This study aims to assess the necessity of updating the Intensity-Duration-Frequency (IDF) curves used in Portugal to design building storm-water drainage systems. A comparative analysis of the design was performed for the three pre-defined rainfall regions in Portugal using the IDF curves currently in use and estimated for future decades. Data for recent and future climate conditions simulated by a global/regional climate model chain (ECHAM5/MPI-OM1/COSMO-CLM) are used to estimate possible changes of rainfall extremes and its implications for the drainage systems. The methodology includes the disaggregation of precipitation up to sub-hourly scales, the robust development of IDF curves and the correction of model bias. Obtained results indicate that projected changes are largest for the plains in Southern Portugal (5 -33%) than for mountainous regions (3 -9%) and that these trends are consistent with projected changes in the long term 95 th -percentile of the daily precipitation throughout the 21 st century. We conclude for the need to review the current precipitation regime classification and change the new drainage systems towards larger dimensions to mitigate the projected changes in extreme precipitation.