Buildings must maintain habitable conditions during prolonged winter power outages, yet the seasonal impact of passive construction properties combined with active energy interventions remains insufficiently quantified. This study examines how thermal mass, slab–ground coupling, renewable generation, storage, and control strategies shape thermal autonomy during extended winter blackouts. The analysis uses full-scale data from two identical residential buildings differing in thermal mass and slab–ground conditions to calibrate and validate a reduced-order model for a full heating season under sustained energy unavailability. Five increasingly complex energy intervention scenarios are evaluated, with construction type serving as the foundational passive strategy shaping all active interventions. Performance is assessed by cumulative hours below 12, 15, and 18°C, alongside seasonal energy input for the most advanced scenario. Under fully passive conditions, the ground-coupled masonry building accumulated approximately 310 hours below the 12°C survival threshold across the heating season, compared with approximately 2450 hours in the lightweight timber-frame building, despite identical steady-state heat-loss coefficients. The effectiveness of every active intervention layer depends on this passive thermal reserve: identical storage capacity reduced hours below 12°C by 71% in the masonry building but only 45% in the lightweight one. Thermal energy storage combined with anticipatory preconditioning produced the largest single improvement across all scenarios, reducing hours below 15°C to approximately 25 and 190 in the two buildings respectively. Maintaining 15°C habitability throughout the season requires approximately 60% more electrical energy in the lightweight building, representing a hidden energy cost of low thermal inertia absent from current performance certification. In the examined temperate climate, passive resilience and decarbonisation align design choices that improve thermal autonomy during blackouts also reduce long-term carbon emissions.