In the context of climate change and accelerated urban growth, the adoption of effective strategies for sustainable stormwater management has become essential. Excessive soil sealing increases surface runoff, leading to frequent urban floods and overloading conventional drainage infrastructures. This study assessed the influence of green roofs and permeable pavements on stormwater runoff and the design of a public drainage network in a 2.1-hectare urban allotment located in Portugal’s Pluviometric Region B. Six drainage scenarios were simulated with different combinations of roof typologies and pavement types: (i) pitched roofs with conventional pavements (Cₘ = 0.70), (ii) pitched roofs with sustainable pavements (Cₘ = 0.61), (iii) flat roofs with conventional pavements (Cₘ = 0.65), (iv) flat roofs with sustainable pavements (Cₘ = 0.55), (v) green roofs with conventional pavements (Cₘ = 0.59), and (vi) green roofs with sustainable pavements (Cₘ = 0.50), being Cₘ the weighted average runoff coefficient that expresses the fraction of rainfall converted into surface runoff. The corresponding peak discharges were 380.06, 328.49, 350.62, 298.67, 320.53, and 268.68 L/s, respectively. A progressive reduction in runoff and peak flow was observed across scenarios, with the most sustainable configuration (Scenario 6) achieving a 29.31% decrease relative to the conventional configuration (Scenario 1). From an economic perspective, the most sustainable scenario had the lowest drainage network cost (EUR 32,092.76 + VAT), compared with EUR 35,919.82 + VAT in the conventional scenario. These findings demonstrate that sustainable drainage measures not only reduce surface runoff and mitigate urban flooding but also enable more cost-effective and resilient stormwater management systems, thereby contributing to the transition towards climate-adaptive cities.