Buildings are designed for a specific climate, yet they often have a lifetime of 100 years. Climate change may require a building to operate over a range of climatic conditions as a result of the impact of global warming. This effect is compounded by the fact that many of the climatic data currently used for design purposes are some 30 years old. This paper investigates the impact that climate change over the last two decades may have on the design and performance of buildings by examining the specific effects of changes in temperature and solar radiation on one particular design variable, energy use. The results indicate that the temperature and solar radiation in the London region of the United Kingdom have changed significantly in the last 15 years, and that the climatic data currently being used for energy design calculations lead to inaccuracies in predictions of energy use of up to 17%. However, evidence is presented which suggests that a proportion of the benefits of milder winters may be taken as improved comfort instead of reduced energy use. It is concluded that the climatic data used in these calculations should be reviewed urgently, as they may be leading designers to adopt solutions which will be inappropriate for future use.