This paper presents the ALLAN.(TM)Simulation software(1), the modelling approach using this tool and the experimental validation carried out for a dwelling subjected to actual indoor and outdoor conditions. ALLAN.Simulation is a software designed at the Gaz de France Research and Development Division and developed with the aid of the Compagnie d＇Ingenierie et de Systemes Informatiques. It is a general software for the description and simulation of dynamic systems. Using this software, a library of models for building simulation has been developed and validated; the complete model of a two-room flat is then assembled and tested against real data obtained on a Gaz de France experimental building. Thanks to the model modularity allowed by the ALLAN.Simulation tool, it is possible to perform validation analysis of the models. While this study exhibits some deficiencies in either the measurement procedures or the modelling analysis, the overall results show the capabilities of the software as a tool for building-performance studies.