Upward extensions in London

To investigate the extent to which upward extensions can contribute to meeting London's housing needs, and to assess any barriers affecting their delivery.

• Mansard roof extensions are a common feature of British streetscapes and a means of increasing living space in a way that “imposes very little visual burden”. They were fairly common practice in the eighteenth and nineteenth centuries, but the practice became less common in the twentieth century with many local authorities eventually prohibiting them altogether.
• More recently, planning policies – both nationally and in London – encourage the development potential of the ‘airspace’ above existing buildings for new homes. The National Planning Policy Framework (NPPF) includes a clause (124(e)) explicitly permitting mansards on appropriate properties, under particular conditions.
• Estimates from 2018 suggest that residential rooftop developments offer the potential for up to 180,000 new homes in London. Others suggest the potential for upwards extensions lies in the potential to enlarge existing homes rather than adding new ones.
• Several examples of local design codes encouraging upward extensions and/or mansard roofs have demonstrated the potential for adding more bedrooms in London. Since Haringey Council’s “House extensions in South Tottenham” supplementary planning document (SPD) was introduced in 2013 at least 200 extensions have been carried out.

Since Tower Hamlets adopted design guidelines for mansard roof extensions in five conservation areas in 2017, the average annual number of mansard planning applications and approvals increased dramatically.

• In London boroughs, conservation areas do not automatically prohibit roof extensions, but they raise the bar on design and typically strip away permitted development rights via Article 4 Directions. Consequently, even standard loft conversions and mansard roofs usually require full planning permission in certain boroughs including Islington. Local authorities restrict these rights to protect historic streetscapes, rooflines, and original architectural details (like traditional butterfly roofs).
• The GLA issued Small Site Design Codes guidance in 2023. This provides information on how to prepare area-wide housing design codes on small sites for a range of character types.

• To what extent can upward extensions / mansards to existing buildings help address the housing shortage in London?
• What policy approach should the London Plan take towards upward extensions / mansards as a means of increasing housing supply?
• What are the barriers to wider adoption of upward extensions /mansards and options for addressing those barriers?
• To what extent are upward extensions an opportunity for councils and housing associations to meet the needs of tenants living in street properties?