Energy Efficiency in Buildings has been gaining increasing relevance in the past decades due to the raise of the energy consumption in the building sector as a consequence of the modern way of living with higher comfort patterns. In Europe, buildings are responsible for 40% of the energy consumption, which turns them into an important target for carbon emissions’ reduction [1]. Energy efficiency should be a main concern not only in new buildings, but also in the existing ones, which have poor energy performances. In Portugal most of the building stock was built before 1990, date of entrance into force of the first thermal regulation. Therefore, most of these buildings need intervention to improve not only the living environment, but also their energy performances. To evaluate the best measures, that fulfil the minimum requirements established for the energy needs with the lowest costs, the European Commission released the Commission Delegated Regulation (EU) Nº 244/2012 that establishes a comparative methodology framework for calculating the cost-optimal level for minimum energy performance. In order to verify the renovation potential of these buildings, based on the Delegated Regulation methodology and taking advantage of a renovation process in course, a social housing neighbourhood called Rainha Dona Leonor was analysed. This analysis allowed obtaining the optimal levels for different renovations measures. The results show that it is not possible to establish a direct connection between the optimal level for the building envelope and the building as a whole, without considering the influence of the building systems.