This study exploresthe ecological potential of flat roof systems by integrating rainwater harvesting and reuse into sustainable renovation strategies. With a focus on green architecture, the research addresses how flat roofs can be transformed into multifunctional elements that contribute to climate adaptation, resource efficiency, and urban resilience. The work aligns with the European Green Deal’s ambition to modernize the EU’s building stock while addressing environmental challenges such as water scarcity, biodiversity loss, and urban heat islands. Conducted within the framework of the VEGA project “Transformation of Existing Buildings into Sustainable Buildings – The Ecological Potential of Flat Roofs,” the study evaluates innovative solutions for collecting and reusing rainwater, particularly for irrigation of rooftop and surrounding green spaces. Through simulations based on real-world conditions, the research demonstrates the technical feasibility and environmental benefits of decentralized rainwater reuse in urban settings. Findings show that retrofitted flat roofs equipped with rainwater harvesting systems can reduce stormwater runoff, support vegetation growth, and lower ambient temperatures, contributing to more sustainable and livable cities. The study underscores the importance of integrated design approaches and highlights how small-scale architectural interventions can play a significant role in the broader transition toward ecological urban transformation.