Achieving Net Zero Case Study: transforming the BT Building into a net zero building

On 23 June 2020, the City of London planning committee granted permission for the transformation and redevelopment of the BT Building in the City of London. Through extensive energy efficiency measures, and by repurposing and reusing a significant part of the existing building, this project aims to be one of London’s first net zero carbon buildings.

As 40% of the UK’s carbon emissions are attributable to the built environment, the sector needs to be pursuing more aggressive rates of decarbonisation. Transformation and reuse of existing structures and materials, both of which are fundamental to the proposals for the BT Building at 81 Newgate Street, offer substantial energy savings compared with the high energy costs of demolition and building a brand new building.

Led by the Development Manager, Pella, the team includes architectural practice Kohn Pedersen Fox Associates (KPF), design consultancy Chapmanbdsp and planning consultants Montagu Evans. The building is currently in the advanced stages of design; work is expected to start on site in 2021, and the office-led scheme is due to be completed by 2025.

Transforming BT Building: what new building will look like
Proposed design for the redevelopment of the BT Building. Image courtesy of Montagu Evans.

Why repurpose rather than demolish?

The original building was built to a high standard, by the Government Property Agency (GPA), less than 40 years ago. This existing quality makes it a good candidate for retrofit, as does its location, barely 200 metres from St Paul’s Cathedral. This location means its height is heavily constrained by several GLA-prescribed London View Management Framework views towards the cathedral’s dome. The City also has its own policy of ‘St Paul’s Heights’, which places a further limit on building heights.

As a result, completely redeveloping the site, with no prospect of replacing the current building with a ‘tall building’, did not make sense to team collaborating on the new design.

“A previous scheme for the site included demolition and new build, but we approached the project with the assumption of transformation. The existing building is extremely robust and well-made, which was a great argument in favour of reuse and against demolition – saving a huge amount of carbon, construction time and waste.” – John Bushell, KPF (from Architects’ Journal, 2020)

Transforming the BT Building: reuse and repurposing

The designs propose reusing the existing Portland stone façade and granite cladding, wherever possible, by carefully removing it and then re-purposing it for use on the new facades. Reusing approximately 70% of the original structure, including existing basements and parts of the original superstructure, reduces the development’s embodied carbon (the total greenhouse gas emissions generated to produce a building) and also reduces construction time, traffic and waste.

Designing out waste and pollution, and keeping products and materials in use, are two of the key principles of the circular economy, as defined by the Ellen MacArthur Foundation. The team behind the new design will monitor the demolition, reuse of materials and new construction to make sure that the site is delivering on these circular economy principles.

Because concrete hardens over time, the building’s current foundations are capable of accommodating new loads. This means that much of the existing structure can be retained but also extended, to offer more floorspace. Infilling the existing central atrium allows for a complete reconfiguration of the floor plates. The proposals will create a total of 75,730 sqm (gross internal floor area) across 17 floors, an increase of 58% on the existing building.

Transforming the BT Building: energy efficiency measures

As well as repurposing some of the existing structure and materials, the building will achieve net zero operational carbon (the carbon created by the use of energy to heat and power a building) through a number of energy efficiency measures. These include:

  • photovoltaic panels (solar panels) at roof level
  • air source heat pumps and chillers
  • passive design strategies such as high-performance fabric, openable windows for mixed mode ventilation and others.

Consultants Chapmanbdsp are leading the work on the energy efficiency initiatives in the building. The ideas they’re exploring include:

  • designing for performance: detailed operational energy modelling to account for all energy uses, and different schedules and management factors
  • undertaking an energy strategy and sustainability assessment
  • reducing overheating created by the façade, thus reducing the cooling demand
  • improving natural ventilation and allowing for more natural light on the office floors.

By doing detailed calculations on the building’s projected energy in use, Chapmanbdsp are aiming to futureproof the development and enable it to meet the ‘Paris Proof’ target  in order to be zero carbon by 2050.

The operational performance results from the detailed calculations will inform the whole life carbon of the development, to offer a net zero carbon office in the truest sense. In attempts to further reduce the reported carbon savings, Chapmanbdsp are looking to incorporate hybrid solutions to improve the efficiency of the proposed system and controls to meet the designed performance in practice.

The scheme has been designed to achieve a carbon reduction target of a 51.2% improvement on current building regulations. This exceeds the current London Plan target of a 35% reduction, which means the scheme won’t have to make a ‘carbon offsetting’ contribution.

There will also be 1,934 sqm of roof terraces and a green roof. These features will help to ‘green’ the City, contribute to local biodiversity and air quality improvements, as well as make the building a more enjoyable space to spend time in.

Transforming BT Building: what new building will look like
Proposed design for the redevelopment of the BT Building will help to make the City of London greener and boost biodiversity. Image courtesy of Montagu Evans.

Thinking long term

90% of the scheme (67,802 sqm) will be offices, but the proposal also includes a gym, a swimming pool and up to 4,398 sqm of retail. The office part of the development has been designed to seek a minimum “Excellent” BREEAM rating with an aspiration to achieve an “Outstanding” rating. (BREEAM is a sustainability assessment method for evaluating a building’s environmental, social and economic sustainability performance, using standards developed by BRE.)

The retail areas have been designed to a similar standard but can only achieve a minimum BREEAM “Very Good” rating. This is because the BREEAM ratings require developments to assume that the energy performance will be poor until the tenants move in and ‘fit out’ the retail units to suit their needs.

This highlights the importance of commercial landlords working with their future tenants to encourage behaviour change, so that the building’s high sustainable credentials continue to be maintained – a point that was made in both our Climate change needs behaviour change and The cost of action and the cost of inaction webinars. At 81 Newgate St, measures will be in place to influence future tenant fit-outs, and the operational energy and carbon emissions of both the retail and office elements – such as sharing energy performance information with tenants to influence how they use the space.

Designing the space to be flexible and adaptable is also critical. The planning application has included different ground-floor layouts for the retail space, to do as much as possible to future-proof the scheme and anticipate retailers’ needs in four to five years’ time. Given the impact that Covid-19 has had on high street retail, it’s important to think about, and be prepared for, how the retail sector might change in the coming years.

Our Achieving Net Zero case studies have showcased inspiring examples of best practice, from low-cost interventions to large-scale infrastructure. We want to raise the profile of schemes and organisations doing a great job of working towards the Net Zero ambition, highlighting what we can learn from them. 

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