Urbanization alters the natural hydrological balance by increasing impervious surfaces, which reduces infiltration and amplifies peak runoff. To address these impacts, several Italian regional regulations introduced the concept of hydraulic and hydrologic invariance, ensuring that post-development runoff conditions remain comparable to pre-development ones. This study aimed to evaluate how the application of a single regulatory framework—the Lombardy Regional Regulation (R.R. 19 April 2019, n. 8)—affects the design of detention volumes when applied to different climatic contexts across Italy. The research focused on understanding how variations in rainfall characteristics influence the sizing of stormwater detention systems under a uniform set of design criteria, in order to assess the robustness and adaptability of the regulation beyond its regional scope.