Thermally active retaining walls can be designed both as support for excavation and as shallow geothermal systems, but their thermal behaviour cannot be readily evaluated at the preliminary design stage because it relies on simultaneous interaction between the shape of the wall, its thermal conductivity and the temperature difference between the ground side and the excavated side. In this work we develop a surrogate model for peak heating season heat exchange of planar retaining walls using a numerically generated database which contains three kinds of cases: the short-duration heat-exchange cases, the geometry-response cases with deep walls and the conductivity-response cases. The purpose of the modelling is not to substitute computationally intensive simulation but to generate a transparent design relationship which would highlight and quantify the major factors involved. The analysis demonstrates that short-duration heat exchange estimates are primarily controlled by the near-wall thermal condition and wall thermal conductivity whereas moderate variations in the external boundary conditions yield relatively minor perturbations. Among all the geometric parameters which describe retaining wall in the seasonal design space the main factor is the composite value of \((H/L)/D\). The Results and Discussion sections continue the analysis of the fitted response using sensitivity derivatives, elasticities, threshold thickness analysis, material compensation analysis and uncertainty propagation. Conductivity analysis indicates that within the explored range the thermal resistance of the wall plays much more important role than the thermal resistance of the soil. A regressional model which is based on the excavation ratio, wall thickness, wall depth, soil conductivity and wall conductivity replicates the assembled design database with high accuracy (\(R^2=0.997\)) and preserves good prediction power in the leave-one-geometry-out cross-validation mode (\(R^2=0.985\)). It can thus be concluded that the equation derived is useful for evaluating the viability of deep retaining walls through thermal analysis at an early stage.