Because of its population growth, attic conversions are holding an important position in Vienna. However, an attic conversion is a complex construction project. This thesis performs a case study in multiple case design about four finished attic conversions, which were started in Vienna in the last ten years. The goal of this study is to generate cost figures and evaluate influences on construction costs. A survey is made, using the structure of the ÖNORM B1801-1:2015, about the construction costs and further parameters of the projects. Based on the gained data, cost groups are made and analyzed, which divide cost in structural, contentual and functional aspects. While influences on the cost groups, considering the surveyed projects, could be found, it was for the most part not possible to evaluate their quantitative impact on construction cost. Cost figures could be formed, but they show a considerable margin of variation, especially in increasing level of detail.