Densification by vertical extension (VE) of buildings has become more common in Swedish cities and architectural discourse, partially due to a Swedish law implemented in 2002 concerning three-dimensional property division. By restoring and recreating functions, VE allows for the preservation of the morphological and architectural identity of existing buildings. However, design and construction above an existing building is complex, partially because of the uncertainties in the existing building. This makes it interesting to view VE from a risk perspective and this paper is a first attempt to do that. It summarises a qualitative, exploratory study aimed to increase the understanding of VE from an architectural design perspective. The empirical material was collected in open and in-depth interviews with eight architects from different companies, all with experience of Swedish VE projects. In this research, the perspective of a two-sided uncertainty is applied as threats and opportunities. With that perspective opportunities and threats concerning architectural design of VE are interpreted and translated into design risks. One finding is the construct that VE can be understood by viewing it as a system of components: the existing building (host), the interface between the new and existing building (interface) and the extension per se (extension). The unique nature of the interface controls many of the solutions applied and is therefore a crucial piece in the overall system puzzle. In addition, it was identified that architects managed a lot of uncertainties in their role, uncertainties mainly related to there being a host building to be extended. The main contribution of this paper is that the understanding of VE can be improved by mapping opportunities and threats by source; host, interface and extension.