The research of this paper provides a useful insight into the many barriers leading to building services overdesign, within the context of NHS hospitals. The issue of overdesign in building services is a systemic problem, whereby numerous contributing factors manifest into an issue that inevitably leads to poor system performance and excess costs. A key factor leading to oversizing is the excessive and uncoordinated application of design margins across the various stages of a building services project. Poor communication between project stakeholders is another significant barrier that inhibits the distribution of information between design groups; unknown requirements, system redundancy and poor system specifications further add to the problem. There are many complex interrelationships associated with the building service design process in hospitals, with external stakeholders adding to the complexity. This points to the importance of effective communication between stakeholders and clear contractual terms between NHS Trusts and external private sector organisations. Many of the barriers identified within this paper are by no means limited to building service systems but also impact on a range of other engineering disciplines.