As evidence of climate change increases, so too are the effects of weather events and climate variability on civil engineering projects becoming increasingly recognised and understood. The changes indicated by the UKCIP02 scenarios, for example, suggest a number of threats and opportunities for the built environment. The Ensembles project will develop an ensemble climate prediction system for use across a range of timescales (from seasonal to decadal and longer) and spatial scales (from global to regional and local). The outputs from this European Commission funded project will allow further development, within a probabilistic framework, of tailored scenarios for urban areas developed as part of the UKCIP/EPSRC Building Knowledge for a Changing Climate programme. Examples of these projections for London Heathrow are presented; they illustrate the uncertainties in projected changes in extremes for urban and other sites. Modelling work that indicates the importance of representing urban areas within climate simulations is also presented.