Potential for the vertical extension of existing buildings

Vertical extension” refers to the addition of new storeys above existing buildings and is increasingly recognised as a means of generating new useable floorspace whilst reducing resource extraction, waste generation and associated carbon emissions. Despite this, understanding of the potential for vertical extensions remains limited, with existing work representing a series of fragmented perspectives from neighbouring contexts. The work contained within this thesis thus takes a more holistic and context-centric perspective to further understanding of the potential for future space provision through the vertical extension of existing buildings.

First, through a sequential explanatory mixed methods study, the relative importance of key drivers, barriers and enablers of vertical extension are assessed. Economic drivers are revealed to be most influential, with environmental considerations simply representing a secondary benefit. The most inhibitive barriers identified are typically technical, such as the identification of reserve capacity, but are not thought to indicate infeasibility. Recommendations to enable or promote extension include enhanced education of various stakeholders national and sub-national policy amendments.

Next, the influence of common design inefficiencies on utilisation and reserve capacity in steel framed buildings is considered. Column utilisation is found to vary greatly with design and appraisal approach, with an average reserve capacity of 10% being found even in least-weight Eurocode designs. This is found to facilitate extension without strengthening in 9%-36% of cases, depending upon structural form and use, with further reserve capacity and extension potential being found in buildings designed to superseded codes and using defensive or conservative approaches.

Finally, a bottom-up assessment framework is developed and applied to assess the potential for housing delivery through permitted development vertical extension across nine cities in England. Over 2 million properties are identified as eligible for extension, with this being capable of generating 176 km2 of new useable floorspace and housing almost 3.8 million people. The spatial distribution of this within and between the considered cities is also assessed, revealing the majority of extension potential to be within suburban areas.

Together, this work offers the first holistic and context-centric perspective of the vertical extension of existing buildings. It illustrates a significant potential to meet housing needs through vertical extensions and that, although a number of barriers remain, reserve structural capacity is likely to be present within a large number of buildings. Remaining barriers are also shown not to be indicative of infeasibility, and instead may be mitigated through adjustment of national or regional policy.