Urban environments are increasingly vulnerable to climate change, with extreme weather events expected to become more frequent and severe. This paper addresses sustainable urban development and the importance of stormwater retention, integrating adaptation and mitigation strategies. It evaluates the publicly funded Hungarian “Green City” program’s water management, focusing on blue-green infrastructures. The 198 implemented projects in the program were assessed for green credentials, vegetation concepts, and rainwater retention using public databases of real municipal data and Google Earth spatial analyses rather than hypothetical scenarios. A lifetime climate change impact assessment with sensitivity analysis was conducted using two case studies from the “Green City” program, highlighting the benefits of prioritizing rainwater over tap water for irrigation. The study proposes a three-pillar—environmental as operational carbon footprint, economic as extended net present value (NPV), and social as accessibility and recreational benefit—evaluation method for urban blue-green developments. It found that many projects rely on tap water irrigation, thus resulting in higher lifetime carbon emissions. The financial assessment of carbon footprint within the extended NPV method emphasizes the need for improved green area irrigation strategies. By modernizing irrigation practices and implementing effective rainwater retention measures, blue-green infrastructures can significantly reduce carbon dioxide emissions while improving long-term economic performance and social benefits through improved usability. The research offers valuable insights into the role of blue-green infrastructures in urban development to combat climate change. The combined three-pillar framework integrating LCA to assess green projects is a transferable decision-support tool that can be adapted to locally available data, advocating the use of rainwater over tap water to achieve environmental, social, and economic benefits. Unlike earlier studies that used hypothetical scenarios, this research relies on the implemented projects of the “Green City” development program with their observed designs and available real data, thus providing a framework for urban blue-green implementations to integrate sustainable practices and effectively address the challenges posed by climate change.