Modelling of district heating networks is subject to many challenges, e.g. computation time and complexity. Since district heating networks occupy the largest share of investment in urban heating, this paper presents a workflow for automatic modelling of district heating networks in an existing CityGML-based urban simulation platform, using the greedy Prim’s algorithm for optimized pipe routing and an empirical logarithmic function for optimized pipe sizing. A validation for the workflow shows, that it is able to size the inner pipe diameters for an existing district heating network with a deviation of less than 7% from its actual values. After optimizations were applied, a smaller deviation of 5.3% could be achieved. After a manual correction for limited inaccuracies in pipe routing has been done, the total pipe length deviated by ca. 8% from the case study network. This paper thus concludes that the workflow provides reliable results along important parameters of a district heating network. Given its low demand for data availability and speed, it provides stakeholders with results to steer decision making on new or to-be-extended heating networks early during the planning process. According to the authors’ knowledge, no comprehensive work was dedicated to the validation of similar methodologies using real-life case studies with existing district heating networks.