Room for lots more homes over London Picture 1 Room for lots more homes over London Picture 3 Room for lots more homes over London Picture 4 Room for lots more homes over London Picture 5

Rooftop Development | Riette Oosthuizen and Natalya Palit

Work is in progress for reformulating the London Plan, setting a new policy context for accommodating London's projected growth. Exploring the Green Belt as an option for new development appears to have been parked for now, shifting the focus to London's insides.

London’s skyline is changing with a staggering amount of new and proposed tall buildings, and more estates are in line to be regenerated, but yet the demand for more homes grows. Latest predictions for housing need in London range from between 50,000 to 80,000 homes per annum, depending on the source. However, in reality housing delivery amounts to approximately half of this, reaching 31,984 homes in 2014/15 in London. The undersupply of new homes, twinned with the increasing challenge of affordability provides a real challenge for all involved, in determining the ‘how’ and ‘what’ of new housing supply. The redevelopment of brownfield land alone will not deliver the required level of growth. As professionals working in the housing industry, it is evident to us that every potential source of land needs to be explored. Policy measures need to be adopted to facilitate the delivery of housing by a much wider range of players, including small and medium-sized businesses who are often priced out in bids for land across London. There is huge potential within the existing fabric of London, with a large volume of research illustrating this. This includes HTA Design 's work on 'Supurbia' - our model for intensifying London's suburbs by building on their inherent qualities, using a resident-led development approach combined with a design framework.

London First and Savills study 'Redefining Density' (September 2015)1 also demonstrated that many of London's well located areas with good PTAL ratings are at densities below the appropriate level set out in the density matrix of the London Plan. Following new research, we have undertaken, we believe that there is further untapped potential within existing buildings that are suitable for upward extension. The idea of 'rooftop development’ is not a new concept. People have been extending their homes and properties by building on roofs through the conventional planning process for years. However, this type of development currently delivers less than two per cent of new homes per year, illustrating there are still barriers that make the process less than straightforward. The Department for Communities and Local Government and the Greater London Authority consulted on permitted development rights to extend upward earlier this year, showing the wider recognition that there is potential on London’s rooftops to deliver additional self-contained units. The outcome of this consultation is yet to be published. In HTA Design’s response to this consultation, we particularly emphasised the importance of adopting policy measures that would support good design. Permitted development rights can sometimes come with the risk that the resultant additions fundamentally harm the streetscape. In contrast, specific policy support for rooftop extensions, supported by careful design guidance, or a Local Development Order with a design code attached, could result in more desirable outcomes. Little work has been done to date to establish an accurate picture of the potential of London’s rooftops. A number of different figures have been quoted in the press, ranging from 500,0002 extra rooms, 130,000 to 140,000 new homes, or even 630,000 new homes3 if building takes place over public buildings such as hospitals and schools. Apex Airspace Development, a company set up exclusively to promote, procure, and deliver rooftop apartment living across the Greater London area, approached HTA Design in April 2016 to produce a detailed study of the potential for rooftop development in the Borough of Camden. Our brief was to accurately establish the true potential for rooftop extensions through a practical assessment of available rooftop space, and to identify a number of units that could be delivered based on this. Camden is a unique borough in that approximately 24 per cent of the borough's housing stock is Council owned, more than half of the borough consists of Conservation Areas and 27 per cent of the borough is covered by open spaces. It is by any conservative estimate fairly constrained and therefore acts as a good starting point to estimate potential.

In setting out to do our research, we were particularly aware of typical planning, design and technical constraints that impact on the potential for sites to be suited to extension, including:

• the structural capacity of the building to support additional loading;

• the feasibility of extending existing lift cores or adding a new external lift cores;

• the presence of restrictive telecommunications equipment and other services at rooftop level;

• the local character, and

• planning considerations such as Conservation Areas, local and London-wide sensitive views ;

• good access levels to public transport; and,

• the need for car parking provision. Using satellite imagery, we identified 475 rooftop sites across Camden suitable for development.

These sites excluded any building stock that was subject to planned or future regeneration. These sites amounted to 198,660 sqm of developable rooftop space. Taking an average of 60sqm per home, and utilising 75 per cent of the suitable floorspace, we calculated that rooftops in Camden have potential to deliver 2,485 new homes. This amounts to 28 per cent of the London Plan 2015 housing target for Camden. We identified a number of building typologies that might be suitable for rooftop developments, subject to these constraints; design solutions for these are illustrated as part of this article. From our findings in Camden we extrapolated the typical development density of 1.14 home per hectare to the whole of London, which delivers 14,330,080 sqm of rooftop space. Using an average of 60 sqm per home, this amounts to 179,126 new homes or 42 per cent of the London Plan 2015 target for housing.

The study acknowledges that this extrapolated figure is approximate, but as the case study borough was fairly constrained, we believe it could be a fair reflection of the potential for rooftop development across London. The research has also been presented to the GLA, who received it positively and who indicated they would welcome further London-wide research to inform evidence to underpin new policies and guidance related to rooftop development. Rooftop development offers a wide range of benefits which can include: increasing opportunities for small and medium sized developers and construction companies, providing a potential new funding stream to assist affordable housing delivery if done at scale, and using offsite manufacture to speed up the delivery process, amongst a range of benefits for existing residents and leaseholders living in suitable buildings. There is significant potential within London's built fabric for intensification. Small sites have traditionally not been the focus in terms of their contribution to new housing supply, particularly as often these sites can be the most difficult to guide through the planning process.

We need adequate policy support for this type of development in London, with recognition that with careful design measures, this could be an altogether sustainable way to grow London, keeping existing communities in place and resulting in local and wider economic benefits.