This paper presents a methodology for the development of a transformation strategy for large-scale district heating systems (DHSs). Using this methodology, a roadmap for Hamburg’s municipal DHS is derived with an exemplary scenario for a climate-neutral heat supply in 2050. The city of Hamburg is currently in the process of transforming its municipal DHS towards climate-neutrality until 2050. So far, a blueprint for the development of such a long-term strategy does not exist. The presented scenario is based on an analysis of the future heat demand and of the regional renewable heat potentials. In the resulting generation mix, the expected heat demand in 2050 can mainly be supplied by industrial surplus heat, large heat pumps and biomass. To meet the city’s climate goals, fossil fuels will have to phase out and even waste incineration needs to be reduced drastically. After the application to the DHS of Hamburg, the methodology and results are transformed into a general approach for municipal heat planning that can support the transfer to other cities. Most importantly, strategic heat planning should be integrated into municipal administration to guarantee a flexible planning process. The paper also reveals shortcomings of currently available methodologies and tools for strategic heat planning, such as spatially resolved forecasts of the heat demand and regional potentials for renewable heat sources, which should be tackled in future scientific research projects.