The principle of hydraulic and hydrological invariance was introduced by the Lombardy Region through the legislative system (see Regional Law 15 March 2016, no. 4 and Regional Regulation 7/2017) with the aim of reducing the hydrological impact of territorial development. Sustainable rainwater management requires solutions that are fully integrated into land use planning and regulatory tools (in particular urban plans, area plans, building regulations, sewage regulations and discharge legislation) and in the technical projects of the settlements and infrastructures. Their implementation therefore requires close collaboration between various stakeholders and an in-depth knowledge not only of the technical aspects but also of those of a cultural and social nature