In Computer-Aided Design (CAD), the idealization process reduces the complexity of the model of a solid object, thus resulting in a simplified representation which captures only the essential elements of its shape. Form features extraction is a relevant issue in order to recover semantic information from an idealized object model, since such information is typically lost during the idealization process. An idealized model is usually composed of non-manifold parts, whose topology carries significant structural information about the object shape. To this aim, we define form features for non-manifold object by extending the taxonomy of form features provided by STEP . We describe an approach for the identification of features, which interact with non-manifold singularities in the object, based on a decomposition of a non-manifold object into nearly manifold components and on the properties of the graph representing such decomposition.