Odour assessment competence and the methods used

An odour assessment for planning has to answer one question: is the proposal likely to cause an unacceptable level of odour at nearby sensitive receptors, and if so what mitigation makes it acceptable. The odour assessment author needs to match the method to that risk, and the competence expected differs sharply between a structured qualitative appraisal and a dispersion modelling study.

Commercial occupiers meet this most often with kitchen extraction, food production, waste handling and livestock. The environmental health officer, not the planning officer, is usually the person whose view decides the application.

What the assessment has to establish

Four things, in order.

The source: what is emitting, at what rate, at what height and temperature, under what operating pattern, and whether emissions are continuous, intermittent or event-driven.

The pathway: how odour travels from source to receptor, which depends on discharge height and velocity, building downwash, local topography and prevailing wind.

The receptor: who is exposed, how sensitive they are, and how often. A dwelling window, a school playground and an industrial yard are treated differently.

The effect: assessed using the framework professional guidance sets out, conventionally summarised as frequency, intensity, duration, offensiveness and location. Those five factors are what an environmental health officer will check, and an assessment that reports only a modelled concentration without addressing offensiveness and receptor sensitivity is incomplete.

Qualitative and quantitative methods compared

The choice of method is itself a judgement that should be stated and justified.

A structured qualitative risk assessment works through source, pathway and receptor systematically, applies the five-factor framework, and draws conclusions using defined descriptors rather than numbers. It suits small and well-understood sources: a single kitchen extract, a modest food unit, a bin store. Done properly it is rigorous, and it is not a shortcut.

Dispersion modelling predicts odour concentrations at receptors using meteorological data, emission rates and terrain, and reports results against a benchmark expressed in odour units. It suits larger or more contentious sources, permitted installations, and cases where the qualitative method cannot resolve the question.

The second risk deserves attention. A model is only as good as its emission rate, and where that rate is taken from a literature range rather than from measurement, reporting results to two decimal places gives an impression of precision the input does not support.

Competence for modelling

Dispersion modelling is a specialist skill. The practitioner should be able to explain their choice of model, the meteorological dataset and the years used, how building downwash was handled, how the emission rate was derived and what sensitivity tests were run.

Membership of the relevant air quality or environmental institutes is a reasonable filter, alongside demonstrable project experience with the same source type. There is no statutory licence for this work, so ask for the evidence rather than the letters. The parallel questions asked of other specialists are set out in our note on writing a brief for a specialist planning report.

For a qualitative appraisal, the relevant competence is different: experience of the process being assessed, familiarity with the mitigation available, and the ability to carry out and record a structured odour survey in the field.

Sampling, olfactometry and field survey

Where an emission rate has to be measured rather than assumed, dynamic olfactometry is the established method. Samples are collected from the source in inert bags and presented to a trained panel under controlled conditions, producing a concentration in European odour units per cubic metre. The method is defined in a British and European standard, and the laboratory should be accredited for it.

Field assessment is the other evidence route. A trained assessor records odour at fixed points over a defined period, noting intensity on a standard scale with wind direction and weather. It is the only method that tells you what is happening now, which matters where the application concerns an existing operation or a complaint history.

Both are worth commissioning before the design is fixed. Measuring after a system is installed leaves no options except replacement.

Existing operations and complaint history

Where the application concerns a business already trading, the complaint record is evidence and the officer will have it. Pretending otherwise damages the whole report.

Ask the environmental health team for the history early, under the usual information request route, and address it directly. An assessment that says the extract was extended in March, the two complaints predate that work and no further complaints have been received since, with dates, is far stronger than one that omits the subject.

The same applies to a statutory nuisance history. Odour capable of being a statutory nuisance under the Environmental Protection Act 1990 is dealt with under a separate regime from planning, and an abatement notice sitting on the premises will shape how the council approaches the application whether or not it is a planning matter.

How environmental health reviews the work

Expect the officer to go to four places.

The operating assumptions, because an assessment based on eight hours of operation is worthless if the business runs sixteen. The receptor list, because omitting an upper-floor flat window or a school boundary is the most common gap. The mitigation, because it must be specified in enough detail to be conditioned. And the residual effect, because a conclusion of no significant effect after mitigation needs to say what happens if the mitigation underperforms.

Officers also look for the maintenance regime. Carbon filters and ultraviolet systems degrade, and an assessment that relies on them without a stated replacement interval and a maintenance record is relying on something the council cannot enforce.

Specifying mitigation that can be conditioned

Mitigation belongs in the assessment, written in terms a condition can reproduce.

1. Discharge point position, height above roof level and vertical efflux with no cowl restricting it.
2. Efflux velocity, stated as a minimum in metres per second at the design flow rate.
3. Abatement, named by type and performance, with the stage sequence described.
4. Maintenance intervals and record keeping, with a named responsible person.
5. Operating hours and any process restrictions relied on in the assessment.

A condition drafted from that list is enforceable. A condition requiring odour control to the satisfaction of the authority is not, and is regularly struck out on appeal.

Where the proposal involves a commercial kitchen, the extraction case sits in a Ventilation and Extraction Statement alongside the Odour Assessment, and the two should be written together rather than by different hands.

PlanScope drafts both to a policy-aware structure and flags with a placeholder every input it cannot verify, including emission rates and survey results, which have to come from measurement or from a suitably qualified author. Councils usually expect that author to hold relevant professional membership, and professional review is strongly advised. Reviewers can be approached through the professional directory, and the wider question of checking someone else's technical work is covered in reviewing a specialist report you did not commission.

For specialist technical reports, qualified professional input may be required.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.