Problem Definition and main objective: Current prefabricated building envelope systems utilised for energy reduction renovations are expected to be a long-lasting solution. In practice, the building envelope systems are not adapted to facilitate future updates due to changing regulations or changing standards. Furthermore, in most renovation systems the material or component with the lowest service life dictates the service life of the complete system, leading to materials being disposed of before they reached the end of their potential service life. The goal of the thesis is to design a system that is adapted to these issues by being a flexible system that separates functionalities in separate boxes and layers, and therefore is able to upgrade or replace individual components when necessary to ensure the system’s long-term functionality. Study Design: Literature review, followed by the formulation of a strategy and a design. Design effectiveness is confirmed through application on a case study. Long-term functionality is confirmed through transformation of the case study application based on formulated scenarios. Setting: The thesis is part of a series of ongoing research projects associated with the 2ndSkin project. The case study is an apartment block located at the Soendalaan in Vlaardingen. Results: A design concept for an external building envelope system for energy reduction renovation of Dutch post-war apartments elaborated in a 1 to 5 detail scale. The final design incorporates an adaptable aluminium frame system with module boxes containing different functionalities, which can be slid into the main frame. The system is finished with a clamp system with cladding, functioning as the protecting layer for the system.