This study uses a bottom-up model of over 200,000 cases to assess Spain's residential building stock under future climate scenarios for the years 2050 and 2080 (SSP1-2.6 and SSP5-8.5), aiming to inform the critical need for climate adaptation planning in the building sector. The novelty of our approach lies in the simultaneous comparison of real, sensible energy demand (free-running simulation) against the official, restrictive energy certification procedure, an approach not previously performed for the entire Spanish stock. The analysis yields two major contributions that challenge current policy. First, we observe a significant shift in energy use: cooling demand surges-doubling by 2080 in vulnerable areas-while heating demand declines, resulting in a net change to total final energy demand of less than 4% across all scenarios. Second, Spain's current energy certification procedure underestimates 2022 total demand by approximately 5%, primarily by underestimating cooling needs by 18%, a discrepancy that will worsen with climate change. As a result, it fails to capture real discomfort, especially in summer. A key policy insight is that retrofitting the entire stock to current building code (CTE) U-values would halve total demand and cap future cooling at today's levels. The Mediterranean region, with half the national stock, is most at risk, facing cooling demand increases of up to 138%. Therefore, a national strategy must prioritize retrofitting the oldest single-family homes in southern Mediterranean areas.