"The European Council, during the March 2007 meeting, focused on the need of improving energy efficiency in the European Union and asked for a prompt response to the priorities defined in the ""Action plan for energy efficiency"". The Plan identified the important energy saving potential in building costs and especially in residential building. This way, it is possible to gather that there is still a great unrealized economically convenient potential for energy savings in buildings. Nevertheless, it is really important that this operation follows a correct cost-benefit analysis, based on a method conforming to the new Directive 2010/31/EU. In fact, this Directive states that the measures to improve further the energy performance of buildings should take into account cost-effectiveness. This research is a preliminary approach for the methodological application of the EU Directive, with the intent of giving a model for the member States during the implementation process. In particular, the model approaches issues connected to the ""major renovation"" of existing multi-family residential buildings owned and managed by the State. Starting from the building energy needs, the model evaluates the effects of a building element renovation, pointing at energy savings and socio-economic costs during the intervention lifecycle. The acceptable maximum cost of the renovation intervention is analyzed considering the range of validity of the cost optimal curve for the minimum energy requirement. The report is completed by a selection of case studies taken from the Italian social housing context in order to validate the methodology studied."