What a wind microclimate assessment really costs

Wind microclimate assessment cost is set by the number of configurations tested, not by the number of pages written. A single proposed scheme in a wind tunnel is one price. The same scheme plus an existing baseline, plus a mitigation variant, plus a cumulative case with two consented neighbours, is four times the tunnel time and four sets of model parts.

Which is why the scoping conversation is where the budget is decided, usually months before anyone is appointed.

When an authority asks for wind work

Wind microclimate evidence is asked for on tall and large buildings, on schemes creating new public space, and on sites where an existing wind problem is known. In London the tall buildings policy in the London Plan 2021 requires wind effects to be addressed as part of the assessment of a tall building, and boroughs apply it through their own tall building policies and design guidance.

Outside London the trigger is less formal. It comes from the design chapter of national guidance, from local policy on amenity in new public spaces, and from the case officer's judgement about whether the massing is likely to accelerate flow at ground level.

The assessment itself is judged against pedestrian comfort and safety criteria. Those thresholds are set out in the Lawson comfort criteria and their application to new spaces is covered in wind microclimate criteria for a new public space, so this article deals only with what each stage of the work costs and why.

What a desk based screening buys

The first stage is a qualitative appraisal by an engineer who looks at the massing, the height relative to its surroundings, the local wind climate and the proposed ground-level uses, then states whether a quantitative assessment is needed.

Screening is the smallest fee in the sequence by a wide margin, and on a mid-rise building in a well-enclosed urban block it can be the only fee. It is also the stage most often skipped, because applicants assume a tall building needs a tunnel test and commission one directly.

Commission the screening even where you expect it to escalate. A screening note that defines the likely problem areas lets you specify the quantitative work tightly, and it gives the authority something to scope against at pre-application.

Computational modelling and its cost drivers

Computational fluid dynamics sits between screening and physical testing. What drives its cost is not the software licence but four modelling decisions.

The extent of the surrounding context included in the domain, because every additional street block adds mesh. The mesh resolution near the ground plane, which has to be fine enough to resolve pedestrian-level flow. The number of wind directions analysed, since directional statistics are normally built from a set of sectors covering the full compass and each sector is a separate run. And the turbulence approach, where a steady-state calculation is far quicker than a time-resolved simulation but is less reliable in the separated flow around building corners, which is exactly where comfort problems occur.

Wind data is a smaller but real cost. Directional and seasonal statistics come from long-term records at a nearby meteorological station, adjusted for terrain, and that data has to be bought or licensed.

The honest limitation is worth stating in the report. Steady-state modelling tends to under-predict gust effects, and several authorities with tall building experience will accept it for screening and masterplan work but ask for physical testing before determining a consent.

Physical tunnel testing and the model

Tunnel testing is the expensive stage and the model is a large part of why. A rigid model of the proposal, typically at a scale around 1:300, has to be made to measure, along with the surrounding context out to a radius of several hundred metres mounted on a turntable.

The cost sits in four places:

• model fabrication for the proposal and the proximity context, which is skilled physical work
• instrumentation, since each measurement location at pedestrian level needs a probe and each probe needs wiring and calibration
• tunnel occupancy time, charged by the session, multiplied by the number of wind directions and configurations
• analysis and the combination of measured speeds with the directional wind statistics to produce comfort categories

Measurement point count is the variable applicants can influence. Points are placed at entrances, on terraces, in the open spaces between buildings, at crossings, at bus stops and anywhere seating is proposed, and each one is a line in the results table. Agreeing the list with the authority before the model is built avoids paying for a second visit to capture a terrace somebody forgot.

Erosion techniques, which use a surface coating to visualise where flow is strong, cost less per configuration than probe measurement and are useful for comparing massing options early. They do not produce the numerical comfort categories a determination needs.

Mitigation iterations and the second test

The pattern that blows budgets is predictable. The first test identifies two or three locations failing the comfort criteria, the design team adds canopies, screens and planting, and the mitigation has to be tested because the report cannot claim an improvement it has not measured.

That second visit means new model parts, another block of tunnel time and a revised analysis. On a scheme with several failing locations it can approach the cost of the first test.

The way to avoid it is to put plausible mitigation into the first model as a tested variant rather than as a proposal. A canopy at the main entrance, a screen at the exposed corner and a row of mature trees in the right place cost very little to include as a second configuration in a single tunnel visit, and a great deal to add as a return visit six weeks later.

Cumulative scenarios with consented schemes

Authorities with an active tall building pipeline routinely ask for a cumulative case: the proposal tested with neighbouring consented but unbuilt schemes in place.

Each cumulative scenario is another configuration, and the proximity model has to be built twice, once as existing and once as consented. On a site with two or three consented neighbours the question of which combinations are tested is worth negotiating hard, because the difference between two scenarios and five is most of the fee.

Ask for the authority's list of schemes to include in writing at pre-application. Ask also whether a consented scheme that has not started on site needs to be in the baseline, since that is a judgement rather than a rule.

Reporting for a committee audience

The reporting fee is modest, and under-specifying it is a false economy. A tunnel report full of tabulated mean speeds and exceedance percentages will be summarised in two sentences in the officer report, and those two sentences decide how the issue is presented to members.

What earns its cost is a set of plan diagrams showing comfort categories by season across the open spaces, a clear statement of which locations fail and what mitigation resolves them, and a short non-technical summary. Specify those deliverables in the appointment rather than hoping for them.

Scoping the brief so one test is enough

Five decisions, made before appointment, control most of the cost.

1. Freeze the massing. Testing a design that is still moving guarantees a retest.
2. Agree the measurement point list, the configurations and the cumulative schemes with the authority in writing at pre-application.
3. Include mitigation as a tested configuration in the first tunnel visit.
4. Use screening and computational work to narrow the question before the tunnel, not instead of it.
5. Name the report deliverables, including the plan diagrams, in the brief.

Councils expect a suitably qualified author for wind work, and professional review of any draft is strongly advised, since the comfort categories are the output the whole landscape design rests on. PlanScope drafts the narrative documents that sit around the technical evidence across its 66 report types. For specialist technical reports, qualified professional input may be required.

See PlanScope pricing: one free report, £50 per report, or Professional at £150 per month for teams producing supporting documents at volume. Enter your postcode on PlanScope to see which reports your site 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. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.