In February, the City of London voted unanimously to approve plans for Seventy Gracechurch, a development for Ontario Teachers’ Pension Plan and Stanhope PLC. The 33-storey tower comprises the adaptive reuse of a former department store, with a new office structure built above. The design exemplifies KPF’s “Keep and Add” approach to adaptive reuse and aligned well with the owners’ aspirations for a transformation scheme and a market leading, sustainable redevelopment. 

An earlier scheme, for demolition and rebuild, was consented in 2020. On purchasing the site, Stanhope and Ontario Teachers’ set ambitious targets for an overall step change in the sustainability agenda and gave the design team the time to work through this more complex problem and define its optimal solution. 

The team—KPF, Robert Bird Group, Arup, and Turner & Townsend alinea—worked towards the objectives of a significant reduction in embodied carbon through the retention of the existing building structure, alongside a full reappraisal of the building envelope and services. 

Approximately 60% of the existing building’s structure will be retained and a new tower will be added above, increasing the existing GIA by more than 560,000 sqft. Additionally, at least 120 tonnes of steel beams from demolished storeys will be reused in the new tower. There is a rigorous discipline to how this is done – a careful arrangement of the new build tower allows it to be supported by a combination of reinforced existing columns and a new core threaded through the original building. The additional loads are spread across the footprint of the site with a new piled raft foundation constructed within the existing basement. 

There is no compromise in the floorplates – it is a sustainability imperative to create buildings of long-term, flexible, and robust utility, along with great care over carbon while making the new and old work together. The impact of every option has been carefully calculated and finessed to minimise impact to whole life carbon. The cost consultant and sustainability consultant worked together in line with both RICS V1 and V2 methods of carbon measurement. 

As modelled, the scheme has achieved a 14% reduction in up-front embodied carbon and an estimated 27% reduction in whole-life cycle carbon when compared to the previously consented scheme (RICS V1) and a calmly resolved design that assimilates very demanding criteria. It has been a very satisfying process. 

This approach demonstrates the need for a close creative relationship between the structural engineer, sustainability consultant, building services engineer, façade engineer, and architect, where all can contribute disciplined analysis and imaginative proposals. We believe that this combination makes for the most rigorous and interesting buildings. 

KPF’s portfolio illustrates our exploration into the most effective way to add a building on top of an existing structure. In New York, One Madison and Hudson Commons demonstrate a similar approach to Seventy Gracechurch, with a new tower added above a retained structure. In London, South Bank Tower, Panorama St Paul’s, and Shoreditch Works all demonstrate the “keep and add” approach. We continue to learn as we investigate each existing structure and understand how we can then create a new composite building incorporating it. 

As Arup Sustainability regularly remind us, structure and envelope account for about half of the embodied energy of a project – so the next step is to reuse what we can of the envelope. At Panorama St Paul’s we have reused over 90% of the stone, and we are exploring the same at Seventy. We are working with Arup on further projects to expand the scope of reused materials. 

There are many routes to a more responsible use of resources while allowing our cities to evolve. We believe that a key approach will be to “keep and add,” which combines exemplary embodied carbon guardianship with robust, vigorous, and sustainable growth.