Building energy regulation, as part of an energy policy, is launched for governing energy-efficient building design with an ultimate goal to attain mitigation of climate change. In most of the current building energy regulations, there is no requirement for assessing the environmental effectiveness of the energy-efficient measures involved. Building professionals cannot ensure that the energy saving can offset the extra embodied energy consumed for compliance with a design regulation. This study investigates the environmental effectiveness of regulatory requirement on energy-efficient building design. A Code of Practice for Overall Thermal Transfer Value in Buildings implemented in Hong Kong was used as a case study. Through a survey and energy simulations, it revealed that implementation of this mandatory building design regulation in Hong Kong is successful in terms of both saving in building operating energy and recovering in embodied energy of the extra building materials used. Policy makers are advised to consider incorporating an assessment phase into the development of building energy regulation to examine the environmental effectiveness of regulatory requirement on energy-efficient building design. It is envisaged that the methodology developed in this study can be applied to other cities for evaluating the environmental effectiveness of implementing a building energy regulation.