1 Undershaft

Original design by Avery Associates, to the immediate right of 122 Leadenhall Street

In January 2015, early plans emerged of a replacement office building for the St Helen's tower in Undershaft within London's Square Mile.[3] The proposal, named 1 Undershaft, was designed by Avery Associates who began working on the scheme in collaboration with the then owner of the site Simon Halabi in 2010.[4] At 270 metres (885 ft), it would have become the third tallest building in London and the United Kingdom behind The Shard and the stalled Bishopsgate Tower.[5]

In July 2015, details of a revised scheme by the new owners of the site, Aroland Holdings, were reported. The plans were for a skyscraper of 304 m (997 ft) designed by Eric Parry Architects.[6] According to some reports, the design could be "modelled on Cleopatra's Needle".[7]

In December 2015, the new design was released for a tower of 294.6 metres (966.5 ft) with 73 floors.[8][9] Subject to planning permission, it was set to become the tallest building in the Square Mile when completed and the second tallest building in London and the United Kingdom behind The Shard.[5] A consultation process took place in autumn 2015.[10] On 8 February 2016, a planning application was submitted for the development,[11] with a decision expected to be made in September 2016.[12]

However, a revised planning application was submitted by the developer which reduced the proposed height by 4.66 m to 289.9 m (951.2 ft), due to possible interference with the flight paths of the nearby London City Airport. Each floor has been reduced in height by 50 mm and structural floor beam depths have been changed. In addition, the level of the soffit has been decreased and the viewing gallery height has been reduced, which was intended to be double height. Despite its height reduction, the proposed height will still make 1 Undershaft the second tallest building in London and the United Kingdom upon completion.[13]

1 Undershaft would replace the St Helen's tower, pictured above

Following a recommendation by planning officers for approval, the scheme was approved by the City of London Corporation on 28 November 2016, with 19 votes in support and two against and given final approval by Mayor of London Sadiq Khan on 12 December 2016.[14][15][16] The start date for construction of the scheme has not yet been decided, but building work is expected to be finished anywhere between six and ten years from its approval date,[17] with demolition of the St Helen's building currently on the site expected to take 18 months and construction of 1 Undershaft due to take between three and four years.[18]

The proposed skyscraper is rectangular in shape and slightly tapers as it gets higher.[19] Developer Aroland Holdings originally wanted 1 Undershaft to be taller than the proposed height. However, the height has been limited by the Civil Aviation Authority (CAA) to avoid intruding on flight paths. In addition, the building's crown, which was intended to resemble Cleopatra's Needle, was not accepted by City planners who wanted "a less demonstrative top. They didn’t want another overt shape".[19]

The tower is designed to be built 10.5 metres off the ground in order to create public space underneath the building. To make room for the public space, the core will need to be positioned to the side of the tower. As a result, bronze diamond shaped external cross bracing will be required, giving the building its nickname The Trellis.[8][20][21]

A public square is also part of the proposed scheme with 2,178 m² (23,444 sq ft) of retail space below ground level.[1][19] The top of the skyscraper is set to have London's highest viewing gallery free for public access (which could include a museum run by the Museum of London),[22] and a restaurant.[8][19]