This paper applies cost–benefits analysis (CBA) and transaction cost (TC) theory to systematically evaluate the costs and benefits of implementing the green building economic incentives, with focused study on the Gross Floor Area (GFA) Concession Scheme in Hong Kong. The data of costs and benefits indicate how the GFA Concession Scheme motivates stakeholders and how much it benefits the built environment, which provides a solid foundation for the improvement of the GFA Concession Scheme. Expert interviews were conducted to verify and complement the new CBA framework and provide empirical evidence for policy-makers and researchers to better understand the allocation of costs and benefits. The results show that the effectiveness of the GFA Concession Scheme is readily justified even if it has caused a lot of extra transaction costs and actual costs. A 10% GFA concession attracts developers to enter the GB market but discourages them to go for a higher level of GB. It is the right time to differentiate the GFA concession to promote a higher level of GB.