Opinion
•The social value of carbon in residential construction
Opinion piece: Should we impose stricter carbon targets on luxury housing?
The pilot UK Net Zero Carbon Building Standard published in September 2024 gives welcome clarity on embodied carbon limits. However, the standard only offers a single set of targets for homes. Luxury housing, with its generous internal floor areas, large spans and high-end finishes and fittings, uses a large share of carbon per capita. Maintaining our current housebuilding rate has been predicted to exceed our carbon budget by 2036ii. Extensive refurbishment and adaptation of existing large detached residential properties must be prioritised over demolition and new build across the industry. And if building new is the only viable option, then luxury homes could be subject to stricter carbon targets, creating an opportunity for this sector to innovate and decarbonise.
As built environment professionals, we facilitate the consumption of materials to create a built environment that improves and enriches people’s lives and allows society to flourish. However, we know that consumption of materials emits carbon and is damaging to our planet. Therefore, the goal must be to maximise socioeconomic function for as little carbon as possible. This begs the question: what should we be building with our limited budget? This article explores this question through the lens of residential construction.
Current metrics for assessing buildings measure carbon intensity through the unit of embodied-carbon-equivalent per m2 of floor area. It stands to reason that larger buildings will generally emit more carbon, when stated as an absolute metric due to their larger floor area. This carbon metric is decoupled from the social value that the constructed floor area provides to society. If we consider residential buildings, their function is to provide safe and warm shelter for their occupants, but many properties go beyond this with extra-large rooms and private amenities such as swimming pools and cinema rooms. These expensive luxury additions are carbon intensive and provide limited additional function to society. This exposes the underlying wealth inequality that is at the heart of a climate crisis where the richest income decile in the US and the EU emits approximately 16 times more than the poorestiii.
Plotting the upfront carbon (A1-A5) of a building using traditional carbon intensity metrics (kgCO2e/m2) shows a very different result than if the upfront carbon is normalised for equivalent social function. Both metrics are plotted in Figure 1 and Figure 2 respectively based on data from a handful of projects from HTS’s bespoke carbon counter. The projects were selected based on subjective definitions of “Luxury” and “Economy” that describe either high-end projects with private amenities or projects that contain units of affordable housing or shared ownership.
The metric for social function in this study is the number of bedrooms. Bedroom data is more readily available than number of inhabitants per home and, while the number of bedrooms is not a direct substitute for social amenity, we can assume that the function of a bedroom is for a person to sleep in. Therefore, a home with a greater number of bedrooms can provide greater social utility by housing more people than an equivalently sized home with just one or two bedrooms and more excess space, i.e. the higher the GIA per bedroom the less socially efficient a project is.
Figure 1 – The structural embodied carbon (A1-A5) for Economy and Luxury homes measured using the industry-standard carbon-per-floor-area metric compared to the total built area of the project.
Figure 2 – The total built area vs the total structural embodied carbon (A1-A5) a project has, normalised to total number of bedrooms, for equivalent social function.
Figure 1 shows that the Luxury projects tend to be smaller buildings (such as individual houses) compared to the Economy projects (which range from houses to include entire blocks of flats). The figure also suggests that larger buildings are more carbon efficient, but this needs further investigation. One of the Luxury projects and three of the Economy projects may be described as “low carbon” as they achieve less embodied carbon for their structure than the 2024 IStructE SCORS target of 255 kgCO2e/m2, (the SCORS target is used here as it is independent of building typology).
When the data is normalised to provision of bedrooms, in an attempt to quantify the social function of the projects, a different picture appears, as can be seen in in Figure 2. The general trend is unsurprising and shows that projects with more normalised floor area have more normalised carbon. The median of the Economy data points is 16 tCO2e/bedroom and of the Luxury data points is 84 tCO2e/bedroom showing that Luxury homes have 5 times the carbon footprint per bedroom than Economy homes. The data shows that Economy housing uses little more space than the UK’s minimum requirement (socially efficient) whereas luxury housing is much more spacious (socially inefficient).
Recommendations
Building Luxury housing appears to conflict with housing a growing population within net zero climate targets. The embodied carbon consumed to make these buildings could house up to five times as many people in Economy housing. New buildings should be low in carbon for their function; for housing this means either being socially efficient (such as the Economy housing in Figure 2 where the carbon used is providing a lot of function) or they need to be carbon efficient to make up for their social inefficiency (i.e. lowering the carbon of the Luxury data points in Figure 2 such that they sit in the bottom right quadrant of the graph).
The industry should:
- 1 Provide luxury accommodation within repurposed and refurbished existing building stock
- 2 Consider setting stricter carbon intensity targets for new build luxury housing to drive innovations in lower carbon home design and materials, in particular bio-based materials
References
i. Rodrigues, L., Delgado, J. M. P. Q., Mendes, A., Lima, A. G. B. & Guimarães, A. S. Sustainability Assessment of Buildings Indicators. Sustainability (Switzerland) vol. 15 Preprint at https://doi.org/10.3390/su15043403 (2023).
ii. Drewniok, M., Dunant, C., Allwood, J., Ibell, T. & Hawkins, W. Modelling the embodied carbon cost of UK domestic building construction: Today to 2050. Ecological Economics 205, (2023).
iii. IEA, Cozzi, L., Chen, O. & Kim, H. The world’s top 1% of emitters produce over 1000 times more CO2 than the bottom 1%. https://www.iea.org/commentaries/the-worlds- top-1-of-emitters-produce-over-1000-times-more-co2-than-the-bottom-1 (2023).
iv. Hackney Council. First 2021 Census data release from the Office of National Statistics (ONS) – Detailed Briefing for Hackney Staff and Partners (Public Version – July 2022). Hackney Council Briefing https://docs.google.com/document/d/1jxgNWzp9k9lPKefbn6lDszL8zTtbTBNsmEZGTSPDtzw/edit (2022).
v. World Population. https://www.worldometers.info/worldpopulation/#google_vignette.
vi. United Nations. Population. https://www.un.org/en/globalissues/population#:~:text=Our%20growing%20population&text=The%20world’s%20population%20is%20expected,billion%20in%20the%20mid%2D2080s.
vii. Shukla, P. R. et al. Climate Change 2022. Mitigation of Climate Change. Summary for Policymakers. www.ipcc.ch (2022).
viii. IEA. Global Status Report for Buildings and Construction. https://www.iea.org/reports/global-status-report-for-buildings-and-construction-2019 (2019).
