In the context of buildings’ harmonization with the European Union requirements and environmental challenges, the development of smart tools could be an important step towards energy efficiency improvement in the building sector. This is particular true for Greece, which has recently incorporated the Directive 2002/91/EC on the energy performance of buildings, as well as the related procedures and modalities. With respect to the above, the main aim of the paper is to present a smart tool for assessing energy performance of buildings. In particular, the proposed system’s operation is based on the optional installation and operation of an automated system of sensors and meters, for monitoring the building energy consumption and the combination of control scenarios, so as to decrease energy consumption.