Because of climate change, present-day buildings will be subject to more extreme conditions by the end of the century. Indoor overheating will increase in summer, resulting in severe thermal discomfort or significant cooling needs to avoid heat stress. Existing buildings therefore need to be adapted. However, current renovation strategies are generally aimed at reducing heating energy consumption. This study presents a methodology for assessing the impact of climate change during summer and heatwave periods on indoor overheating of an existing building within an urban context. The proposed methodology is applied to the case study of a French urban dwelling within an energy-efficient renovated building. The thermo-aeraulic dwelling model built on TRNSYS and CONTAM, as well as the urban heat island model implemented on UWG, are calibrated and validated thanks to a field measurement campaign of a multi-family building in Lyon during the summer of 2022. Results show that thermal discomfort will rise sharply by the end of the century, and could reach extreme levels in the worst emission scenario. Use of windows and solar protections allows to reduce the percentage of time over 28 °C from 40 % to 54 % depending on the future scenario. Ventilation by opening windows, which is the most commonly used cooling passive strategies, continue to be essential to reduce indoor overheating in high insulated building under future climates. However, its efficiency is very dependent of occupant behaviour.