The application by the school proposes the demolition of existing sports facilities and other buildings and the construction of a 7,269 square metre, three-storey sports building, a 3,675 square metre science building, plus new car parking and landscape changes.
Much of the development site, some 4,600 square metres, would be built on designated metropolitan open land (MOL), which has the same protected status in planning policy as green belt.
The London Borough of Harrow originally approved the plans in January last year, but reversed the decision the next month following a direction from Khan to refuse them.
The Greater London Authority (GLA) had concluded that the building was "inappropriate development within Metropolitan Open Land" and would cause "substantial harm" to the openness of the MOL "by reason of its excessive footprint and its location". This harm "is not clearly outweighed by other considerations" and the very special circumstances needed to justify development "do not exist", it added.
However, yesterday, Jenrick approved the application in line with the recommendation of a planning inspector, who found that the necessary "very special circumstances" do apply.
Jenrick concluded that the benefits of the proposals would outweigh adverse impacts on the MOL, as well as a conflict with the council's heritage policies.
He agreed with the inspector that the proposal would be inappropriate development on the MOL, and therefore harmful.
However, the inspector’s report said that he did not find the proposal would have an adverse visual impact.
Inspector Cullum Parker said: "The footprint is not ‘excessive’ when one considers that there is a certain level of need that the sports building will have to provide.
"This is need that no main party argues is not required to be provided."
Parker criticised evidence provided by the London mayor proposing alternative concepts, which he said "literally do not stack up even under the rudimentary assessment".
His report concluded that the provision of educational facilities, plus the provision of 1,300 hours of free access to state schools nearby amounted to "very special circumstances".
Others included compliance with the council’s supplementary planning document for the school, which proposed the site for a sports centre.
The council has also proposed a land swap, identifying another parcel of land to be designated as MOL in compensation for the loss caused by the proposed school. The inspector considered this to be another very special circumstance, along with the lack of alternative sites.
The inspector also concluded that the development would not harm the historic environment.
He said: "The lack of overall architectural formality and unity at Harrow School is an interesting and distinct feature in the context of the foundation and design of public schools and their expansion in the C19 as a building type.
"The more organic evolution of Harrow with its intimate and entwined relationship with the town is thus distinctive within this building type and a key element of both its architectural and historic interest."
Jencrick said that although the plans were not in accordance with the heritage policies of the development plan, "in view of the significant public benefits outweighing the harm in line with the heritage test in paragraph 196 of the National Planning Policy Framework, the proposal accords with the adopted development plan when considered as a whole".