Centralized solar thermal systems represent the future in the European district heating sector. The price of produced heat is almost constant, technology is clean and emission free, available all over Europe. In the summer months, system can completely replace other thermal energy sources and when combined with the seasonal thermal storage it is possible to store heat for winter periods. Large centralized heat systems were started to be used in Europe in the early 1980s, and today in Europe there are 199 such systems of rated power greater than 700 kWth. Most of these systems are located in Denmark, where the largest facility of that kind in the world is located. A techno-economic analysis of the solar thermal system with the seasonal thermal storage that would take place in Zaprešić was made within this paper, taking into account current heat consumption. System simulation was performed in EnergyPLAN. The criterion of determining the volume of the thermal storage was not to reject any heat produced in solar collectors. Several scenarios have been made, each with a different production of heat in solar collectors and its corresponding storage. The best solution was shown to be a solar heating plant which produces 64% of heat consumed. The heat price from solar energy is 45.13 €/MWh for optimistic investment cost prediction, i.e. 58.48 €/MWh for pessimistic investment cost prediction. In addition, a simulation of a system operation consisting of solar heating plant with seasonal thermal storage, cogeneration plant and natural gas boilers was performed. The integration of solar heating plant with the seasonal thermal storage has been shown to be financially viable. In the optimistic scenario IRR stands at 9.39%, while for the pessimistic scenario IRR is 6.29%.