Refurbishing the existing building stock is an acknowledged issue in the building industry. Even though awareness has been raised, the design phase of refurbishment projects is often problematic. The decisions taken in the early stages of the design determine the final result; however, the assessment of the environmental performance only happens at the end of the design process.This paper discusses an approach to the designing of refurbishment projects, as a way to energy-efficiently upgrade the residential stock. Based on a case study multi-residential building of the post-war period in Germany, we assess the impact of the retrofitted building components into the environmental performance of the building. The different options are systematically organised into categories, creating a “toolbox”. The compilation of different “tools” composed the refurbishment strategy. In this way, the impact of the refurbishment was evaluated in the early design stages. The toolbox supported the decision-making process of the design, resulting in integrated strategies that improve the performance of the building.