Khan called in housebuilder Barratt’s application for the 15-hectare scheme earlier this year, after the London Borough of Barnet’s planning committee refused permission against the advice of its officers.
The application sought full planning permission for the redevelopment of the National Institute for Medical Research site in the Mill Hill ward, in the northern part of the London borough.
The site, which is primarily used for research and development purposes, falls within the green belt and the Mill Hill Conservation Area.
In a statement, the mayor’s office said that when Khan first saw the plans in December 2016, they included 20 per cent affordable housing - 92 homes, all of which were for shared ownership.
When the scheme was later referred to the mayor for a decision on whether to call it in, it included an additional offer of £4.56 million towards off-site affordable housing, the statement said.
It added that, following the mayor’s decision to call the application in, the scheme now includes a new offer of 40 per cent (by unit) affordable housing, split 30-70 (by habitable room) between homes at social rent levels and those for shared ownership.
A Greater London Authority (GLA) officer’s report said that, as a result, the scheme is eligible for the "fast-track route" set out in the mayor’s affordable housing and viability supplementary planning guidance.
The SPG states that, where applicants are meeting a 35 per cent threshold for affordable housing provision alongside an appropriate tenure mix and with no public subsidy, they would receive a fast-track approach with no viability assessment being required by the GLA.
The report said that, since the mayor’s decision to take over the application in May 2017, GLA officers have "sought to ensure full compliance" with the SPG. "The baseline affordable housing commitment has subsequently been increased to 35 per cent by habitable room (40.2 per cent by unit), and given the tenure split secured … is eligible to be dealt with under the fast-track route," the report said.
The report added that the proposals would not constitute inappropriate development in the green belt. "The scheme involves complete redevelopment of a previously developed site, which would not have a greater impact on the openness of the green belt and the purpose of including land within it than the existing development and is therefore not inappropriate," it said.
Khan said: "Delivering more of the genuinely affordable homes Londoners need is one of my top priorities as mayor and I will use all the tools at my disposable to do so.
"This development offers a significant number of high-quality homes which will be available through shared ownership, to help people struggling to buy a home on the open market.
"I have also been able to secure new homes at social rent levels within the development, which is key to helping Londoners on low incomes and to making sure we build a mixed community here."