One of the top priorities of European countries is to reduce energy consumption and greenhouse gas (GHG) emissions in the built environment. To reach this goal, urban renewal processes are seen as a core strategy towards a sustainable built fabric, given the fact that a large proportion of the cities of tomorrow is already standing. Indeed, there are still considerable potential energy savings to be made in European countries in general, and in Spain in particular, a country in which most residential buildings were constructed before 2001 based on very low thermal regulations compared to the current ones. Therefore, these buildings require large amounts of energy to ensure a minimum indoor comfort. Considering the global context of the country and the current economic crisis, which is particularly affecting the construction sector, the future lies in the renewal process of the built environment, responsible for 40% of the primary energy consumption in the country. The originality of this research is to apply the cost-optimal methodology at the territorial scale, using statistical and population census data and taking into account the 12 climatic zones of the Spanish territory. Initially, the cost-optimal methodology was proposed by the European Union to study different building retrofitting scenarios. It consists in a multi-criteria assessment that allows comparing various levels of intervention under different macro-economic scenarios, in terms of the cost- effectiveness of the strategies, and energy and environmental savings. This thesis presents the application of the cost-optimal methodology at the territorial scale in order to estimate the energy saving potential of residential buildings constructed before 2001. The assessment methodology is implemented in an Excel- based strategic decision-making tool, aiming to help select the best strategy for achieving the European requirements according to the nearly Zero Energy Building (nZEB) standard. Moreover, this research proposes a new systematic approach linking the different scales (regional, city / district and building) through energy simulations based on different levels of detail (LoD). Starting from the representation of the existing building stock using Archetypes, a bottom-up extrapolation (also known as scaled-up methodology) is applied, using the results obtained from the energy simulations performed by the parameterization of each Archetype. The application of Excel-based strategic decision-making tool allows the user to make two types of studies: (I) At the building scale, a comparison of different improvement package strategies, for one type of housing in a particular climatic zone. (II) At the territory-scale, a comparison of different improvement package strategies to have an overview of all types of buildings and all climatic zones simultaneously. The results include economic and financial aspects, consumption and energy savings, associated GHG equivalent emissions, and the results of the life-cycle analysis (LCA) and cost (LCC). These outcomes allow verifying if the proposed scenarios could reach the objectives for nZEBs. Ultimately, this research shall contribute to helping firms in the construction sector adapt their business plans towards the promotion of urban renewal projects by improving existing buildings, while also supporting stakeholders of urban planning in making strategic decisions at medium/long -term towards a sustainable urban renewal process of the building stock.