The aim of this work is to develop a bottom-up methodology that can assess energy efficiency and carbon dioxide (CO₂) mitigation strategies in the existing building stock. The work concerns the European building stock and is part of an overall analysis of how the European energy system can be transformed to be more sustainable. This paper reports on the validation of the methodology through its application to a sample of 1400 buildings representative of the Swedish residential stock based on data from 2005. Energy and CO₂ assessments are performed; all the resultant potentials are presented as a function of costs for the energysaving measures studied. The results obtained for Sweden are compared to results available in the literature and discussed in relation to the further application of the methodology to the European building stock as a whole. Such application must take into account national differences in energy supply systems (heat and electricity) and characteristics of the building stock. In addition, the use of sample buildings is data intensive, and data may not be available for some European Union member states. Thus, the next step should be to further develop the model so as to be able to also use archetype buildings instead of sample buildings, allowing the use of national statistics and general information on building characteristics as model input. Transaction costs and interactions of demand side and supply side should also be included in the cost calculations.