This study investigates how social capital can be developed to enhance team effectiveness in project-based cross-functional design teams within building team delivery methods. With this method, the contractor, client and other parties are involved early in the design phase to integrate their expertise into the design. Building on literature on social capital and team effectiveness, a sensitizing framework was created and empirically tested through a qualitative multiple-case study of three construction projects. Social capital is conceptualized as a dynamic and embedded emergent state, comprising of the structural, relational and cognitive dimension. Team effectiveness is described as the direct outcome of social capital encompassing both social actions and the intrinsic values experienced by team members. The study reveals that balanced development across all three dimensions of social capital, facilitated through continuous feedback loops and deliberate team practices, is essential for enhancing team effectiveness. The findings show how imbalances, particularly when the structural and cognitive dimension are neglected, can undermine team and project performance outcomes. A theoretical framework is proposed, emphasizing the need for clear roles, expectation management, critical dialogue and adaptive mechanisms. This research contributes a conceptual model for analyzing social capital in PBCFDTs and provides guidance for both practice and future research, suggesting longitudinal studies and broader application across project delivery methods. In short, the study shows that effective teamwork is not coincidental but built through deliberate social action enabled by social capital.