The cost estimation at the pre-project stage provides an important decision-making indicator for the future of the project. With a preliminary cost estimation, project participants can make financial decisions and cost control. The aim of this paper is to propose a model for estimating the costs of facade systems before the pre-design stage, using the GAM (Generalized Additive Model) method. The commonly used method for the valuation of facade systems is based on individual calculation. Such valuation process is complicated and time consuming. For this reason the search for a new forecasting method is justified. The database developed for modelling purposes includes 61 cases of real costs of system façade execution for public buildings. Each case is described by 16 parameters (namely, input variables). The average absolute percentage error (MAPE) was used to assess the model, which takes the value of 14,26% for the generalized model with a logarithmic binding function and 11.77% for the model with an identity binding function. On the basis of the studies and the results obtained, it can be concluded that the constructed model is useful and can improve the process of forecasting system façade costs at the pre-projection stage.