Andrew Morrison, Director and Project Lead, at Hopkins Architects discusses the challenges and opportunities of reducing carbon in heritage projects.

The carbon reducing challenges and opportunities of heritage projectsAndrew Morrison, Director and Project Lead, at Hopkins Architects discusses the challenges and opportunities of reducing carbon in heritage projects. 

South Molton spans three and a half hectares between the Bond Street Elizabeth Line station to the north and Claridge’s to the south.

It is currently the West End’s largest mixed-use development project, delivering 370,000 sq. ft of new office space, private and onsite affordable housing, a 5* hotel opportunity and new cafes, shops, restaurants, community and leisure space, all stitched together with an improved public realm.

At the heart of the project is a 267,000 sq. ft. joint venture between Grosvenor and Mitsui Fudosan UK, that will deliver, two best-in-class office buildings, providing corporate HQ opportunities in one of the West End’s most sought-after locations.

The entire project is situated within Mayfair’s Conservation Area, and its development has necessitated balancing the requirements for a contemporary office building with respect for its heritage setting and adjacent listed buildings.

Working through a part retrofit, part new-build scheme with existing and listed buildings gives rise to many challenges. Below we examine two of these.

1. Steel Frame and Embodied Carbon

In contrast to developing a new office building with a repetitive floor plate and good floor-to-floor heights, working in a heritage context often requires a sensitive and nuanced approach to massing, often resulting in a complex structural frame to accommodate setbacks. As the majority of embodied carbon within a building is associated with the structure, transfers to deal with setbacks make the tasks of carbon reduction much more acute.

The sensitivity of building within a conservation zone placed a natural planning constraint on floor-to ceiling heights for South Molton of 3600mm. Whilst technically achievable, this required heavy fabricated steel sections. Following the successful planning approval, Grosvenor and the design team worked in partnership with Westminster City Council to undertake a detailed impact assessment on the visual effects of increasing the floor-to-floor heights of the upper setbacks to 3700mm, thereby giving an additional 100mm to the structural steel zone and enabling lighter rolled steel sections to be used. The embodied carbon saving of 4000tCO2e was presented to WCC alongside a series of townscape assessments, to weigh up the impact of the overall building height increase of 500mm. Acknowledging the carbon benefit, the compromise to the historic setting of the listed buildings was deemed acceptable and approved under a Minor Material Amendment application.

In general, facades represent approximately 15% of the embodied carbon of a building, so we developed a retention strategy that led to almost half of the existing facades being retained and minimising the amount of new facades required. Two buildings, Brookfield House and 56 Davies Street, were particularly challenging. The windows would have clashed with the newly proposed floor plates, set to accommodate modern commercial requirements. Both buildings have certificates of immunity from Listing, and whilst they could have been removed, there was a reluctance to demolish these robust Portland stone facades.

We proposed to dismantle and rebuild these facades, stretching the height of the windows proportionally to align with new floor plates. This also allowed us to construct new modern insulated walls, within the same overall façade zone. A re-use strategy was developed to split the existing stone in half, maximising material yield and providing additional Portland stone to be used in the extended facades, and whilst challenging, presented circularity, heritage, cost and embodied carbon gains.

Throughout the project, having collaborative conversations with the planners, the structural engineers and the clients has enabled real carbon reductions to be made that have helped balance the demands of the project within the site constraints of Mayfair, while maintaining and enhancing the character of the area.