The built environment is a major direct contributor to carbon dioxide emissions and therefore to global warming. Half the worldís population now lives in urban areas, which account for 75% of all energy use. There is thus great theoretical potential to reduce global carbon dioxide emissions by improving the performance of the built environment specifically. In this context, this paper reports on ongoing work to create a global index that gauges the capacity of countriesí built environments to move towards the zero-carbon ideal. Commissioned by the RICS, the index aims to bring together key emission reduction factors that fall within the direct competence of the built environment sphere at a range of scales (urban area down to individual building) and across a range of activity sectors (domestic, business, industrial, transport). It is envisaged that the index will be relevant to those with an interest in promoting, complying with or investing in carbon emission reduction measures. Specifically, it is targetted at the property and development sectors as well as spatial planning bodies. The paper focuses on conceptual and methodological issues encountered in the index framework design and data identification stages of the project, ahead of its completion at the end of 2008.