Thermal resilience of buildings under extreme climatic and infrastructural stress is shaped primarily by architectural decisions made before any active system is installed, yet passive strategies are typically evaluated in isolation, without reference to the decision hierarchy that determines their reliability under disruption. This study presents the first integrated empirical synthesis of passive thermal resilience and energy performance across three decision orders, defined by reversibility and operational dependence, using full-scale experimental data from two research facilities in a temperate European climate. The first comprises two residential test buildings with contrasting envelope mass and ground coupling, monitored during summer heatwaves, winter heating outages, and normal operating conditions across multiple seasons. The second comprises five rooms of identical geometry within an occupied building, tested under combinations of phase change materials, external shading, and night ventilation during summer heat events. First-order decisions embedding structural thermal mass and floor–ground coupling eliminated all overheating indicators during extreme summer conditions and maintained safe indoor temperatures for over 60 hours during winter blackouts, operating independently of occupant behaviour or energy supply. The winter energy penalty of 12.2% emerged only in the latter part of the heating season, as subsoil heat retained from summer delayed conductive losses, and life-cycle assessment confirms a carbon performance reversal after 15–16 years favouring the ground-coupled configuration under all climate scenarios. Second-order PCM integration reduced peak temperatures by 1.0°C and cooling energy by 1.6%, compared to 2.7°C and 12.3% achieved by structural thermal mass, demonstrating that retrofit additions cannot replicate fabric-embedded strategies. Third-order strategies combining shading, night ventilation, and thermal mass reduced peak temperatures by up to 7.4 K when all three operated simultaneously, but each proved insufficient individually: shading was effective during the day but limited at night, while night ventilation reduced nocturnal temperatures markedly but had little daytime impact. The timing of passive strategy implementation within the design process thus has direct consequences for thermal resilience, operational energy use, and long-term carbon performance.