Set within the context of China's growing need to develop a cleaner and more efficient energy supply infrastructure, this paper describes a procurement process for evaluating and reducing the carbon dioxide emissions arising from operational processes within buildings during the design stage when key decisions affecting long-term energy demand are often made but frequently not understood. The focus of the study is a case-study building, the 8275m² two-storey arrivals/departures building, Satellite 3, at Stansted Airport, UK. The client set a challenging design performance target of a 20% net CO2 reduction below British Airways Authority's (BAA) current most energy efficient building, Satellite 1, Stansted. This reduction was to be achieved without compromising the BAA corporate standards on passenger and staff comfort perceptions or appreciable increases in capital costs. The carbon assessment methodology involved the following stages: • Establish baseline CO2 footprint and target from real-time energy data for Satellite 1 of 103kg CO2/m²/a. • Review of the 'base-case' building performance assessment for Satellite 3 using BAA's standard specification criteria. • Assess the impact on energy demand and CO2 emissions of a number of variations to the 'base-case' design parameters ranging from solar shading, insulation levels to control systems and occupancy patterns. • Develop an evaluation methodology that iteratively balances capital cost and revenue expenditure with environmental benefits to enable informed choices to be made by the client on where the most 'environmentally effective' design responses lie. Outcomes: • A 23% net CO2 reduction by design was achieved at no additional capital cost. • Learning and environmental costs benefit methodology disseminated to the BAA supply chain and adopted at Gatwick and Heathrow Airports. • CO2 is now a new key performance indicator for design efficiency within BAA.