The perception of water as a resource is undergoing dramatical change. As a consequence of climate change, on the one hand, drought even in temperate latitudes and the lowering of groundwater levels in many areas is an issue that now affects almost everyone [1,2]. On the other hand, the increase in extreme weather events and climate-related natural hazards poses a risk to more and more people [3,4]. It is not only the first United Nations Water Conference in almost 50 years in 2023 that is drawing attention to what has become a universal truth: water must be managed.

One source of water which has a significant impact on the sizing of water infrastructure systems is rainwater [5,6,7]. Storm water discharge to protect other infrastructure from damage, such as that seen, for example, in Los Angeles or Tokyo, is only one question for urban planners [8,9,10]. The integration of rainwater into the urban water cycle is a subject of water-sensitive urban design [11,12,13], with concepts such as green roof technology, living walls, and sponge cities as instantiations [14,15,16,17,18].

Rainwater harvesting (RWH) and use is a cornerstone of many of these concepts and contributes to influencing both peak water demands and storm water runoff [19,20]. In addition to simply installing a rainwater tank on a property, understanding neighborhoods or districts as local, decentrally managed water grids offers the possibility of resource balancing, as participants with low consumption or additional municipal buffers can make their harvest available to other participants in the network as needed [21].

Thus, when considering an RWH system as a design object, the questions are, first, how to size it according to the provision and consumption of rainwater [22], second, how to make it robust to changing requirements and premises over a long service life [23], and third, where to set the system boundaries for maximum benefit to all stakeholders. Current design practice and standards such as EN 16941-1 [24] tend to focus on …