London, like many global cities, has adopted density as a strategy to address a range of global and urban challenges, many of which are interconnected. Overall, higher urban densities are considered intrinsically more efficient in terms of land use, resources, and embedded infrastructure. For London, the higher-density strategy is visible as tall (and very tall buildings) are inserted into its historic, low-rise urban fabric. However, the wider benefits and costs of this policy have not been fully addressed. For example, apart from those associated the generic efficiency credentials of individual buildings the impact on the ambient environment experienced by those outdoors and even on other proximate buildings are not generally addressed.  The Covid 19 pandemic has shown that the high-density residential buildings in neighbourhoods with limited accessible outdoor space are especially vulnerable to public health events, such as the transmission of disease and heatwaves. 

Nonetheless, tall buildings have a dynamic effect on the surrounding landscape, for example, loss of sun and daylight to adjacent buildings and the ground, alongside changes to street scale ventilation, and in turn air quality. The magnitude of these effects depends on both the ambient atmospheric conditions and the dimensions and layout of buildings in a neighbourhood, but the impacts depend on the function of the urban space. For example, while mutual shading may be of benefit to glass-walled commercial buildings with high internal energy loads and a daytime occupation, this will create an unpleasant outdoor environment. Similarly, for a residential neighbourhood, shading will reduce the daylight available to residences and diminish the value of parks and community spaces.  Tall buildings in a low-rise urban setting can yield benefits and our work with the City of London has shown that tall buildings can deflect cleaner cooler air down to street level providing a valuable ventilation function.

As it stands, there is no reference within existing policy to account for the external impacts or criteria to indicate when mitigation efforts are required. Consequently, these issues are often addressed post-construction through ad hoc measures, such as tree planting to overcome predictable outcomes. This lacuna means that the opportunity to maximise benefits at a neighbourhood scale, rather than an individual building scale, are missed.  In part, these oversights not only result from the complexities of these relationships, but from our underestimation of the importance of these effects. To increase our understanding of these built-form outcomes we have developed a series of open research projects that investigates the impacts and challenges of increasing urban density, including those associated with tall buildings. This research includes both field measurements and simulated studies into the dynamic and interdependent relationships between buildings and outdoor spaces. 

This is being explored through a series of debates titled Cities, Climate and Critical Urban Infrastructure, which brings together some of the UK’s leading building and urban experts to exchange ideas and identify our knowledge gaps on how to address this issue and others in the context of our changing urban environments. The series has set the goal of integrating knowledge that is often discipline-based and arcane to develop policy guidelines that are appropriate to creating efficient, comfortable, healthy, and liveable urban environments for now and for the future.    

More details of the next instalment in this debate series can be found here.

Dr Julie Futcher RIBA 
Dr Gerald Mills