This paper investigates the impact of newly published CIBSE weather data on low energy buildings and the likely consequence on design. Emphasis has been given to the design assessment of naturally ventilated buildings. Three sets of weather data are applied and compared: CIBSE Example Weather Year (EWY), CIBSE Test Reference Year (TRY) and CIBSE Design Summer Year (DSY). The investigation has been focused on two locations only: London Heathrow to represent the south region of the UK and Manchester Ringway to represent the north region of the UK. The paper compares the raw weather data in terms of frequency of occurrence of external temperatures, followed by two case studies using a dynamic building thermal simulation program to quantify the impact of weather conditions on services design. Results are obtained by investigating the feasibility of a natural ventilation strategy, and the heating and cooling energy consumption if mechanical means are required to meet the internal comfort levels.