Sustainable design practices are a key component in ensuring that the building and civil infrastructure industry does not damage our natural environment. Green buildings and allied initiatives in other sectors of the built environment are a primary mover in promoting sustainable design practices. An important and dangerous inhibitor of sustainable design practices, however, is the perception among key decision-makers that the additional costs of green or sustainably designed buildings are too high, and that they are therefore not economically viable. This study tested what those perceptions are, and compared the results to (limited) actual cost data and found that the cost premium is less than half of what most decision-makers think it is. Recommendations are then made around the development of- awareness programmes at both undergraduate and postgraduate levels, the need to highlight the necessity for developing further and more accurate data related to green building costs, and the need to establish incentives to drive the take-up of sustainable design practices.