In the context of transition to sustainability, one of the main challenges facing societies today is the supply of water. By integrating different methodological tools and studies we developed the innovative software program Plugrisost® (rainwater, greys and sustainability), a simulation model, that facilitates the economic evaluation and the potential environmental impact of alternative water supplies (rainwater harvesting [RWH] and greywater systems) at different scales of urban planning. This modelling tool contributes to urban water planning for smart cities development. Plugrisost® analyses the optimal design variables, cost and environmental performance of RWH and greywater systems, using tap water production as a reference system for comparison. The use of economic and environmental indicators can make the optimal size of a rainwater tank more restrictive when it is compared to the results regarding the satisfaction of the demand for rainwater. Economic and environmental analysis can help avoid oversizing tanks for rainwater and thus obtain greater benefits. Plugrisost includes estimated cost and Global Warming Potential, as well as other life cycle impact indicators for this purpose. A case study analyzing the implementation of a RWH system in a single-family house in Aveiro, Portugal is included, showing the importance of rainwater availability and demand for the successful operation of simulated systems.