Building performance simulation environments are very complex software systems and the result of many person years of development and testing. The instantiation of a specific building requires additional work, depending on the quality of the user interface. We will show that with today's software engineering environments, the development time can be drastically reduced. Simulators for specific applications can be created in a few weeks and discarded after use. A prerequisite for our development method is the partitioning of the building and its installed systems into simple components that compute and interact at run-time autonomously, as in the real world. In contrast, classical building performance simulators are typically based on global systems of differential equations that model the physical reality and are numerically solved at run-time. Our approach is based on object-oriented modeling. All effects are modeled dynamically and are executed in real-time. New physical effects can be easily integrated. As calculations are triggered by value changes, very short real-time responses are achieved if necessary. Also, fast and accurate responses to external events can be guaranteed. These features are necessary for real-time tests of building automation systems. We will present data showing that our approach results in realistic simulations and show limits of real-time computations.