Stop building carbon-hungry basements

Basements have a lot of downsides – they are almost always expensive, typically increase programme, and often significantly increase a project’s risk profile.

We invariably subject them to extensive value engineering exercises. And, on top of all that, they are also a key driver of embodied carbon.

Significant gains are being made in reducing the embodied carbon of above-ground space, for example building the frame in mass timber or steel produced via electric arc furnace, using recycled aluminium for the cladding and so on. But this means that a typical concrete basement is now up to 50% more carbon hungry than creating the equivalent space above ground.

As the UK’s electricity grid moves to becoming carbon neutral, reducing embodied carbon is becoming more and more important, and we expect it will be the defining the challenge of our industry over the next decade. However, our work on forward-looking projects is showing that meeting the 2030 goals set by LETI and others is extremely tough with today’s technologies and regulatory background.

To meet these challenges, basements will undoubtedly need to improve. This can be partially achieved with evolutionary advancements such as “earth-friendly concrete” and the like, but this would miss the more fundamental opportunity to avoid building basements in the first place.

So why build basements? It is simple viability economics. In central London values will typically be up to five times construction costs, so even expensive basement spaces are worth it if they free up more internal area above ground.

Once the massing is set by the planning discussions with the local authority, there will be an economic imperative to build some basement space. This is even more the case as demands for ancillary space for bicycle store and additional facilities increase.

As all these pressures mount, could the planning authorities take the opportunity to allow for viability at a lower embodied carbon count, rewarding lower carbon solutions, for example, through greater massing if there is no basement.

This would be a challenge given the current planning regime, but illustrates the joined-up conversations the industry needs to have to continue to evolve and meet the embodied carbon challenge.

If we truly want to meet LETI goals then we need to create a regulatory framework which allows for more innovation in building a better business case for net zero buildings.

Jon Spencer-Hall is a partner at Core Five