The draft new London Plan was published for public consultation in December 2017 and an examination in public was held between 15 January 2019 and 22 May 2019.
The use of small sites, defined as those used for the development of between one and 25 homes, forms a key part of the mayor’s proposed strategy for meeting housing need.
However, in a report on the examination of the draft new London Plan made public this week, a panel of inspectors warned that the mayor had been overly optimistic in his assessment of the rate at which small sites would be developed. The inspectors also recommended that the plan’s overall housing target be reduced by almost 20 per cent.
They advised that the plan’s target for the development of small sites should be more than halved, from 245,730 to 119,250 over ten years. As a result, the overall ten-year housing target should be cut by about a fifth from 649,350 to 522,850, which equates to 52,285 homes per year.
Citing "arbitrary" growth assumptions and a dramatic increase in the development of small sites in outer London boroughs, the inspectors said they had been forced to "question whether the targets are realistically achievable. The short answer is that they would not be and hence they are not justified."
According to the examination report, the London Borough of Bexley faces an almost 700 per cent increase in its small sites delivery target.
The draft plan envisages the annual delivery of 24,500 homes on small sites across London, compared to 15,300 homes delivered on small sites every year between 2003 and 2017.
The inspectors noted that their proposed changes represent an acknowledgement that housing delivery in the capital is likely to fall short of the 66,000 identified annual need.
"Of course, it is a major concern that the targets are so far below the assessed need," they said.
"However, the evidence simply does not justify the reliance placed by the mayor on small sites to fill the gap between the two and we are sceptical about the delivery from this source," they said.
"It is difficult to see how the number of deliverable housing units could be increased without consideration being given to a review of the green belt or further exploration of potential with local authorities within the wider South East.
"This would all take time and in our view it is better to proceed on the basis of an adopted plan rather than one that is in limbo."
Among the inspectors other recommendations are a call for the mayor to commit to a green belt review to inform the next edition of the London Plan.
The inspectors also concluded that the draft plan may need to allocate "many hundreds of hectares" of additional land for industrial, storage and distribution uses, including potentially on green belt land.
Khan must now consider the inspectors’ recommendations before submitting his response and a revised plan to the secretary of state for approval.
Deputy mayor for planning, regeneration and skills, Jules Pipe, said: "The mayor makes no apologies for his ambitious plans to build more homes for London – particularly genuinely affordable homes.
"We have to maximise opportunities to meet London’s housing needs and small sites have significant potential to deliver homes Londoners urgently need. These sites, particularly in outer London, must play a greater role."
This article was updated at 16:00 on 22/10/19 to include the above comment from Jules Pipe.