Kitchen extract discharge height above the roof

Termination height is the single design parameter that decides whether a commercial kitchen extract system causes complaints. Filtration removes grease and a proportion of the odour. Height decides what happens to whatever is left. A kitchen extract flue height that stops below the roof line discharges into the turbulent wake on the leeward side of the building, where the air recirculates and brings cooking odour straight back down to the windows above and behind.

The expectation most environmental health officers start from is a vertical discharge at high velocity, terminating at least one metre above the ridge of the building, and above the roof line of neighbouring buildings where they are higher or close enough to matter. Anything lower is a case you have to make rather than a default.

Why height does the work

A flue discharging well above the roof puts the plume into the free airstream, where it mixes with a large volume of air and dilutes quickly. The same flue discharging at eaves level, or through a wall, releases into the recirculating region that forms downwind of any building. Air in that region does not move away. It circulates, and odour accumulates in it.

Discharge velocity matters alongside height. A vertical discharge at a high exit velocity gives the plume momentum, which carries it further before the wind bends it over. A low-velocity discharge, or one fitted with a cowl or a rain cap that deflects the flow sideways, loses that momentum and behaves like a vent in a wall whatever its height.

So the three things an officer looks for are height relative to the roof line, a vertical unobstructed discharge, and a stated exit velocity. Fail any of the three and the odour case has to be rebuilt on abatement alone.

Where the expectation comes from

Government guidance on the control of odour and noise from commercial kitchen exhaust systems is the reference document. It sets out an odour risk assessment that combines the type and scale of cooking with the proximity and sensitivity of receptors, producing an odour control level that determines how much abatement the system needs, from basic grease filtration through carbon filtration, electrostatic precipitation and ultraviolet or ozone treatment.

Height is part of the same assessment rather than an alternative to it. A high odour risk kitchen discharging above the ridge still needs the abatement its risk level calls for. What height buys you is tolerance: a well-sited termination makes the residual odour acceptable, while a poorly sited one means no realistic amount of abatement will satisfy the neighbours.

Set the risk assessment, the abatement specification and the termination design out together in a Ventilation and Extraction Statement. Where the risk level is high or the receptors are unusually close, a standalone Odour Assessment carries the dispersion argument.

Justifying a lower termination

It is done, and it is done on evidence. Four arguments work in combination.

Enhanced abatement beyond the level the risk assessment requires, specified by make and model with the manufacturer's efficiency data, and sized for the actual extract rate rather than a nominal one. Dispersion modelling showing predicted odour concentrations at the nearest receptors within accepted limits, which only means something if the model inputs are the real exit velocity, temperature and location. Separation distance, where the nearest openable window is far enough away and in a direction the prevailing wind does not favour. And a maintenance regime committed to in writing, with filter change intervals, a service log and a named responsible person.

The council will want that last point secured. Expect a condition requiring the system to be installed and operated in accordance with the approved specification, maintained to the stated schedule, with service records kept and made available on request. Some authorities add a review mechanism triggered by substantiated complaints.

Be honest with yourself about the odds. A high odour risk use such as a charcoal grill or a deep-frying takeaway, below flats, with a termination at first floor level, is a scheme that will struggle however good the filtration specification is.

Openable windows, balconies and air intakes

Separation is measured in three dimensions and officers check all of them.

The vertical relationship is the first question: is the termination above the highest openable window within the relevant distance. The horizontal clearance is the second, because a termination one metre above a ridge but immediately beside a dormer window is still a problem. And the worst case is a discharge into an enclosed courtyard, a lightwell or a narrow gap between buildings, where there is no airflow to dilute anything and the odour simply fills the void.

Mechanical air intakes deserve separate attention. A fresh air intake serving flats above, or a neighbouring building's ventilation plant, drawing in extract air is a complaint that arrives on day one and cannot be mitigated by filtration at the source.

Plan the route around those constraints before drawing the flue. Moving a termination two metres along a ridge is a drawing change. Moving it after installation is a new application.

The flue on the elevation

A 600 mm diameter insulated duct running up a rear elevation is a visual intervention, and in a town centre it will be seen.

The options, roughly in order of acceptability, are an internal riser within the building discharging at roof level, an existing chimney flue relined where one exists and is of adequate size, an external duct on a rear or flank elevation with a colour matched finish and a careful route, and lastly an external duct on a principal elevation, which is rarely acceptable.

Internal risers bring their own requirements. Ductwork passing through compartment floors needs fire resistance and fire stopping to satisfy Approved Document B, the duct has to be accessible for cleaning along its length, and where it passes through a residential demise above there is a legal question about rights as well as a technical one. Fan vibration isolation and in-line attenuators are usually needed to keep the plant noise acceptable, and the fan data feeds into a Noise Impact Assessment assessed under BS 4142:2014+A1:2019.

Listed buildings and conservation areas

A flue on a listed building needs listed building consent as well as planning permission, and a flue in a conservation area will be assessed for its effect on the character and appearance of the area.

The designs that get consented share a pattern: an internal route using an existing chimney where one survives, a termination at an existing stack rather than a new projection, a rear or courtyard elevation not visible from the street, and a duct sized as small as the extract rate allows. A Heritage Statement should assess the effect on significance and explain why the route chosen is the least harmful available, with the alternatives considered and rejected.

Where the heritage constraint prevents a compliant termination, the fallback is heavier abatement plus a reduced cooking specification, with the use conditioned accordingly. That is a smaller business than the operator wanted, and it is better established before the lease is signed.

What to submit

Six items, and applications that include all six are usually determined without an odour objection.

1. The odour risk assessment, naming the cooking processes, the extract rate and the receptors.
2. A system schematic showing filtration stages in order, with makes, models and efficiency data.
3. The flue route in plan, section and elevation, with the termination height dimensioned against the ridge and against the nearest openable windows.
4. The design extract rate and the discharge velocity at the termination.
5. Fan acoustic data with the attenuation proposed, for the plant noise assessment.
6. A maintenance schedule with filter change intervals and a named responsible role.

Councils expect a suitably qualified author for the odour and acoustic work, and professional review of any draft is strongly advised. The approach to odour from other sources, where dispersion rather than filtration governs, is set out in our note on odour from waste transfer and composting facilities.

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 project 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.