Reef Group are to create a £100m+ (US$128+, €117+) subterranean mixed-use destination with a focus on healthcare and wellbeing in London's historic Cavendish Square.

Currently an underground car park, the site below Cavendish Square Gardens will be redeveloped to provide 280,000sq ft (26,000sq m) of space across four storeys.

The existing structure will be reused and adapted, with atria, lightwells and a 200m (656ft) pavement light drawing natural light into all levels of the new development.

Located at the centre of major retail, medical and commercial districts in London, the destination will draw influences from each.

There will be also three entrances – one in each area and each with a structural glass entrance pavilion.

At ground level, Reef's aim is to create a 21st-century public square and to restore Cavendish Square to its former glory, with it having been one of London's original Georgian squares first landscaped in 1729.

A pedestrian-first green lung will set the scene for the health and wellbeing focus of the new development, doubling the current amount of public space and featuring new planting, water features and drinking fountains.

Below ground, spaces will range from 5,000sq ft (465sq m) to 250,000sq ft (23,225sq m) and, although no tenants have yet been announced, renderings envisage clinical, spa, fitness and bathing facilities, as well as restaurants, bars and retail spaces.

In particular, the development is intended to expand the footprint of the Harley Street Medical Area collective of independent hospitals, clinics and specialists, with additional space for specialist healthcare facilities.

Stewart Deering, Joint chief executive of Reef Group, said: "Reef have worked closely with Westminster City Council in designing a unique and iconic destination for London. We will invest in excess of £100m in developing a vibrant, sustainable and sensitively designed subterranean mixed-use 'place' that will combine retail, leisure, medical, health and wellbeing.

"The historic Georgian square will be restored to its former glory, becoming more accessible to the public than ever before with improved public realm. Cavendish Square will become an oasis in the heart of London and as well as providing an interesting mix of occupiers, we hope the plans will encourage more public events to take place within the square."

Construction is expected to start in late 2020 and is expected to be completed in 2022.