In the light of market uncertainties and globalization, the capacity of fast and flexible adaption of a chemical plant to changing market conditions becomes a major necessity in future production scenarios. The capacity flexibility of a plant can be influenced by oversizing and numbering up of unit operations. The additional costs for these measures have to be compensated for by the financial benefit arising from the option to adapt to uncertain market demands. Therefore, the additional costs have to be balanced against the gain in flexibility. In this work, methods for the evaluation of capacity flexibility are adapted from the manufacturing industries to enable a cost-effective design of a modular plant. The evaluation methods are used to select modular plant setups allowing the best compromise between costs and flexibility in different market scenarios.