To achieve a sustainable urban and spatial development the focus currently lies on the urban concentration, modernization and redevelopment of existing building stock. This especially opens up opportunities for (lightweight) steel construction, since steel has considerable advantages in comparison with other materials. The use of steel leads to lightweight building elements, high prefabrication, and good recyclability of the structure. Sustainable projects dealing with the building stock require a holistic view on all fields of building construction in consideration of functional, economic, ecologic and socio-cultural aspects. Within the scope of the presented research project an interdisciplinary research group develops integral, sustainable solutions for the modernisation of the building stock with optimized (lightweight) steel structures for building extensions.