At the scale of individual buildings, rainwater harvesting (RWH) performance can be precisely estimated using water balance modeling, which accounts for rainfall input, storage dynamics, and water demand. Simplified city scale approaches, based on average annual rainfall and total water consumption, often overlook variations in roof area, occupancy, and demand patterns, as well as the influence of storage capacity. In this study, a GIS supported methodology is developed to estimate urban RWH potential at the building block scale, while enabling aggregation of results to larger spatial units. The analysis is applied to Statistical Zone 26 in Turin, Italy, which serves as a spatial framework for data integration rather than as the primary unit of calculation.