Vertical extensions (VEs), or adding floors atop existing buildings, have gained popularity as a novel strategy to create new floorspace in cities without consuming land or demolishing existing buildings. However, research gaps exist on the number of floors that can be built using VEs when the excess structural capacity of existing buildings is considered, and how different structural systems and materials influence this. Moreover, limited measurements of the embodied carbon benefits of VEs, as compared to conventional development approaches, exist in the knowledge base. This paper addresses these research gaps by quantifying the spatial capacity of VEs with different structural systems and materials, including concrete, steel, timber, and hybrid systems, with a total of 25 scenarios simulated. It uses life cycle assessment (LCA) to then compare the structural embodied carbon emissions of VEs with demolish and rebuild alternatives. Results show that between one and nine additional storeys could be built atop five base buildings in Sydney, by using their excess structural capacity and when considering vertical loads. This equates to an additional 33–225 % of floorspace. When also considering lateral loads, up to seven additional storeys were viable. VEs were found to have structural embodied carbon savings of between 32 and 212 kgCO 2 e/m 2 over demolish and rebuild scenarios, equal to a 34–94 % reduction. This paper is among the first studies to measure the space potential of VEs with different structural systems and materials, including mass timber configurations, and document multiple embodied carbon comparisons with conventional development. • Twenty-five vertical extensions (VEs) are simulated atop existing buildings. • LCA is used to compare embodied carbon of VEs with demolish/rebuild scenarios. • Between one and nine storeys of additional floorspace was found to be viable. • Embodied carbon savings ranged from 32 to 212 kgCO 2 e/m 2 . • Lightweight structures such as steel and timber provide largest additional floor areas.
Julistiono et al. (Sun,) studied this question.