In the European Union, heating and cooling account for 50% of the total energy consumption and half of the carbon emissions, as 75% of the energy is still supplied by fossil fuels. In the Netherlands, energy transition decision-makers are sailing in the dark in an attempt to realize the heat transition in high-density areas. How can market parties optimize the mix of heat supply technologies, while responding to policies and network requirements? To adequately manage renewable energy on a large scale, flexibility measures need to be in place across all components of the system, from generation to distribution, to supply and demand. This master thesis is part of the TU Delft research project "DEMOSES" and is done in collaboration with the energy company Eneco. The project aims to study to what extent the integration of heat storage and additional heat sources can enhance the flexibility and long-term sustainability of the South-Holland district heating network. This is done by developing and analyzing a district heating network model of South-Holland, and expanding the network with additional heat sources and heat storage. Additional heat sources lead to increased flexibility and sustainability, while additional heat storage primarily increases only the flexibility. Looking ahead to 2030, to realize the heat transition in South-Holland, many additional sources and storage should be added in the following years to realize a flexible, sustainable, and affordable district heating network. However, with the proposal of the Collective Heat System Act, the question is whether investments in additional heat sources and storage are attractive for energy companies when ownership of these assets in the following years cannot be guaranteed.