Need to know: What new guidance on wind will mean for City planners

The City of London Corporation has published the first set of rules governing when wind assessments are required for planning applications.

City of London: new rules on wind assessments published
City of London: new rules on wind assessments published

What is new about these guidelines?

The City of London Corporation’s new wind microclimate guidelines provide the first framework for assessing the impact of planning applications on wind conditions. They establish a set of rules and technical guidance for how and when wind assessments are required for planning applications – and specify that the impact of wind conditions on cyclists must be considered too.

Buildings between 25m and 50m tall will now need to be subjected to computer modelling or wind tunnel testing, while those beyond 50m require both, each carried out by a different independent consultant. Applications must acknowledge the cumulative effects of the proposed building together with existing approved buildings.

How do you assess wind impacts?

Wind tunnel testing is carried out by building a scale model of the development in its current and future surroundings, then moving air around the model. Small probes on the model building and surrounding public realm measure wind speed. Meteorological data are used to determine wind speed and direction in different seasons.

Computational fluid dynamic (CFD) modelling follows the same principles but uses a computer model to simulate the effects of wind on a digital model of the building and its surroundings.

Wind speeds are assessed against the intended uses of the site to determine whether the conditions are acceptable, using certain "comfort criteria". For instance, for sites where there is "frequent sitting" (cafés, restaurants etc.) a mean wind speed of 2.5m/s is deemed acceptable. Where there is "occasional sitting", such as in general public spaces, 4m/s is acceptable.

How will this affect tall building design in the City?

The guidelines encourage developers to "address wind microclimate matters... before designs are finalised". Early consideration of wind conditions and a willingness to work iteratively, with designs refined in response to findings regarding wind effects – is key to ensuring that developments are fit for purpose and more likely to be consented.

Through repeat testing and design, careful use of massing and placement of open spaces, problematic areas can be designed out and mitigation designed in, the guidelines say. 

They add that the shape and orientation of buildings, and the placement of active frontages, seating areas and entrances, should take account of wind speed and direction, as well as building’s effect on adjacent land uses and buildings.

They also say that trees and other landscaping features can all be used to provide localised shelter, but care should be taken to ensure that an apparent solution does not exacerbate conditions elsewhere.

What are the implications for existing buildings?

It is clearly more cost effective and better practice to design tall buildings with wind considered from the start, rather than trying to retrofit expensive – and sometimes unsightly – mitigation. The guidance is intended to make iterative design and assessment the norm. However, mitigation techniques such as implementing additional hard or soft landscaping can minimise adverse wind effects from existing buildings.

Planning applications associated with existing buildings, for instance for additional storeys or extensions, will require wind assessment to ensure that mitigation is considered where required.

What does this mean for national planning policy?

Where London leads, others tend to follow. It is now common to have planning policy focused on specific requirements for tall buildings in other areas. Wind is often one of the factors that design teams are asked to consider. It is possible that other city authorities will adopt similar guidance to the City of London’s or apply some of its principles to their pre-application process.

Wind tunnel testing is costly, so we may see an increased use of the more cost-effective CFD outside central London, where budgets may be tighter.

Neil Purvis is senior environmental planner at Barton Willmore

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