Abstract The cost-optimal analysis represents a useful tool to stimulate the refurbishment of existing buildings, highlighting the minimal global cost that private or public users have to sustain in order to attain appreciable energy savings in the building life span. The identification of precise interventions on the building-plant system could steer legislators to the emanation of appropriate financial incentives in order to make the refurbishment of existing edifices more attractive. However, the cost-optimal solutions cannot be generalized as they are strongly influenced by the initial conditions and by the climatic context. In this paper, a traditional social housing unit, realized in Italy during the 1970s, largely widespread and strongly energy-consuming, was analysed in two different climatic zones belonging to the Italian territory. Suitable interventions were identified for the reduction of energy requirements for heating, cooling and domestic hot water production. These analyses were carried out by using the same simulation tool for energy and economic performances, highlighting the deviances among the cost-optimal solutions, the n-ZEB target and the minimal interventions for the containment of energy consumptions imposed by the current Italian legislation for buildings subjected to refurbishment.