The Planning and Regeneration Committee of the Greater London Authority Council met on Wednesday 8 July 2A026 to discuss the potential and challenges of upward extensions in London. The committee heard that upward extensions could significantly increase housing capacity, with estimates suggesting potential for around 180,000 homes across London. However, significant barriers remain, including regulatory complexities, costs, and the need for careful consideration of heritage and local character.

The committee explored the potential for expanding housing capacity in London through upward extensions to existing homes and buildings. Experts highlighted that while there is significant spatial potential, practical and regulatory hurdles currently limit its realisation. Discussions also covered the benefits and limitations of upward extensions, with a focus on how local authorities are attempting to manage these through design codes and planning policies.

The Potential of Upward Extensions

Experts presented varying estimates for the potential of upward extensions. Dr Riëtte Oosthuizen of HTA Design suggested that, based on a 2016 study, there could be potential for around 180,000 homes across London by utilising 75% of roof space with modular construction. However, she cautioned that post-Grenfell regulations, such as the need for second staircases and sprinklers in buildings over 18 metres, add complexity and can lead to suboptimal living conditions. Dr Charles Gillott from WSP UK & Ireland cited a University of Sheffield study estimating around 500,000 eligible properties in Greater London for vertical extensions under permitted development rights, potentially housing an additional 1.2 million people. Nicholas Boys Smith MBE of Create Streets presented research suggesting a potential of 140,000 to 320,000 homes in London through upward extensions, with a significant number of additional bedrooms (300,000 to 680,000), which he stressed are crucial for reducing homelessness.

Barriers to Delivery

A key limitation identified was the complexity and cost associated with the current planning and regulatory regime. Nicholas Boys Smith highlighted barriers to entry, including the need for planning consultants and architects, and the financial risk involved, which he argued makes the potential uncomfortably modest. Dr Charles Gillott echoed this, noting that the success rate for permitted development prior approvals for extensions is only around 50%, suggesting the process is too stringent. He also pointed out that the types of buildings most suitable for upward extensions (pre-1948 terraced houses) are often excluded from PD rights.

Heritage Considerations

Claire Brady from Historic England acknowledged that heritage should not necessarily be a barrier to upward development. She noted that many historic buildings, particularly masonry structures, can accommodate additional storeys. Historic England's report, Good Growth, Heritage and Housing, explores various forms of intensification, including upward extensions, as part of the solution for housing in London. However, she stressed the importance of sensitive design that respects the character of heritage assets and local areas, with design codes needing to be adapted to local contexts.

Benefits for Residents and Communities

Several speakers highlighted the benefits for existing residents. Michael Ritchie of Tower Hamlets explained that mansard roof extensions allow growing families to stay in their borough, contributing to community cohesion. Richard Truscott of Haringey noted that their SPD in South Tottenham was driven by local demand from the Haredi community, helping to relieve overcrowding and prevent families from having to move. Nicholas Boys Smith argued that increased housing supply through upward extensions would put downward pressure on rental prices and benefit those seeking bedrooms, including those at risk of homelessness.

Design Codes and Local Approaches

The discussion emphasised the importance of locally tailored design codes. Michael Ritchie detailed how Tower Hamlets has developed design codes for five conservation areas to ensure consistency and control the visual impact of mansard extensions, while also requiring facade improvements and contributions to streetscape enhancement funds. Richard Truscott explained that Haringey's SPD for South Tottenham focuses on additional floors for pitched-roofed terraced houses, rather than mansards, and is specific to the area's character. Both acknowledged the need for detailed work and adaptation to local contexts, particularly for heritage assets.

Sustainability and Environmental Impact

Ash Bhatti of Apex Airspace Development highlighted the environmental advantages of upward extensions, noting that they avoid demolition and utilise recycled materials, thus reducing embodied carbon. He also pointed out that they often eliminate the need for decanting residents. Nicholas Boys Smith further emphasised that the greenest building is the one that already exists, and that upward extensions leverage existing embodied carbon, avoiding the significant carbon footprint associated with demolition and new build. Dr Charles Gillott added that a high number of small extensions has a lower embodied carbon impact than a small number of large extensions.

Viability and Future Policy

Viability remains a significant challenge, particularly for affordable housing delivery. Michael Holland of Populo Living noted that while upward extensions are a valid approach, delivering them in occupied buildings is a major constraint, and it is often more economically viable to build on new sites. He suggested that the GLA could help boroughs gain confidence in upward extensions by providing guidance on suitable areas and approaches, potentially using digital mapping and AI. The committee also discussed the need for policy changes to simplify the process, with a call for increased resources and knowledge at all levels of planning. The potential for street votes and the role of local development orders were also raised as mechanisms to encourage development.