The performance of a large infrastructure project depends on not only technical design choices, but also contractual and other economic arrangements. These choices and arrangements interact in the context of uncertainty to result in the project’s realized performance. Large infrastructure projects such as desalination plants are thus multi-dimensional design problems in which the dimensions can be broadly categorized into either the technical or institutional domains, creating the need for “dual-domain design”. This paper describes the concept of dual-domain design for infrastructure in the context of desalination projects in the Kingdom of Saudi Arabia. It demonstrates the results of an analytical model that relates design choices along some technical and institutional design dimensions to plant economic performance. The analysis shows that plant design can be optimized subject to an uncertainty profile of water demand, and is sensitive to technology type, output capacity and potentially to price/contractual terms embedded in the delivery mode. The lens of dual-domain design thus provides a richer understanding of the relationship between project design and potential performance. Next steps can include multi-attribute assessments of performance (energy, environmental impact, etc.) as well as a greater variation in contractual forms in the institutional domain of design.