An eleven year old constructed wetland system for storm water treatment at Stammersdorf near Vienna has to be redesigned. The small sand trap and the usage of loamy material as substrate for the main layer of the reed bed led to a rapid clogging of the system and to a very limited elimination rate. To obtain data for parameters (catchment hydrology, loading rates) for the new design, online measurements at the inlet of the system of water flow and some physical parameters were undertaken as well as sampling and analysis of the inlet water (TSS, TOC, NH4‐N). The new design includes two parallel ditches (each 1380 m3) which are a combination of settling and retention ponds. The regulated outlet of the ditches is now treated in the main treatment zone which is a horizontal subsurface flow constructed wetland (1500 m2) with coarse sand (1/4 mm) as main layer. The final structure is a polishing pond (800 m2) after which the effluent is recharged into the groundwater.