The building construction sector in the UK is responsible for generating about 49% of the carbon emissions in the country, which is primarily due to the embodied carbon emissions generated from manufacturing construction materials such as concrete or steel. Reducing the embodied carbon is timely required to achieve the net-zero goal by 2050. To lower the carbon footprint of buildings, the circular economy principle has attracted significant attention from the industry, government and academia for its potential to effectively decarbonise the material manufacturing and building construction sectors. Different circular economy approaches are identified, including retrofitting end-of-life buildings to fit the new building requirements or creating the new buildings with materials recycled from the demolition of the end-of-life buildings. However, it is uncertain about the reductions in carbon footprint from using these approaches for buildings and the more effective approach for decarbonisation. This research aims to investigate the reductions of environmental impacts achieved with the two approaches, as retrofitting existing buildings and recycling demolished concrete for new construction, to achieve higher environmental benefits. Comparative life cycle assessment has been conducted to analyse the life-cycle embodied carbon and other environmental impacts of the new building retrofitted from an existing case study building and that of the new building constructed with the concrete materials recycled from the demolition of the original building. Besides, the life cycle assessment for the new building constructed with recycled concrete materials also incorporates the use of carbon sequestration technology in the recycling process. The results demonstrate that the circular economy approach through structural retrofit provides better environmental performance due to the much lower consumption of new construction materials.