The Water Framework Directive (WFD) under the guidance of the precautionary principle sets out standards to guarantee high quality water services for European citizens. This creates pressure on European cities to update and renew their water infrastructures in accordance with EU Law at great financial cost. Cities within the Union try to bridge this financial gap with a variety of approaches. This paper presents the cases of London and Milan, both of which were subject to legal proceedings for breaching the Urban Waste Water Treatment Directive. By example of these two cases, this article details how the precautionary principle affects urban water infrastructure provision, and how the regulation of the primary risk of pollution can both trigger innovation and create secondary risks within the highly integrated urban water infrastructure sector. The London case focusses on an individual infrastructure project and shows how its financial framing has compromised the final outcome, while the Milan case presents a longer-view perspective that shows how structural changes in the urban water infrastructure sector have enabled an environment for sustainable financial innovation. The role of transparency and good local governance practices are emphasized for a successful implementation of the precautionary principle requirements in a city’s water sector. Managing this process effectively can result in meaningful social innovation for urban water infrastructure provision.