The creation and management of buildings over their lifecycle involves the cooperation of many organizations, which broadly fall into a client domain and AEC (architecture, engineering, construction) domain. While this mix of expertise is essential, the ineffective management of the boundaries between these organizations can undermine building lifecycle performance. This paper explores client–AEC interactions at the project development and handover stages, with a view to discovering insights into client–AEC interface management for effective building lifecycle integration (BLI). The concept of boundary objects provided the theoretical framework to discuss findings from two case studies on the project development phase of a private finance initiative project, and the asset development process in a repeat client organization. The findings suggest that there are different emphases in boundary crossing activities at different stages, with boundary roles that relate to decision-making and authority to commit resources being more relevant at the project development stage, whereas the need to explain meanings appear to be more relevant at the handover stage. AEC professionals in client organizations play a crucial role in bridging knowledge boundaries about buildings, but this professional/functional strand to BLI needs to be effectively managed alongside the organizational boundaries, since the authority to resource BLI efforts resides within organizations.