Infrastructure asset managers have the increasing challenge not only to maintain efficiently the functionalities and quality of their infrastructures, but also to upgrade them for the increasing demands and to achieve added value to their assets (Hertogh, Bakker, van der Vlist, & Barneveld, 2018). Moreover, infrastructure projects in densely populated areas, such as The Netherlands, suffer from increased complexity due to interdependencies of infrastructures which impact performance of projects (NGinfra, 2017, pp.1-3; Hertogh et al., 2018). Infrastructure operators are limited to handle such complex dependencies individually, as they need to collaborate closely with other asset owners in the entire life cycle of projects, especially in the design phase, to execute the projects successfully and add value to their infrastructures.In this light, inter-organizational collaboration (IOC) and multi-actor perspectives are driving growing attention among management scholars and practitioners to understand the complex nature of collaborative networks (Keast & Hampton, 2007; Keung & Shen, 2013; Dietrich et al, 2010; Storm, 2018). Currently, literature on IOC has many different approaches; however, its dominant focus has been within a framework of ‘problem owner’ – ‘problem solver’ relationship or an ‘owner-to-contractor’ perspective. This research project is conducted from an ‘owner-to-owner’ perspective or a horizontal collaboration perspective. The objective of this research is to investigate the required criteria to assess levels of horizontal collaboration between infrastructure owners in the design phase of projects by identifying appropriate frameworks in literature that focuses on social relationships and interdependencies, such as collaborative network and resource dependency theories. The expected result of this research study is to deliver an Inter-organizational Collaboration Assessment Tool (ICAT) to assess their levels of horizontal collaboration. The research is guided by the following research question: How can integrated collaboration between infrastructure owners be assessed on infrastructure projects? The research project will be examined through the lens of exploratory case study. The results reveal that asset managers need the knowledge of new soft mechanisms of social and cultural collaboration to deal with the increased complexity and dependencies of their infrastructure projects. Clients need to pay attention to their collaborative capacity at three different levels: individual, relational and organizational capacity to improve project performance.