Building developments involve complex decisions and the increased significance of external effects has further complicated the situation. Society is not just concerned with economic growth and development, but also the long-term affects on living standards for both present and future generations. Certainly sustainable development is an important issue in project decisions. Using a conventional single-dimension evaluation technique such as CBA to aid decision-making is no longer adequate. A much more sophisticated model needs to be developed to handle multi-dimensional arrays of data. The development of a sustainability index is a way to address multiple criteria in relation to project decision-making. Using a sustainability index will greatly enhance the assessment of external effects generated by construction activity, realise sustainable development goals and thereby make a positive contribution to the identification of optimum design solutions.