A research project for the Flemish Environment Agency (VMM) revised the hydrological design parameters, which are currently used in the guidelines for the design of urban drainage systems. The revision involved extrapolation of the design rainfall statistics, taking into account the current knowledge on future climate change trends (till 2100). Uncertainties in these trend projections have been assessed after statistically analyzing and downscaling a large number of climate model simulation results. Based on the climate scenarios and related changes in rainfall statistics, changes in flood frequencies of sewer systems and overflow frequencies of storage facilities have been quantified. Also the change in storage capacity, required to exceed a given overflow return period, has been calculated, for a range of return periods and infiltration or throughflow rates.