The new CIBSE design summer years (DSYs) for London Weather Centre, Heathrow and Gatwick in London are now available for three baseline years: 1976, 1989 and 2003. This study tested how these different design summer years impacted the assessment of overheating in a naturally ventilated office in London. Two office designs were tested, an uninsulated and one retrofitted with insulation and night cooling. The choice of baseline year impacted the level of overheating for both the uninsulated and retrofitted models. Tested in the more severe years, 1976 and 2003, the offices experienced the highest levels of overheating. When an office was retrofitted and night cooled, the choice of location had more of animpact on overheating due to the urban heat island effect. London Weather Centre and Heathrow experienced higher levels of overheating than Gatwick. The study highlights the need for designers to carefully consider how the differences between the weather files will impact their overheating assessment depending on their buildings’ fabric and ventilation design.