In this paper, we propose modelling objects using extrusion-based representations, which can be used to complete partial point clouds. These extrusion-based representations are particularly well-suited for modelling basic household objects that robots will often need to manipulate. In order to efficiently complete a partial point cloud, we first detect planar reflection symmetries. These symmetries are then used to determine initial candidates for extruded shapes in the point clouds. These candidate solutions are then used to locally search for a suitable set of parameters to complete the point cloud. The proposed method was tested on real data of household objects and it successfully detected the extruded shapes of the objects. By using the extrusion-based representation, the system could accurately capture various details of the objects' shapes.