During a design competition held in 2010, a preliminary design was made for an extension on top of the Oval tower. The Oval Tower is an existing building with a height of about 100m. However, following that competition, this design was never put into practice. According to experts in the field, this project still has a great potential. It was therefore chosen to further investigate the feasibility of this extension project and the feasibility of extended high-rise building in general. In the general building process, one of the first steps is to analyse whether a project is feasible, concerning the structural feasibility. The objective of this master’s thesis is to give insight in the multitude of factors which determine this feasibility and make-ability of an extension on top of an existing high-rise project. For the determination of these factors several reference projects have been investigated. Both existing research projects as well as existing extended buildings have been taken into account. Subsequently a Quick Scan of three buildings has been made. Using the results of the Quick Scan and the results of the research into existing projects and buildings several key factors were derived, of which an initial diagram was composed. In this diagram a division was made between a technical, a functional and a financial inventory. It was chosen to mainly focus on the technical feasibility factors in the further research. The most important technical factors were found to be: use of materials, loads, level of safety, load bearing capacity of structural elements and construction methodology. Further investigation has been done throughout these factors. As a starting point for the calculations on the Oval Tower case study the diagram was used. Using this diagram and remaining data, calculations were made for the existing Oval Tower structure. It was shown that the core and the columns subjected to wind loads are the leading structural elements. Next to that it turned out that various other elements contain overcapacity. Using the findings on the present overcapacity in the Oval Tower structure, three alternatives for the extension were derived. The design alternative that posed minimal structural adaptations was eventually chosen to be most suitable. For this design alternative three construction methods were compared with the help of a trade off matrix. Investigating the three construction methods through this matrix led to a choice on the most appropriate building method. A detailed plan for this construction method was composed. Which factors determine the feasibility of an extension on top of a high-rise building? This question and its corresponding answer is the main topic of this master’s thesis. A diagram was composed in which the most important factors are shown and explained. Using the diagram as a starting point, a case study for the Oval Tower was conducted. The intention of this case study is to answer the question whether it is structurally feasible to extend the Oval Tower with two floors and a crown on top of the existing structure. Considering the Oval Tower extension case study it can be concluded that it is structurally possible to extend the tower with two additional floors as well as a crown structure on top. However the research also showed that structural strengthening of several building elements is necessary. For the functional and financial inventory several assumptions have been made, additional research on these inventories is needed.