The Planning and Regeneration Committee met on Wednesday 8 July 2026 to discuss the potential for upward extensions in London to increase housing capacity. The committee heard that while there is significant spatial potential for upward extensions, practical and regulatory barriers currently limit their delivery.

The committee explored the benefits and limitations of upward extensions, with a particular focus on mansard roof extensions and airspace development. Discussions also covered the role of design codes, the impact on heritage assets, and the challenges of fire safety regulations and resident engagement. The potential for upward extensions to contribute to sustainability goals, such as improving EPC ratings and reducing embodied carbon, was also highlighted.

Upward Extensions in London

The committee discussed the significant potential for upward extensions to increase housing capacity in London. Dr Riëtte Oosthuizen of HTA Design estimated that there could be potential for around 180,000 homes across London through rooftop developments, using modular construction and utilising 75% of roof space. However, she noted that building over 18 metres is complex due to the need for second staircases and sprinklers, which can lead to suboptimal homes.

Dr Charles Gillott from WSP UK & Ireland highlighted that a University of Sheffield study identified approximately 500,000 eligible properties in Greater London for vertical extensions under permitted development (PD) rights, with a potential to house an additional 1.2 million people. He stressed that this figure represents only the potential under PD rights and that a more traditional planning approval route could yield even greater potential.

Claire Brady from Historic England stated that heritage should not necessarily be a barrier to upward development, noting that many historic buildings, particularly masonry structures, can accommodate additional storeys. She mentioned examples of successful roof extensions on heritage assets and highlighted Historic England's publication, Good Growth, Heritage and Housing, which explores various forms of intensification growth.

Nicholas Boys Smith MBE of Create Streets presented research suggesting a spatial opportunity for between 140,000 and 320,000 new homes in London through upward extensions, with a potential for 300,000 to 680,000 additional bedrooms. He argued that the current planning and regulatory regime presents significant barriers to entry, making the practical potential modest. However, he believes that simple fixes in the planning system could unlock this immense spatial opportunity.

Barriers and Solutions

A key theme throughout the discussion was the significant barriers to the wider adoption of upward extensions. Nicholas Boys Smith highlighted the barriers to entry, including the cost of planning consultants, architects, and the risk of not obtaining planning permission. He suggested that simple fixes in the planning system, such as Local Development Orders (LDOs) and design codes, could unlock the spatial opportunity.

Dr Charles Gillott pointed out that the success rate for PD prior approvals for extensions is only around 50%, suggesting the process is too stringent. He argued that the onerous prior approval process leads applicants to opt for full planning permission, which is not significantly more costly. Nicholas Boys Smith agreed, stating that the concept of prior approval is inherently flawed due to a lack of experience among planning consultants and officials.

Claire Brady noted that while Historic England supports design guidance, it needs to be adapted to local contexts, particularly in conservation areas and for listed buildings, requiring an extra layer of detail. She emphasised that borough-led approaches are crucial as they understand local architecture, context, and residents' needs.

Benefits for Residents and Communities

Michael Ritchie of Tower Hamlets explained that their focus on mansard roof extensions was driven by resident demand, allowing growing families to extend their homes and stay within the borough. This contributes to social cohesion and addresses the shortage of family-sized homes. He also noted the increasing need for space due to more people working from home.

Richard Truscott from Haringey highlighted that their SPD in South Tottenham arose from local demand, particularly from the Haredi community, leading to a transformation of two-storey streets into three-storey streets. He stressed that the guidance is intended for single-family dwellings to help relieve overcrowding for large families.

Michael Holland from Populo Living emphasised the benefit of upward extensions for creating larger family homes, such as converting three-bedroom houses into five-bedroom homes, which is more economically viable than new-builds. He also noted the advantage of avoiding decanting residents when working on occupied buildings.

Environmental and Sustainability Considerations

Nicholas Boys Smith argued that upward extensions offer a sincerely green way of growing by utilising existing embodied carbon in buildings, avoiding the carbon footprint of demolition and new construction. He stated that the greenest building is often the one that already exists.

Dr Charles Gillott reiterated the embodied carbon benefits, noting that extensions are often part of wider refurbishments, allowing for fabric upgrades and general renovation. He also highlighted that a high number of small extensions has a lower embodied carbon impact than a small number of significant extensions.

Claire Brady added that upward extensions can also spur the regeneration of historic buildings, bringing forward residential uses and potentially cross-subsidising refurbishment costs.

Design Codes and Resident Engagement

Nicholas Boys Smith stressed the importance of visual design codes that are based on evidence of what local people like, noting that it has become easier and cheaper to conduct visual preference surveys. He warned against elite capture and advocated for codes that represent local preferences, encouraging local councils to develop their own, with a potential for GLA or national level intervention if local authorities do not act.

Claire Brady agreed that design codes should be local and based on character areas, but stressed the need for adaptation to local contexts and the involvement of local communities. Dr Riëtte Oosthuizen added that a wider conversation with the community is needed around design codes, and that residents, particularly younger people, need to be brought on a journey to understand the modern context of development, including challenges like overheating.

Fire Safety and Viability

Dr Riëtte Oosthuizen highlighted fire safety regulations as a barrier, particularly the need for second staircases and sprinklers in buildings over 18 metres, which can be difficult to retrofit in existing properties with low floor-to-ceiling heights. Nicholas Boys Smith noted that for typical terraced and semi-detached houses, these regulations may not be triggered, making upward extensions a more straightforward option.

Michael Holland acknowledged that viability is a significant challenge, especially when dealing with occupied buildings and evolving regulations. He suggested that deep retrofits with added floors, combined with new-build quality and warranty provisions, could build resident confidence. Ash Bhatti noted that while challenges exist, they are not insurmountable, and that experience gained from delivering projects leads to improvements.

The Role of the London Plan

Michael Ritchie suggested that while the London Plan provides broad policy support, more directional impetus from a higher authority might be needed to encourage councils to proactively develop upward extension policies. He noted that Tower Hamlets' focus is on tackling overcrowding through other means, making roof extensions a sideshow.

Richard Truscott felt that house extensions should remain a local council matter, but acknowledged the need for more resources and specialist knowledge. He suggested that the London Plan could potentially offer guidance on appropriate typologies and sustainability measures for different property types.

Conclusion

The committee heard that upward extensions offer significant potential for increasing housing supply in London, providing benefits for residents, communities, and the environment. However, a range of barriers, including regulatory complexity, cost, viability concerns, and the need for robust resident engagement and sensitive design, need to be addressed to unlock this potential. The discussion highlighted the importance of local design codes, clear guidance, and potentially a more streamlined planning process to encourage wider adoption.

The meeting concluded with the committee noting the report and discussion, and delegating authority to the Chair, in consultation with party group lead members, to agree any output arising from the discussion. The committee also noted its work programme and the additional activity undertaken since its last meeting. The next meeting was scheduled for 4 September 2026.