The council approved planning via a virtual planning committee.
The development will be 280,000 sq ft across four storeys below ground level.
Cavendish Square London will have floors of up to 80,000 sq ft and floor-to-ceiling heights of up to 8m.
Cavendish Square transformation
The transformation will provide specialist wellness facilities for London’s private healthcare system.
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Cavendish Square health and wellbeing facility
Construction is expected to start next year, with completion due in 2023.
Stewart Deering, joint chief executive at Reef Group, said: “Reef Group has worked closely with Westminster City Council to design an iconic destination for London and we are delighted to have secured planning consent for the scheme.
“We will invest more than £150m in developing a vibrant, sustainable and sensitively designed place with a core focus on health and wellbeing.”