The last few years have seen the growth of a movement in architecture and construction aimed at retrofitting existing buildings instead of demolishing and retaining them. This has been focused in particular on the concept of ‘embedded carbon’ as the sector tries to become greener.

New construction techniques may be helping to make this possible, with curtain walling contractors among those involved. Because this method of using steel and glass is relatively light, it can enable more levels to be built on top of existing buildings without overwhelming the foundations.

A good example of the use of glass in a retrofit is about to appear in the City of London, where planning permission has just been granted for the redevelopment of 70 Gracechurch Street,  Construction Enquirer reports.

The scheme will see the existing ten-storey building, the former Marks and Spencer Offices, being largely retained and topped with another 32 levels in three glass towers, providing 550,000 sq ft of office space in what will become a 155 m (508 ft) skyscraper. Work is expected to commence in 2028.

Because the extra height will almost quadruple the number of floors, some work will need to be done to strengthen the weight-bearing capacity of the building. This will involve new vertical columns in the heart of the structure and new basement slabs on a concrete raft. 

The increasing use of retrofitting has led to Bulding.co.uk creating the Retrofit of the Year category at its annual awards. Its 2024 awards shortlist included YY London, a retrofit of a first-generation Canary Wharf building.

Its cylindrical structure features a glass walled exterior and two extra floors, outdoor space on 12 of the 13 floors and a BREEAM Excellent rating for its environmental performance. While it did not win the award, it was an exemplar of how structural glass can play a role in retrofit projects.