The energy retrofit of social housing buildings in Italy is a big challenge, for their poor energy performance and their large diffusion, but it is affected by several problems mainly due to lack of funds. Therefore, a solid methodological base to achieve optimal energy levels, considering the best balance with the costs, can be useful for a cheaper approach to their energy performance improvement. The cost optimal methodology indicated by the European Directive 2010/31/UE is here applied on a social housing building, located in the northern Italy, in order to demonstrate how it can be used as a supporting decision tool for refurbishment interventions on existing residential buildings, when limited investments from Public Authorities or privates are involved. A series of energy efficiency measures are defined in order to identify different improvement scenarios, related both to the envelope and to the technical systems. After a first step calculating the primary energy consumption and the Global Cost in accordance with the EN 15459:2007, the best costs/benefits ratio is evaluated among all the hypothesized scenarios. The results of the research are expected to be a stimulus for the definition of specific refurbishment plans for the energy efficiency increase of social housing.