The post-war building stock represents 33% of all residential buildings. A common characteristic is that they are poorly insulated compared to the current Dutch building regulations. This research will focus on post-war walk-up apartments, they account for approximately 8% of the total residential building stock. 70% out of this 8% is social housing. All housing corporations agreed that in 2020 the residential portfolio should have an energy label which is at least B. The refurbishment of post-war walk-up apartments is really needed to achieve this energy label. The 2ndSKIN approach is a prefabricated system for the renovation of walk-up apartments. This approach contains a zero-on-the-meter concept, where building services are integrated into the façade. The 2ndSKIN approach is still too costly and the zero-energy target is only met in specific conditions. To optimize the retrofit of residential buildings different energy systems should be applied. The approach can then be used in different urban contexts, which will result in upscaling the refurbishment of walk-up apartments. Literature research on several existing renovation approaches and different energy systems has been conducted. Together with the analysis of post-war walk-up apartments, it resulted in four main energy systems. These concepts have been simulated with the software Uniec2.2. The goal of the simulation was to achieve the BENG regulations (Bijna Energie Neutrale Gebouwen [Almost Energy Neutral Buildings]). In order to achieve the BENG regulations, a low and high impact variant have been designed. These variants differ in insulation value and how ventilation is provided. The conclusion of the simulation provided direct input for the design phase. A case-study is used to apply the outcomes of the simulations in detail and to design an approach where it is possible to accommodate different energy systems. The outcome of this research is a prefabricated renovation approach for post-war walk-up apartments that is applicable to accommodate energy saving measures with different energy systems. Building services are integrated into the façade and different building services units can be placed in the backyard to have different energy systems in the building. With the new building services and the increased insulation value of the building envelope, it is possible to fulfil the BENG regulations with a post-war walk-up apartment building. Except for one concept, all other concepts are even zero-energy. The different energy concepts and variants for the building envelope have been included in a decision-making diagram. With this diagram, housing corporations or homeowners associations can choose between the different concepts, based on three different goals. Also, some considerations concerning the choice between the low and high impact for the building envelope are made.