Existing classification systems linked to the environmental performance of buildings provide limited added value for practitioners. A survey among Swedish construction entrepreneurs showed that there is a real demand for better formulated criteria and clearer guidance. At the same time, critical investigation of requirements based on fixed average values for primary energy factors (such as in the EU Environmental Performance of Buildings Directive) shows that they are insufficient to provide guidance towards environmental sustainability building practices. They fail to take into account a number of methodological issues, including seasonal and hourly variability of energy supply and demand, and the future evolution of energy mixes. This is illustrated in the case of Sweden. The outline of an Open Classification System, currently under development, is then presented. This system focuses on methodological transparency and validity, as well as ease of use for practitioners. It addresses specifically issues where other existing systems were found to be lacking, and its methodology will be assessed to ensure that it provides optimal guidance towards environmentally sustainable practices. The system is based on three criteria: the energy resource index and global warming potential, calculated with attributional and consequential life cycle approaches, and a heat loss factor to assess the building's energy performance independently from the supply side.