Exposure to extreme heat increases the risk of developing heat-related illnesses. Climate change and urbanisation are increasing the global population exposed to extreme heat, posing a threat to public health. Despite the fact that people spend about 90% of their time indoors, indoor temperatures are much less documented than outdoor temperatures. Therefore, we will monitor indoor temperatures in 100 homes in Amsterdam, the Netherlands, and explain the temperatures based on factors related to neighbourhood design, building morphology and resident behaviour. Based on the unique results, we will make recommendations for climate-sensitive urban design to reduce indoor heat stress.