Engineering design has a major impact on downstream performance, but is often guided, in both procurement and solution type, by intuition rather than a solid science base. Engineering is also increasingly collaborative, crossing organisational boundaries. However, the long-term close partnerships, as often proposed, are difficult to achieve in the context of individual engineering projects. Building on concepts related to investments in specific assets, this study describes the underlying characteristics of engineering design investments and then prescribes the relational investment and contract forms to select them, thereby contributing to our understanding of the engineering design and relational investments that can and should be made. We undertook multiple case study research, which is presented in two phases. The descriptive phase identifies positive outcomes for solutions based on off-the-shelf designs with fixed price mechanisms, for solutions based on adaptive designs with target cost mechanisms, and for bespoke solutions contracted either on cost-plus mechanisms or, if in bite-sized pieces, on fixed price mechanisms. Negative performance outcomes were found for adaptive solutions with Fixed Price mechanisms. A prescriptive phase, yielding a visualized model, then offers guidance for relational investments and contract mechanisms that are suitable for different engineering designs. Applicability zones and three potential transitions to challenge and guide current practice are developed to inform decision making. Managerial relevance statement Complex engineering projects are increasingly collaborative, involving multiple organisations, and involve innovative aspects. Sometimes these innovative aspects are developed especially for a single customer from fundamental research and development activity, whereas other times it may be possible to adapt previous engineering solutions. Clients of such projects must establish the conditions for project success through robust procurement approaches; on the one hand, contractors will be concerned that the risk and reward profile of their contracts is appropriate; on the other, there is little guidance for clients as to how to establish the right approach. Our paper focuses on establishing appropriate governance, including contractual forms and relational types, for different engineering design categories. This is operationalized through the concept of specialized investments. The descriptive model offers insights for practitioners into how governance is approached in a number of cases. The final visualized prescriptive model offers guidance for decision makers. Using our model, appropriate levels of relational investment, as well as a contract mechanism, can be aligned with different engineering design categories. Overall, we propose that the model forms an intellectual