Climate change is an urgent global challenge. Buildings significantly contribute to energy consumption and CO2 emissions. To accelerate decarbonization, ATES systems, integrated with 5GDHC networks, offer a sustainable approach. These systems enable seasonal thermal energy storage and bidirectional energy exchange. However, designing these complex combined systems in urban areas poses significant challenges, including integrating dispersed ATES wells, managing phased construction, and navigating dense underground infrastructure. This study investigated a biologically inspired approach, utilizing the network optimization capabilities of slime mould (Physarum polycephalum). A simulation model was developed and verified, incorporating key slime mould behaviours like efficient pathfinding, to generate optimal thermal network configurations. Using the TU Delft Campus as a case study, this research explored the model's capacity to streamline the design of sustainable and efficient heat distribution systems. Key findings indicate that the visual differences in layout between the best results and all results are minimal. In conclusion, this research successfully developed and applied a slime mould growth model for optimal thermal network design, specifically for district heating systems combined with ATES in an urban environment. The model proved effective in generating a network designs that minimize CapEx.