The Central European building stock needs sustainable renovation to reduce energy consumption needs and dependency on fossil fuels for comfort heating. Although accurate thermal envelope assessment is critical for designing energy-efficient retrofits, not only is the influence of model granularity under-explored in the Hungarian context, but previous studies have shown serious neglect of the incorporation of thermal bridges and related thermal anomalies by professionals. Our literature review highlights that although actual losses are estimated to be up to 35% compared to initially expected due to the neglect of thermal bridging, a tiny percentage of energy performance certificates consider their effects. Our case study investigates the impact of varying levels of detail (LODs) in building information modelling (BIM) on the thermal performance analysis of a “Cube House”, a common 1970s single-family home typology in Hungary. The LOD variation involves both geometrical and information-wise granularity of the models. We have authored the BIMs with Archicad 28, and we used the IFC 4.3 model schema to transfer a STEP-based geometry to COMSOL Multiphysics, where we have modelled steady-state heat transfer for the cases. We have shown how the numerical model can be translated to a graphical calculation methodology to derive quantitative and qualitative performance metrics for a thermal envelope. We used the current Hungarian energy legislation as the baseline for comparison: we compared the regulated thermal transmittance correction factor’s ( \(\zeta \) =0.3) regulated value for this building geometry and assigned it to the simplest LOD1 case. The studies showed a clear impact on the usability of the level of detail in BIM models for thermal bridge assessment. They resulted in surprisingly high correction factors for both LOD2 and LOD3 cases. We have calculated the area-weighted \(U_d\) -values for both LOD2 ( \(U_{d,\textrm{LOD2}}=0.387 \mathrm {\frac{W}{m^2K}}\) ) and LOD3 ( \(U_{d,\textrm{LOD3}}=0.327 \mathrm {\frac{W}{m^2K}}\) ), and found that the corresponding correction factors are equal to the regulated \(\zeta =0.3\) value’s 499.99% in the case of LOD2 ( \(\zeta _{\textrm{LOD2}}=1.500\) ), and 370.13% in the LOD3’s case ( \(\zeta _{\textrm{LOD3}}=1.110\) ). Our proposed geometrical interpretation benefits architects and façade engineers working in early-stage designs, when there is still room for meaningful design iterations.