Optimizing consumptions in the field of civil construction led to define energy labels for residential buildings. To calculate the building energy demand the EPgl was determined, i.e. the annual consumption per m² of primary energy. This paper examines the technical solutions useful to optimize the energy demands for heating during space-heating season and domestic hot water production (thanks to energy analysis softwares as MC11300 and TRNSYS) and, at the same time, to take into account the financial issues those interventions implied. The total inside heated surface of the building case study is 1204.00 m², hence the inside heated volume is about 3250.80 m³. Besides the more traditional interventions concerning the building envelope and its systems, the paper examined the performance of a system obtained through the combination of a cogenerator (CHP) and a heat pump (HP), thus, substituting the conventional boilers of the buildings. CHP+HP solution increases the most the energy label of the building (from a D class with EPgl = 59.62 kW h m⁻² year⁻¹, to an A class, with EPgl = 25.64 kW h m⁻² year⁻¹), determining an annual energy cost saving of 3,114 € year⁻¹, allowing to amortize installation costs (54,560 €) in a reasonable payback period, i.e. 15.4 years. This innovative solution in the residential sector can be realized through retrofit interventions on existing buildings, hence it leads the current dwelling towards nZEB with a remarkable benefits for the environment.