving the concepts. The project clearly demonstrated that extended detention can be an effective tool in removing suspended solids from stormwater compared to other proprietary devices. It also shows that less frequent maintenance will be needed to maintain the overall level of performance. While the efficiency of the structure is around 75 percent removal, approximately 17 percent of the solids discharged can be attributed to resuspension. Several refinements show promise in significantly reducing the resuspension loading which could increase the overall efficiency to over 80 percent with maintenance intervals in excess of twelve months. For this reason the project is being extended another year to further refine the performance and develop better design guidance.