Method
A series of whole life carbon comparisons were studied for a hypothetical office building to explore whether, in carbon terms, it is better to retrofit an existing building or to demolish and build new. The hypothetical existing building assessed was a large, 6-storey square plan block (60m x 60m) with an internal courtyard and a basement level. The basement and foundations were retained and reused, and strengthened where necessary, across all scenarios (including new build). This is typical of the existing building stock we are dealing with on a variety of London projects. The existing building is specified to the standards of a London office building typical of the 1960s, utilising a reinforced concrete frame, precast cladding and outdated mechanical, electrical and plumbing systems (MEP). The extended options had a 100% net area uplift of the base case.
Seven scenarios were assessed in total. The retrofit examples followed our sliding scale of intervention, or ‘three Rs’: refresh (light refurbishment), repurpose (deep refurbishment, with courtyard infill) and reimagine (vertical extension). A number of new build scenarios were included for comparison: a baseline new build, an intermediate new build, an extended baseline new build (not illustrated below), and an extended intermediate new build.
The new build scenarios are based on the intermediate and baseline office designs within UK Green Building Council’s ‘Building the case for net zero’ report. The report also has a ‘stretch’ target; however this has been omitted from this study, as we wanted to evaluate the 2025–2030 range to reflect the current market position. The deep refurbishment achieves an EPC A rating, while the light refurbishment only achieves a C, reflecting the MEES (Minimum Energy Efficiency Standards) interim milestone for 2027.
Each of these scenarios was conceptually designed and a whole life carbon assessment covering modules A–C was conducted. A mixture of Environmental Product Declarations and benchmark data were used to conduct the embodied carbon assessments, with DesignBuilder software used to conduct the operational energy estimates over a 60-year lifespan for all scenarios.
Findings
As expected, the scenario that resulted in the lowest whole life carbon was the light refurbishment scenario, which involved minimal alterations to the existing building and instead focused on improving the thermal performance of the building fabric, replacing MEP and retaining the internal courtyard. Exploring an option of retaining the internal courtyard in the deep retrofit scenario showed a benefit on the operational carbon, with a saving of 25kgCO2e/m2 over the 60-year lifespan compared to an infill scenario.
While these scenarios have the lowest carbon numbers, it’s important to note that these options may not bring the building to a level which many landlords are looking for when redeveloping their buildings, or what tenants and their agents are looking for in a workplace. The retrofit options may meet BCO refurbishment criteria but not the new build equivalent. We’ve found that retrofits with no additional area often lose floor space to new amenities, such as end-of-trip facilities and enlarged lobbies. Without adding additional lettable floor space, it’s difficult to get these schemes to stack up financially.
Operational carbon made up the largest proportion of whole life carbon in all scenarios, varying from 63% to 79%, although this contribution was shown to decrease under optimistic electricity grid decarbonisation scenarios. Fluctuations in operational carbon are minimal between the different design scenarios. This is partially due to the deep floorplate of the hypothetical building – typical of these kinds of buildings in London – which restricts opportunities for daylight and natural ventilation.
The study highlighted that most of the embodied carbon emissions result from the construction stage, except in the refurbishment case, where in-use carbon has a significant contribution. However, this is still considerably less over a building’s life than an equivalent-size new build.
These use stage emissions are largely from the replacement of the MEP. This finding, while not unexpected, highlights the importance of conducting a whole life carbon assessment to identify carbon hotspots. To lower the impact of replacing MEP, we can apply circular economy strategies to facilitate remanufacturing, re-use, and repair rather than replacement and disposal.
For a building of this initial massing size, the study concluded that a retrofit approach results in significantly lower whole life carbon numbers than demolition and new build. The demolition and new build schemes would need to achieve a whole life carbon of less than 750kgCO2e/m2 to equate to an energy-efficient retrofit. That said, the result is more nuanced for the vertical extension option, where only an 8% improvement in whole life carbon would make a new build comparable with the retained and extended option. This is due to the structural improvements and quantity of new elements in the extended options. The inclusion of circular strategies, such as recycled steel, with the new build could also result in the new build options being lower than the extended retrofit in practice. It’s also worth noting that our study used a large area increase, but a more modest area uplift would have favoured the retrofit and extend scenario.
Part 3 of this retrofit blog series will dive into the findings in more detail and our recommendations for dealing with buildings of this scale.