A district cooling system is a centralized cooling supply system providing air conditioning to a set of buildings located in the same district. Designing and sizing such a system is very complex, as both the initial construction cost and the operation cost of the cooling system during its entire life must be considered. We first propose a modeling approach aiming at formulating this combinatorial optimization problem as a mixed-integer linear program of tractable size. We then extend a previously published hierarchical decomposition technique in order to find the optimal solution in an efficient way. Finally, we provide preliminary computational results based on a real-life case study located in China.