A warming climate, increasing frequency and severity of extreme heat events, and the urban heat island (UHI) effect, are all expected to intensify thermal loading on buildings. This paper examines how the heat island affects space-conditioning loads within urban and suburban office buildings, and how the trend of replacing traditional heavyweight facades with lightweight alternatives can affect the magnitude and timing of the heat island experienced, as well as building energy use. This paper addresses this through simulation studies of street canyons based on the urban Moorgate and suburban Wimbledon areas of London. The results showed that including the heat island in dynamic thermal simulations to have an adverse effect on annual space-conditioning for urban canyon scenarios including stone facades, as well as a glazed alternative. With the suburban condition, a modest decrease in annual space-conditioning was shown for buildings with brick facades, while a white-painted timber alternative showed a marginal increase. The study demonstrates that the trend in urban centres to replace heavyweight building facades with lightweight insulated ones can increase space-conditioning loads, and thereby adversely affect the heat island to create a warming feedback loop. Within a suburban context however, the same change decreased space-conditioning loads to present a beneficial effect. The study in turn stresses the significance of accounting for heat island loads when estimating urban and suburban energy use, for which a combined simulation approach has been presented as an analysis pathway.