8 December 2021

Reaching net zero means changing cities – and more demolitions


It might sound like a paradox, but cities are good for the environment. By constantly changing, and using density to share land and infrastructure between more people, urban areas reduce the pressure humans put on the natural world. And it’s not just the countryside that feels the environmental benefits of urbanisation – the climate does too. On average people who live in UK cities emit four tonnes of carbon every year, while those in non-urban areas emit six tonnes.

Nevertheless, urban areas do still account for 45% of the UK’s carbon emissions, so if Britain is to get to net-zero by 2050, decarbonising our cities will be a big part of it. More climate-friendly urban living will be a key part of this mix – indeed, we can get a quarter of the way to the 2050 target just with densification and related transport changes in cities.

In a practical sense, many of the things that need to change are buildings. Carbon emissions from buildings, including homes and commercial property, have long been understood as a problem, and a particularly British one at that, as we have the oldest – and therefore among the least energy efficient – housing stock in Europe. Accordingly, various initiatives including heat pumps, district heating, and insulation to reduce emissions from buildings will be needed to help decarbonise Britain’s cities.

But demolition has attracted ever more criticism as a source of carbon emissions. A recent successful planning application by Marks and Spencer to knock down and redevelop their Oxford Street store sparked a debate on Twitter, with some advocating that ‘retrofitting’ the existing building would be more climate-friendly.

The idea is that the production of construction materials such as concrete and steel releases carbon dioxide into the atmosphere. ‘Whole-lifecycle’ analysis of buildings often shows that this ’embodied carbon’ from construction can be significant, and globally it accounts for 11% of greenhouse gas emissions.  If the aim is to improve the quality of the space, then retrofitting the building with insulation, better seals, and new technology may be a cost-effective option, both for the climate and in terms terms of capital expenditure and energy costs.

However, this is not the full story. Retrofit may well be the right choice for some sites, but that’s a decision best made according to individual circumstances, rather than a blanket policy. We certainly should not be making it difficult or controversial to build brand new buildings in urban areas and we are still going to need to demolish buildings as part of a Net Zero strategy, for three reasons.

First, strictly speaking, it’s new buildings that are responsible for releasing new embodied carbon. The embodied carbon released in the construction of existing structures is already flying around in the atmosphere. The act of demolition does emit a small amount of carbon from the energy expended, but it does not release embodied carbon. 

This matters as a number of UK policy problems – most obviously, the housing crisis – can only be solved with more buildings. If a certain amount of gross floorspace is to be added to a city regardless, whether it is built on a greenfield site or a brownfield site that needs to be demolished makes no difference in terms of the embodied carbon released from new construction. In other words, while embodied carbon is released in the construction and replacement of buildings, the embodied carbon in old buildings is effectively a sunk cost.

Yet an exclusive focus on retrofit risks uneconomic decisions that do not reduce the emissions of embodied carbon that are required to solve other policy problems. For instance, London Councils recently proposed spending £98 billion on retrofitting all 3.8 million existing dwellings in London. When the capital has a severe housing shortage that can only be addressed with more homes, it is difficult to justify spending that much money on housing without building a single extra house – especially when new homes can be built to much more efficient standards than retrofitted buildings, and new technology can and will reduce the embodied carbon emitted from construction.

That brings us to the second reason: there are limits to what retrofit can achieve as there is not much easy fruit left to pick. Two thirds of dwellings with a loft already have loft insulation, and while insulation might be rather technically straightforward for most of the 20.4 million homes in Britain with cavity walls, it is much harder to insulate the other 8.5 million homes with solid walls that are also much less energy efficient. 

In fact, reducing the operational carbon emitted from the running of buildings will require some of the existing stock to be replaced, even if that results in a one-off increase to embodied carbon. To take a simple and conservative example, the Government has an ambition to ensure as many homes as possible reach the Energy Performance Certificate (EPC) Band C as part of the strategy to reach Net-Zero

Doing some simple calculations with the buildings in the EPC domestic register shows that roughly three in five homes in England and Wales had ratings below C. But 16% of homes – a quarter of those below C – were assessed to have a potential rating below C, which means even if retrofitted they still would not meet the Government’s ambition. Some of these 3.8 million homes will need to be demolished and replaced by much more efficient stock if the operational carbon emitted by their residents to remain comfortable every day is to be reduced.

Third, even in areas where retrofit might look a sensible option, it may still make sense to demolish and rebuild. Why? Because urban buildings are not islands – they are located within cities, and the decisions we make about them have spatial effects on individual people and the national economy. Demolishing structures at the end of their lifespan in cities to build denser and taller makes it more viable for more people to use climate-friendly transport, as it makes the national economy more urban.

The climate benefits from increased density within cities are particularly important for the UK, even though it is already highly urbanised. Centre for Cities research shows that while 66% of people in big European cities can travel into their city centre by public transport within 30 minutes, only 40% of people in large British cities can. The reason is British cities are much less dense than their European counterparts, and our preponderance of low-rise housing forces people to live far from public transport and rely on cars. Infill development between existing houses will help address this and achieve higher densities, but densification can only be achieved at scale with some demolition.

Even electric cars with low operational emissions do not eliminate this trade-off, because cars themselves release embodied carbon when they are manufactured. If an old building near a railway station is demolished and rebuilt with more homes at a higher density, every household that chooses to buy one fewer car, even if electric, as a result saves at least 20 tonnes of CO2. When some pure retrofit schemes are saving just 1.5 tonnes of carbon per dwelling replacing old homes with brand new, energy-efficient ones will be the climate-friendly option.

Unfortunately, whole lifecycle analysis of buildings in urban areas rarely considers the whole lifestyle effects of urban density on transport emissions. If the UK tries to minimise the total number of demolitions rather than the amount of carbon we release, we risk the unintended consequence of actually increasing emissions and making Net Zero harder to achieve.

Cities have lower carbon emissions because they are places of change. Over the centuries, their residents have been able to build, demolish, and rebuild to achieve more efficient use of their resources, especially people and land. The wealth that urban economies create due to this efficiency and growth will be crucial for financing the path to Net Zero and the mitigation of climate change.

But to fully decarbonise, cities will need to demolish some buildings. Especially when England is currently demolishing fewer than 10,000 homes a year, cities are currently not changing enough to hit our climate goals. Urban areas will need more builders, architects, developers, and engineers to do so, and they in turn will need new innovations and technical skills to decarbonise construction. But if cities are to change to the degree tackling climate change requires, then some of their buildings will need to change too.

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Anthony Breach is a Senior Analyst at the Centre for Cities.

Columns are the author's own opinion and do not necessarily reflect the views of CapX.