The CO2-free heat supply of buildings in cities is one of the greatest challenges for architects and engineers. The use of waste heat and renewable energy sources as a heat source requires the implementation of new technical solutions with significantly different system requirements. Low-temperature heating systems and sector coupling offer the most promising approaches for this. However, due to their complexity, design, and implementation, especially in existing buildings, require elaborate simulation models from the areas of heat and power supply, building structure and occupancy as well as system control. This paper uses a practical example to show how existing tool chains can be used in the planning process through intelligent tool coupling. They even enable engineers to integrate and evaluate new system components during the entire modernization phase with respect to all existing system requirements. The aim is to use digital simulation-based planning tools.