A significant part of the buildings in the downtown of Budapest were built at the turn of the century, the condition of the load-bearing structures is usually adequate, but the secondary structures and mechanical systems are outdated, and from an energy point of view the building stock is mostly outdated and inefficient. In case of historical buildings, energy renovation is not a simple, self-evident process, as it requires a differentiated approach and striving for the optimum. This study provides an overview of the different building structures, analyses the possibilities of energy renovation, and formulates a proposal on the extent and method of interventions. Using the example of a case study, it quantifies the amount of the energy improvement that can be achieved by implementing different renovation scenarios. Energy calculations carried out on the selected sample building demonstrated that deep renovation can result in significant energy and CO2 savings. It has also demonstrated that, in addition to the comprehensive whole building envelope upgrades, the modernization of the mechanical systems is also required in order to achieve significant improvements in the energy performance of the building.