The navigation of pedestrians can be regarded as their movements from one unoccupied space to another unoccupied and connected space. These movements generally occur in three types of environments: indoor, outdoor, and semi-bounded (top-bounded, and/or side-bounded) spaces. While the two former types of spaces are subject to most of the attention, the latter (semi-bounded) also presents a valuable impact on the navigation behaviour. For example, top-bounded environments (e.g. roofs, shelters, etc.) are very popular for pedestrian navigation since a top structure can offer protection from harsh weather, rain, or strong sun. However, such semibounded spaces are completely missing in current navigation models and systems. This is partly explained by the fact that modelling the space, which is by defining a three-dimensional boundless and extensible component (mainly out of the indoor environment), is a very challenging task. In this paper, we propose a structure-based approach for top-bounded space extraction in the built environment, relying on 3D models. Thanks to the rapid expansion and availability of 3D city models, our approach can help to account for such type of spaces in 3D pedestrian navigation systems.