A holistic approach for the simulation of evacuations from buildings in cases of fire and toxic gas spread is developed within the German project iSiGG to achieve high reliability in fire safety planning. Its essence is in the mutual interaction of the domains of crowd simulation, pollutant gas spread simulation (CFD) and Building Information Modeling (BIM), embedded in a coherent IT system. The conceptual basis of this system is provided by a dynamic multimodel ensuring interoperability of all system components and supplying simulation tasks with the necessary building and environmental data. More importantly, it allows to take into account various possible changes of the state of building elements, which may be caused by inhabitants or by the building control systems and can lead to strong changes in the simulation models. The simulations themselves are coupled on numerical level through a shared Voxel Model in a co-simulation approach.