Buildings are responsible for over a third of global energy-related carbon dioxide (CO2) emissions. A significant share of these emissions can be avoided cost effectively through improved energy efficiency, while providing the same or higher level of energy services. How large is this emission reduction potential globally and how much will it cost for society to unlock it? This paper provides answers to these questions, presenting the results of bottom-up research conducted for the Intergovernmental Panel on Climate Change (IPCC), based on the assessment of 80 country- or regional-level mitigation studies throughout the world. First, the paper analyses the findings of these studies in a common framework. Then, it aggregates their results into a global estimate of CO2 mitigation potential. The paper concludes that by 2020 it is possible to cut cost effectively approximately 29% of buildings-related global CO2 emissions, the largest among all sectors reported by the IPCC, representing a 3.2GtCO(2)eq. reduction. Developing countries house the largest cost-effective potential with up to 52% of building-level emissions, whereas transition economies and industrialised countries have cost-effective potentials of up to 37% and 25%, respectively. Energy-efficient lighting was identified as the most attractive measure worldwide, in terms of both reduction potential and cost effectiveness. If this potential is realised, the building-related CO2 emissions would stay constant over 2004-2030. These stabilisation levels (if achieved by all other sectors) would cancel about 3 degrees C temperature increase over the projected period of time. (C) 2007 Elsevier Ltd. All rights reserved.