Abstract The aim of this study was to assess variations in indoor temperatures in London dwellings during periods of hot weather, and the degree to which those dwelling-to-dwelling variations are explained by the thermal characteristics of the dwelling and location within the urban heat island (UHI). Indoor temperatures during periods of hot weather were modelled using the EnergyPlus simulation programme, taking as input data the building characteristics of 15 notional dwelling archetypes broadly representative of the London housing stock, and assessed under warm future weather conditions at two locations within London. Data on dwelling types and characteristics were determined from Geographic Information System databases, national level domestic building surveys and other sources. External weather data were derived from the London Site-Specific Air Temperature model under the UK Climate Impacts Programme (UKCIP) 2002 2050s Medium–High emissions scenario. There was substantial variation in indoor temperatures across built forms. The thermal quality of a dwelling has an appreciably greater effect on indoor temperatures during the ‘hot’ period studied than the UHI itself. The effects of built form and other dwelling characteristics appear to be more important determinants of variation in high indoor temperatures than the location of a dwelling within London’s UHI. This observation suggests that policies aimed at protection against the adverse effects of high summer temperatures may need to focus more on dwelling design and construction than on the amelioration of the UHI.