District heating is of great significance for the Nordic countries due to the high heat demand. The Finnish government has set a national target of carbon neutrality in 2035. This implies a huge challenge and rapid system change. The Helsinki metropolitan area consists of Helsinki, Espoo and Vantaa, and in each city a different district heating company operates, and the technologies planned for decarbonization are different. This research aims to analyze these strategies with respect to carbon dioxide emissions and production costs, assuming different future European Union emissions carbon trading prices. The software EnergyPRO is used to provide least-cost optimal district heating operation solutions. From 2010 to 2030, carbon dioxide emissions from the Helsinki metropolitan area district heating will decrease by about 4.2 million tonnes. However, the average heat production costs are expected to increase considerably by almost threefold; while heat trade between the cities will reinforce the feasibility and decreases the system operation costs and total emissions. Helsinki will import heat, especially from Vantaa waste incineration plants. Higher carbon dioxide prices would reduce the total emissions, increase the total district heating operation costs, and lower the heat imported to Helsinki. As all the cities plan biomass as an alternative to fossil fuels, a higher biomass price would limit its consumption but in-crease natural gas usage the carbon dioxide emissions. In the future, combined heat and power plants will be used significantly less, leading to lost income on electricity sales and profoundly changing the business of the district heating companies.