Wind microclimate assessment for a London tower
A usable wind assessment names its test locations, states the criteria and season for each, and includes consented but unbuilt neighbours in a cumulative scenario.
A wind microclimate assessment london boroughs will accept has three parts: a set of named test locations, results expressed against recognised pedestrian comfort criteria, and a cumulative scenario including consented but unbuilt neighbours. Leave out the third and the assessment will be returned, because a tall building's wind effects depend as much on what is next to it as on its own form.
The requirement comes from London Plan policies on public space and on tall buildings, which ask for the environmental effects of a tall building at ground level to be assessed and mitigated. In practice it means pedestrian level wind testing, and for a referable tower it usually means wind tunnel testing rather than modelling alone.
When an assessment is needed
Three triggers, in descending order of certainty.
A building that meets the borough's definition of a tall building, which varies by borough and is set in the local plan rather than nationally, will attract the requirement through the validation list. A scheme creating or substantially altering a public space, a through-route or an entrance plaza will attract it even at moderate height, because the policy concern is the usability of the space rather than the height of the building. And a scheme next to an existing or consented tower may attract it where the combination is what changes conditions.
Check the borough's validation list and its tall buildings policy, then ask at pre-application. Boroughs differ on whether a desk study and qualitative appraisal suffice at the lower end, and getting that answer in writing saves a wasted commission.
The criteria results are measured against
The established framework in the United Kingdom is the Lawson comfort criteria, which classify locations by the wind speed exceeded for a given proportion of the time and map those classes onto activities: sitting for longer periods, short-term sitting, standing and waiting, walking, and conditions unsuitable for pedestrians. A separate distress or safety criterion deals with strong gusts rather than comfort, and it is assessed against a stricter frequency of exceedance.
Two points matter when reading a report. The criteria are seasonal, so a summer assessment is used for amenity spaces and outdoor seating while the annual or winter case governs safety. And the acceptable class depends on the intended use of each location, so a through-route can sit in the walking class while an entrance or a seating area cannot.
An assessment that reports wind speeds without stating the criterion, the season and the intended use of each location has not answered the policy question.
Wind tunnel against computational modelling
Boundary layer wind tunnel testing uses a scale model of the proposal with the surrounding townscape built to the same scale within a radius around the site, rotated through wind directions, with the results combined with long-term meteorological data for the area. It remains the method boroughs and the Greater London Authority are most comfortable with for tall buildings, because it captures the unsteady flow that matters at pedestrian level.
Computational fluid dynamics has a clear role and a real limitation. It is excellent for design iteration, letting a team test twenty massing options or canopy arrangements in days, and it produces visualisations that explain the problem to a design team far better than a table of exceedance percentages. The limitation is turbulence modelling: steady-state approaches can understate gust effects, and more advanced transient methods are expensive enough that the cost advantage narrows.
The practical approach on a tall building is both. Use modelling during design development to get the massing right, then wind tunnel testing on the resolved scheme for the application. Confirm the borough's expectation first, because some will accept well-documented modelling and some will not.
Cumulative scenarios
Four scenarios are usually tested: existing conditions with the site as it is, the proposed development in the existing context, the proposed development with surrounding consented schemes built out, and sometimes a future baseline with the consented schemes but without your proposal.
The cumulative case is where surprises happen. A consented tower across the road can channel wind into your entrance, or shelter it. Either way, the result that matters for your conditions may not be the one from the existing-context test.
Agree the list of cumulative schemes with the borough in writing before testing. The list is a judgement about which consents are likely to be implemented, and a disagreement about it discovered after the wind tunnel session is an expensive disagreement. Where the site sits inside a designated growth area with multiple schemes in the pipeline, the list is longer and the agreement more important, as our note on Opportunity Areas and what a London allocation changes sets out.
