In this paper we address the question of how the installed base influences the process of creating a national architecture for eHealth. National eHealth solutions empower patients to access information and services and enable more efficient and cost effective ways of delivering care. Based on a case study conducted over a two year period, we trace the design of services between citizens and their GPs in the context of the Norwegian eHealth portal. Our analysis is based on the concept of installed base from Information Infrastructure studies that allows us to focus on the relationship between needs, capabilities and the existing technical components. Specifically, we take a relational view on the installed base and analyse three core decisions shaping the architecture of the infrastructure. These decisions expose the work of creating an infrastructure in relation to how existing technologies are selected and configured and how new needs are identified. This paper contributes to the understanding of Information Infrastructure evolution by taking a sociotechnical view on the creation of IT architectures.