Appropriate design and construction can reduce the environmental impact of buildings over their entire life cycle. Such a prevention strategy can be supported by an evaluation tool. Our approach consists of applying life cycle analysis to the building sector. The use of computers is preferred for an easier comparison of alternative designs and to test possible improvements of a project. We adopted an object oriented approach in order to link models with professional practice. A data structure has been defined, in which a building is decomposed into objects. The corresponding classes include both data (inventories of basic materials and processes) and methods (accounting for fabrication, recycling, etc.). The result of such an evaluation is an environmental profile, constituted from various indicators on natural resources used, environmental effects and waste produced. First sensitivity analysis were performed showing the importance of the utilisation phase with its associated energy, water and waste fluxes. The environmental performance of a building is a result of a sequence of decisions like the choice of the building site, design choices, occupant's behaviour, etc. The potential applications of this research are to inform the various actors about the consequences of their choices, to assess the environmental interest of innovative technologies (e.g. solar components) and to develop new tools for practitioners.