This study evaluates the impact of climate change on the rainwater harvesting potential of a single-family residential building in southern Brazil. Based on daily precipitation data from 19 climate models, the historical (1980–2013), near-future (2020–2060), and far-future (2060–2100) periods were analysed, considering the SSP2–4.5 and SSP5–8.5 emissions scenarios. The average water consumption in the building is 716 l per day, of which 50% is considered for non-potable uses, such as toilet flushing and cleaning. The analysis considered the volume of rainwater that could be harvested and the number of days on which such water would be sufficient to meet the non-potable water demand. The results indicate significant changes in the rainfall regime over the coming decades for the city evaluated, with direct implications for the efficiency of the rainwater harvesting system. These differences are due to variations in total rainfall and distribution throughout the year, which are expected to change in the following decades in southern Brazil. The study contributes to understanding the climate risks associated with urban water security and can subsidise adaptation strategies and integrated water resource management in residential buildings. In this way, it is possible to act with greater knowledge about future climate precipitation models, which helps with resilient and adaptive management.