Despite advancements in microclimate modelling, limitations persist in empirical validation and accuracy of input parameters, which influence simulation fidelity. This study presents a case of assessing the urban heat island effect using ground-truthing-based modelling across heterogeneous urban typologies in Chennai. Simulations under future climate scenarios with envelope upgrades reduced the Universal thermal climate index (UTCI) by up to 16°C in dense canyons during peak summer with limited nocturnal cooling due to residual heat and poor ventilation. The study underscores data-related constraints and contributes an applied framework that embeds microclimate insights into the urban planning process to support resilient, context-specific interventions.