Odour impact criteria in an Odour Assessment
Odour has no single statutory limit in planning. The criteria section chooses the benchmark, applies it to identified receptors and decides everything that follows.
The odour assessment impact criteria section is where a report decides what counts as an unacceptable smell, and it has to do that before any modelling or judgement about your kitchen extract means anything. Odour has no single statutory limit in planning. The criteria are chosen by the assessor from recognised guidance, applied to identified receptors, and then defended to an environmental health officer who has probably dealt with complaints from the same street before.
For a restaurant, takeaway, brewery or food production unit, this section is the one worth reading in the draft. Everything else follows from it.
What the criteria are trying to express
Odour annoyance is not a simple function of concentration. A faint smell of bread is tolerable at levels that would make a rendering plant intolerable, and the same odour at the same concentration provokes complaint in a garden and passes unnoticed on a pavement.
So the criteria have to express an acceptable exposure for a particular odour, at a particular kind of receptor, over a stated proportion of the time. That is three variables, which is why a bare statement that odour will be controlled to acceptable levels is not a criterion.
The section should state the criterion, the receptor type it applies to, the averaging period and percentile it is expressed over, and the source of the criterion.
The FIDOL factors and how they are applied
Practitioners use the FIDOL framework, which sets out the elements that determine whether an odour causes annoyance.
Frequency, meaning how often exposure occurs. Intensity, the perceived strength of the odour when it is present. Duration, how long each episode lasts. Offensiveness, the hedonic tone of the odour, which is the difference between baking and boiling fat. And location, covering the nature and sensitivity of the receiving environment.
FIDOL is a framework for judgement rather than a calculation. Its value in an assessment is that it forces each element to be addressed separately, so a report cannot conclude that odour is acceptable because the source is small without saying anything about how offensive the odour is or how close the nearest window sits.
Offensiveness carries the most weight in criterion selection. Odours from cooking, bakeries and breweries are treated as less offensive than those from waste processing, rendering or intensive livestock, and the acceptable exposure benchmark differs accordingly.
Choosing an exposure benchmark for the receptor
Where dispersion modelling is used, the benchmark is expressed as an odour concentration in European odour units per cubic metre, as the 98th percentile of hourly mean concentrations over a year. Recognised guidance sets progressively tighter benchmarks as offensiveness increases, with the most offensive odours held to the lowest concentration and the least offensive allowed the highest.
The assessment must say which benchmark it has adopted and why that classification fits the odour in question. A commercial kitchen serving a mixed menu including fried fish is not obviously in the same class as a bakery, and the classification decision should be reasoned rather than asserted.
Receptor sensitivity then modifies the judgement. Housing, hospitals, schools and care homes are high sensitivity. Offices and retail are medium. Industrial premises and footpaths are low. A benchmark appropriate for a residential garden is not the benchmark for a service yard.
Where a receptor is unusually exposed, such as a flat with an opening window 6 m from a proposed extract termination, the criterion should be stated for that receptor specifically rather than averaged into a site-wide conclusion.
Dispersion modelling against qualitative assessment
Not every proposal needs a model, and a proportionate approach is part of good practice.
| Approach | When it fits |
|---|---|
| Qualitative source, pathway and receptor assessment | Small commercial kitchens, single extract, few receptors, well-understood abatement |
| Semi-quantitative risk assessment | Moderate sources, several receptors, some uncertainty about dispersion |
| Detailed dispersion modelling | Larger or more offensive sources, complex building geometry, sensitive receptors nearby, or where a qualitative conclusion is contested |
A takeaway in a parade of shops with flats above almost never needs a dispersion model. It needs a properly specified extract system, a correctly located discharge point and a clear explanation of why the design meets the criteria. Councils that ask for modelling on a scheme of that size are usually reacting to a history of complaints rather than to the technical requirement, and that is worth a conversation at pre-application stage.
Sensitivity of the receiving population
The criteria section should describe the receiving environment rather than list postcodes.
Existing odour exposure matters. A new unit on a street with three established restaurants sits in a different context from the first hot food use in a residential parade, and the assessment can reasonably say so, without using existing exposure as a licence to add more.
Complaint history is relevant context where the council has published it or raised it, though an assessment should not speculate about how a particular authority behaves or invent a complaint record. Where the officer raises a history, address it.
The number and type of receptors, their distance, their orientation relative to prevailing winds and the height of the discharge relative to nearby openings all belong in this section, because they determine which criterion applies where.
Abatement and its assumed efficiency
This is where criteria get quietly undermined.
An assessment that concludes compliance on the basis of 95 per cent odour removal has made an assumption that must be evidenced. Manufacturer test data for the specified unit, at the specified airflow, with the specified filter loading. Not a catalogue claim for a different configuration.
State the abatement train: grease filtration, then carbon or other treatment, then discharge parameters. State the discharge height, the efflux velocity and the direction, because dispersion from a vertical uncapped discharge at high velocity is entirely different from a cowled outlet at low velocity, and the second is what usually gets installed.
Then state the maintenance the assumed efficiency depends on. Carbon has a finite life. Grease filters that are not cleaned degrade the whole system. An efficiency assumed in the assessment and not maintained in operation is the most common reason a compliant scheme generates complaints in its second year.
A Ventilation and Extraction Statement covers the system design itself, and the two documents should share the same specification rather than describing different systems.
Conditions that keep the criteria enforceable
Write the criteria so they can be conditioned. That means a measurable or inspectable requirement rather than an outcome.
Expect conditions requiring the extraction and abatement scheme to be installed as approved before the use starts, a maintenance and filter replacement schedule with records kept and available for inspection, and in some cases an odour management plan. Where the council is concerned, a condition requiring a post-installation commissioning report is increasingly common.
Plant noise usually rides alongside odour in the same consultation, and the acoustic assessment of the fan and any external condensers is a separate exercise using different criteria. Our note on rating penalties in a BS 4142 assessment covers how tonal and impulsive characteristics are handled there, and a Noise Impact Assessment will usually be requested on the same application.
A PlanScope Odour Assessment draft sets out this structure, states the criteria framework and marks every concentration, efficiency and distance as still to be confirmed until survey and equipment data is supplied. Councils expect a suitably qualified author for odour work and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What are the FIDOL factors?
Frequency, intensity, duration, offensiveness and location. Together they describe whether an odour is likely to cause annoyance. The framework guides judgement rather than producing a number, and its value is that it forces an assessment to address each element separately instead of concluding acceptability from the size of the source alone.
How are odour exposure benchmarks expressed?
As an odour concentration in European odour units per cubic metre, given as the 98th percentile of hourly mean concentrations across a year. Recognised guidance sets tighter benchmarks for more offensive odours, so the classification of the odour as more or less offensive drives the benchmark that applies.
Does a small takeaway need dispersion modelling?
Rarely. A qualitative source, pathway and receptor assessment is usually proportionate for a single commercial kitchen with a well-understood extract system and a small number of receptors. Where a council asks for modelling at that scale it is often responding to local complaint history, which is worth discussing at pre-application stage.
How should abatement efficiency be justified?
With manufacturer test data for the specified unit at the specified airflow and filter loading, not a general catalogue claim. The assessment should also state the discharge height, efflux velocity and direction, because a cowled low-velocity outlet disperses far worse than the vertical uncapped discharge usually assumed.
What conditions follow an odour assessment?
Typically installation of the approved extraction and abatement scheme before the use starts, a maintenance and filter replacement schedule with records available for inspection, and sometimes an odour management plan or a post-installation commissioning report. Write the criteria so they can be turned into an inspectable requirement.
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.