Historically, sustainable drainage systems (SuDS) have been usually event-based designed, like traditional pipe and storage grey infrastructure, by applying design storm events derived from intensity-duration-frequency (IDF) curves established with rainfall data records. However, this approach has several drawbacks, especially: i) it neglects the dry inter-event periods which play a key role with evapotranspiration for vegetated SuDS like e.g. green roofs, ii) the design storm events themselves are considered as fully deterministic and not as a stochastic input, and iii) storm events based on past records do not account for climate change. The paper presents a more refined approach, named Highly-Informed-Design-Evaluation-Strategy (HIDES), which considers both i) long term simulations with future rainfall and potential evapotranspiration time series generated by means of downscaling IPCC modelling scenarios, and ii) stochastics design events for different levels of performance (day to day rain, major events, extreme events). An application to two green roofs simulations is shown.