With the increasing number of indoor navigation applications, it is essential to have clear and complete conceptual model (in the form of UML class diagram) for IndoorGML. The current version of IndoorGML standard has an incomplete class diagram (incomplete w.r.t. attributes, of which some are appearing in the XML/GML schema), and that provides confusion for the users of the standard. Furthermore, there are some issues related to unclear association names, unclear class names, classes that related to the Primal space and the Dual space, code lists not specific per type (which should have their own code list values), untyped relationships to external object classes, and semantically overlapping classes. In this paper, we propose an enhancement for IndoorGML conceptual model (UML class diagram) to avoid the misunderstanding. We propose a conceptual model that maps the classes of the standard in a better way. This conceptual model is the basis for 1) a database schema when storing IndoorGML data, 2) the XML schema when exchanging IndoorGML data, and 3) when developing IndoorGML applications with an intuitive and clear GUI. Furthermore, the proposed conceptual model provides constraints for more meaningful model and to define more sharply what is considered valid data. This paper briefly reports these preliminary results on the UML conceptual model.