The motivation to adapt a building in terms of an upward vertical extension lies within the need for urban renewable programs. Although this practice has significant supports to make the built environment more sustainable, there are also barriers which invariably concern costs. In the case of vertical extension projects, the constructional and structural implications are greater than for other renovation projects. Hence, the major objective of this study is to identify the design parameters that outline the optimal vertical extension in building renovation with respect to costs and environmental impact, and alongside, to compare the design options of reuse and vertical extension and demolition and new structure. For these purposes, a crossed research was carried out, including a literature review, the analysis of existing case studies, interviews, and at the end a comparative analysis, based on the design of a new case study. First, the literature review presents the 'state of the art' of vertical extension projects from the structural engineer's standpoint. Then, the first list of critical design parameters were identified, on the basis of the analysis of the case studies and the interviews. The last part of this thesis, consists of the investigation of a new case study, an existing vacant building in the Hague, called Astoria. A preliminary structural design was carried out for different scenarios of vertical extension, which were defined with respect to the extent of interventions in the existing structure. Then, a global cost analysis and a Life Cycle Assessment are performed for both design options, presenting the results in comparative graphs. As a result of the structural analysis, the foundation and the shear concrete core were figured out as the critical structural elements during the vertical extension. Moreover, environmental impact and costs analyses highlighted the addition of 4 storeys as the optimal vertical extension for Astoria; further vertical extension requires interventions in the existing structure, which makes the project economically unviable. Concluding, this research indicates five principal design parameters that define the optimal vertical extension, namely, the municipal policies, the foundation of the existing building, the absence or not of testing methods, the floor system of the new block, and, last but not least, a feasibility study, which reveals the relationship between value and costs. For future projects, it is advised to introduce these parameters in the initial design stages, as a quick scan, in order to investigate and optimize the possibilities for vertical extension. As an outcome of the Astoria comparative case study, it is further suggested to introduce vertical extension as a normal practice in building renovation, regarding the multiple benefits and perspectives that have been revealed for both developers and society on the basis of environmental impact and construction costs.