Rain gardens are playing an increasingly significant role in the adaptation of urban areas to climate change, particularly in light of the growing frequency and intensity of rainfall events. The objective of this study was to assess the influence of physical and geographical conditions on the design, sizing, and effectiveness of rain gardens in Poland, with a particular focus on runoff control and regulation in the context of climate change adaptation. A computational tool was developed, integrating the kinematic wave equation with dynamic flow control algorithms, to support the modelling and optimization of infiltration systems. Analysis of meteorological data from 29 stations (covering 37 to 44 years of observations), along with soil infiltration characteristics, revealed a strong correlation between catchment imperviousness and the required infiltration trench capacity as well as the volume of controlled runoff. The highest storage capacity requirements were observed in Mikołajki (52.6–352.8 m³), while the lowest were observed in Elbląg (28.7–187.1 m³). The maximum controlled runoff volumes occurred in Katowice (91.4–213.7 m³) and Mikołajki (56.2–260.8 m³), while the lowest were recorded in Świnoujście (18.4–126.2 m³), Leszno (19.6–87.9 m³), and Poznań (32.7–84.9 m³). The developed tool offers substantial support for enhancing the resilience of urban retention systems, highlights the importance of implementing advanced stormwater management strategies under changing climatic conditions.