Mitigation, in order of effectiveness
Form first. Setbacks and a podium break the downdraught that a flat tall facade drives to ground. Chamfered, rounded or stepped corners reduce the acceleration at the corners, which is where the worst conditions normally occur. Breaking the plan so the building does not present a single tall flat face to the prevailing wind does more than any added element.
Then the ground plane. Deep canopies at entrances, recessed entrance doors, draught lobbies or revolving doors, and porous screens positioned across the accelerated flow all work. Porosity matters, since a solid screen creates its own accelerated flow around its ends.
Planting comes last, and honestly. Trees help when mature and in leaf, which means they do nothing for the winter safety case in their first decade. A mitigation strategy resting on a row of newly planted trees will be challenged, and should be.
Because form dominates, the assessment has to inform the massing rather than validate it. The design-led capacity approach described in our note on optimising site capacity under Policy D3 is the right place for wind to enter the process.
Where to test
Choose locations by use, not by geometry. The list should cover every building entrance including the residential cores and the commercial entrances separately, the cycle store entrance, any outdoor seating or cafe spillout, the podium or roof amenity spaces, the children's play space, the main pedestrian routes through and around the site, the bus stops and pedestrian crossings nearest the building, and the corners at the base of the tower.
Amenity spaces at height deserve particular attention, because a roof terrace or podium that fails the sitting criterion is an amenity space on a drawing and nowhere else. Where the podium carries the play provision, the wind result and the Play Space Assessment have to be reconciled on the same drawing. Daylight and sunlight testing of the same spaces runs in parallel, using the approach in our note on applying BRE 209 daylight guidance in dense London.
Who should do the work and what the report must contain
Pedestrian level wind assessment is specialist work. A Wind Microclimate Assessment should be prepared by a consultancy with boundary layer wind tunnel facilities or demonstrable modelling capability, and councils expect a suitably qualified author with professional review strongly advised.
An AI-assisted draft can structure the document, set out the policy framework, list the test locations and frame the mitigation discussion. It cannot generate wind tunnel results or modelled exceedance data, and no draft should ever present modelled figures that did not come from an actual test. Commission the testing, then build the report around the measured results.
The finished assessment needs the method and software or facility named, the meteorological dataset and station identified, the surrounding model extent stated, the scenarios listed, the criteria defined with the season, a table of results by location and scenario, the mitigation proposed with the retested results, and a residual effects conclusion. Expect a condition requiring the mitigation to be implemented and retained.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your scheme is likely to need.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
When is a pedestrian level wind assessment required in London?
Where a building meets the borough's own definition of a tall building, where a scheme creates or substantially alters a public space, through-route or entrance plaza even at moderate height, and sometimes where a scheme sits beside an existing or consented tower. Check the borough validation list and confirm the expectation at pre-application in writing.
What are the Lawson comfort criteria?
A framework that classifies locations by the wind speed exceeded for a given proportion of the time, mapping those classes onto activities such as sitting for longer periods, short-term sitting, standing and waiting, and walking. A separate distress criterion addresses strong gusts against a stricter frequency of exceedance, and is used for safety rather than comfort.
Is computational modelling accepted instead of wind tunnel testing?
It depends on the borough. Modelling is well suited to design iteration and to explaining effects to a design team, but steady-state approaches can understate gust effects and transient methods are costly. For a referable tower, wind tunnel testing on the resolved scheme remains the expectation in most cases, so confirm the requirement before commissioning.
Why does the cumulative scenario matter so much?
Because a neighbouring consented tower can channel wind into your entrance or shelter it, so the conditions you will actually experience may differ from the existing-context result. Agree the list of cumulative schemes with the borough in writing before testing, since a disagreement found after a wind tunnel session is expensive to resolve.
Can planting be relied on as wind mitigation?
Only with care. Trees reduce wind speeds when mature and in leaf, so they contribute little in their first decade and nothing to a winter safety case. Mitigation should rest on built form first, then canopies, recessed entrances and porous screens, with planting treated as a supplement rather than the solution.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.