Latest European Union programs related to energy efficiency underline the need for retrofitting existing buildings, which are responsible for 40% of EU total energy consumption. Accounting for almost 45% of the existing building stock, the architecture of the second half of the twentieth century represents one of the main targets, becoming a vulnerable category despite its heritage value. This consciousness clarifies the urgent need of developing a new Methodology for Energy Retrofitting of Modern Architecture (MERMA), capable of integrating thermal improvement with architectural preservation. This paper aims to demonstrate the legitimacy of both issues, which share, essentially, the same concerns: supporting a sustainable development by preserving non-replaceable resources, natural or cultural as they are. The MERMA is a cohesive general methodology, based on an interdisciplinary approach. It starts from the architectural and technological inquiry, includes the energy analysis and different project proposition, and uses evaluation matrices to outline the most suitable intervention strategy. As an applicative case study, the first Olivetti office building (1963), by the architects Bernasconi, Fiocchi, and Nizzoli, is chosen. It is located in the industrial site in Ivrea, recently registered in the UNESCO World Heritage List and protected by the Italian law on heritage monument DL 42/2004. By means of the MERMA application, a reduction of 55% of the building's energy demand is assured, ensuring the fulfilment of all regulations' standards, avoiding any derogation, and preserving its heritage value. For high-quality or recognized II post-war building stock, combined energy-saving and heritage preservation is justified and proposed, instead of the sole energy retrofitting commonly applied today.