The two applications were the first to be called in by the mayor of London after they were refused planning permission by their respective borough councils.
Both applications offered high-density schemes near well-connected transport hubs in outer London.
The first at Palmerston Road in Wealdstone included 186 homes of which 41% would be affordable, while the second at Hale Wharf in Tottenham Hale would deliver 505 homes with at least 35% being made affordable.
Mr Khan said that after carefully considering all the evidence, he was confident that both these high-density developments would deliver hundreds of the much-needed, genuinely affordable homes that Londoners needed.
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“We’ve worked with the applicant on the Hale Wharf scheme in Haringey to increase the level of affordable housing and ensure the project will not encroach on our precious green belt, as was the case in earlier designs.
“The development at Palmerston Road in Harrow also offers a high level of affordable housing, which is particularly important as we move towards my long-term strategic target of 50% affordable.”
City Hall planners were able to process the applications quickly and both took around two months to reach the mayor before he made his final decision, which they claim was significantly faster than under the previous mayoralty.
“Both schemes are close to transport links and this is one of the key factors in determining where major housing developments should be built,” added Mr Khan.
“Building the homes Londoners urgently need will mean town centres and suburbs becoming denser, so we expect developers to continue to come up with high-quality designs which don’t have a negative impact on their surroundings.”