Building performance simulation (BPS) is a powerful measure to educate the building design process. However, its use in practice is to a large extent limited to the detailed design stage dedicated to the code compliance analysis of worked out design proposals. BPS is not much used to support the conceptual design stage. To date BPS tools are regarded as pure analysis tools, which do not provide design information. It is hypothesized that, when integrating uncertainty analysis techniques to existing BPS tools, following an incremental research approach, valuable design information can be provided. The article gives an update on the process to extend the capabilities of a tool specifically developed to support the conceptual design stage about uncertainty assessment. The resulting prototype should be capable of providing information about the variation of specific building performance metrics (simulation output) based on the propagated uncertainties of the building specification (simulation input). The main focus was hereby the representation of material properties and their impact on the performance uncertainty of the building concept. The results also indicate the degree of impact, sensitivity, of the building specification parameters on the variation of the performance metrics as a result of their uncertainty.