Project organisations are complex, dynamic, open socio-technical systems, in the domain of building construction designed to complete another, in many cases very complex system – a building. Two research projects in Austria dealt in these two senses with the 'management of complexity': Within the project 'Cost-Benefits of the Integrated Planning' (co-be) sequential and integrated planning processes of a complex building have been compared. A planning experiment with 200 probands showed, that the early integration of experts for building services e.g. increases the job satisfaction and reduces the period needed for the project development, but the quality of results and necessary manpower needed remains more or less identical. 'Interdisziplinäre Forschung zur Energieoptimierung in Fertigungsbetrieben' aims at the integrated optimization of building, machinery and building services. Three strategies dealing with the increasing complexity of integrated optimization have been developed and will be presented.