We propose an approach that enables spatial subdivision of urban areas into the categories "urban core", "urban fringe" and "urban periphery" based on the built-up morphology. These categories represent a transition from urban to suburban areas. We test our method on the four largest cities in Germany: Berlin, Hamburg, Munich and Cologne. Our results suggest, that the approach is transferable and provides a plausible spatial delineation of the three categories. However, further modifications are needed when polycentric regions or small cities are to be considered in future applications.