Ground-source heat pump systems are increasingly promoted as a key technology for reducing operational carbon emissions in buildings, yet their performance is strongly influenced by near-surface ground thermal conditions. In dense built environments, the assumption of undisturbed ground is often invalid, as anthropogenic heat sources continuously modify subsurface temperatures. Among these sources, residual heat losses from basements represent a persistent but still insufficiently quantified thermal input to the shallow ground. This study investigates residual heat transfer from non-insulated basements to the surrounding soil and evaluates its potential relevance for horizontal ground heat exchangers during the cold season. To address the lack of systematic quantitative assessments linking basement heat losses to shallow geothermal applications, two complementary approaches are employed: a three-dimensional transient numerical model simulating conductive heat transfer across the basement–soil system, and a standardized monthly analytical method*. Five basement interior temperature scenarios, ranging from 10°C to 30°C, were examined over the October–March cold-season period, enabling a systematic comparison between detailed numerical modeling and simplified analytical assessment. Results show that heated basements act as stable thermal sources for the surrounding soil, maintaining subsurface temperatures above freezing beneath the basement footprint throughout the October–March period. Total residual energy transferred to the ground increases monotonically with basement temperature, ranging from approximately 5.8 MWh at 10°C to over 24 MWh at 30°C , with peak monthly transfers reaching up to 4,445 kWh in December–January for the highest temperature scenario. The analytical method yields slightly higher seasonal energy estimates than the numerical model, with differences remaining within approximately 2–5% across all investigated cases. The results provide a quantitative assessment of basement-derived residual heat and its spatial influence on the surrounding soil. These findings provide a quantitative basis for treating basement-derived residual heat as a supplementary thermal resource for shallow geothermal systems, and demonstrate that a standardized analytical method can serve as a reliable and practically applicable alternative to detailed transient simulation for seasonal energy estimation in such contexts.