Restaurant extraction ducts: routes and termination

Settle the extraction route before you sign the lease. A restaurant extraction duct has to get from the kitchen to a discharge point at roof level, and on a mid-terrace unit with flats above that route may not exist, may not be within the demise you are taking, and may need both landlord consent and planning permission. Units get taken every month by operators who assumed the flue was a detail for the fit-out stage.

The planning question is separate from the change of use. A duct on an external elevation is development in its own right, and on a listed building it needs listed building consent as well.

Establishing the route before committing

Walk the building, not the floor plan.

What you are looking for is a continuous path from the canopy to roof level with adequate space, a structure that can carry the duct, and the legal right to use it. An internal riser is the better answer wherever one exists, because it avoids the external appearance argument entirely and is easier to insulate and to fire stop. Risers in older buildings are often occupied, undersized or shared.

Where the route runs outside the unit you are taking, three questions follow. Does the lease grant rights to install and maintain services through retained parts? Will the landlord consent, and on what terms? And is the route within the building's ownership at all, or does it cross a neighbour's land or airspace?

Ask for a measured survey of the proposed route rather than relying on the letting agent's plan. Duct sizes come out larger than people expect once the required extract rate is calculated from the canopy area and face velocity, and a 500mm diameter duct does not fit where a 200mm soil pipe does.

Internal riser against external duct

An internal riser keeps the elevation clean, keeps the duct warm, reduces condensation and grease cooling, and usually attracts no planning objection. It costs floor area on every storey it passes and it needs fire stopping at every compartment line.

An external duct is quicker and less costly to install and it is visible from the street or the rear yard. That visibility is where the planning argument happens.

The practical compromise on many terraced units is a duct rising in a rear lightwell or against a rear elevation that is not publicly visible, boxed or painted to match, and terminated above the ridge. Where the rear elevation faces a conservation area, a listed terrace or residential windows, that compromise gets harder.

Insulation matters for performance as well as appearance. An uninsulated external duct cools the extract, condenses grease on the inside face and increases cleaning frequency.

Termination height and nearby openings

The discharge point does more for odour control than any single piece of equipment.

The approach councils generally apply, drawn from national guidance on odour and noise from commercial kitchen exhaust systems, is to discharge vertically at roof level, above the ridge or eaves of the building and of nearby buildings, clear of openable windows, balconies and air intakes. A discharge at low level into a yard, or through a wall grille, is very difficult to defend where there is any residential receptor nearby.

Discharge velocity matters too. A high efflux velocity carries the plume upwards and away rather than letting it fall back against the building, and guidance sets a recommended minimum. A cowl or rain cap that deflects the discharge downwards defeats the whole arrangement, so specify a vertical discharge with rain protection that does not obstruct it.

Check what is above and around the termination. A flat roof with a plant deck, a neighbouring building one storey higher, or a bedroom window in a rear return can each turn an otherwise reasonable design into an objection you cannot answer.

Odour control sized to the risk

National guidance sets out a risk-based approach: assess the odour risk from the type of cooking, the size of the operation and the proximity and sensitivity of receptors, then specify abatement to match.

A low risk operation, perhaps a small café reheating food with a discharge well clear of receptors, may need nothing beyond grease filters and a good termination. A high risk operation, such as a large kitchen frying and grilling with residential windows close by, will need a treatment train: grease filtration, then electrostatic precipitation or equivalent, then carbon filtration, with ultraviolet or ozone systems used in some designs.

Set the odour risk assessment out in the submission rather than simply naming the equipment. Environmental health officers want to see the reasoning that led to the specification, and they want to know who will maintain it.

An Odour Assessment does that work where the risk is high or the receptors are close, and it is a document the environmental health team will read before the case officer does.

Noise from the fans and the plant

Kitchen extract plant is commercial plant, so it sits squarely within the assessment method for industrial and commercial sound, which compares the rating level against the background sound level at the nearest receptor.

The measures are familiar. Select the fan for the duty rather than oversizing it, fit attenuators on the discharge and, where needed, on the intake, mount the plant on anti-vibration mounts, isolate the duct from the structure with flexible connections, and avoid mounting plant directly on a party wall or on a structural element shared with a flat.

Conditions usually fix a plant noise limit expressed relative to background, often with a requirement for no tonal or impulsive character. That limit is easier to meet by selection than by retrofit, so bring the acoustician in while the equipment schedule is still open. A Noise Impact Assessment covering plant is a standard requirement on restaurant applications in residential areas.

Late servicing is a separate argument that catches operators after opening, and the issues around night deliveries and servicing noise are worth resolving in the same application.

Grease, cleaning and access

An extraction system that cannot be cleaned will fail within a year, and the failure shows up as odour complaints.

Design in access hatches at changes of direction and at intervals along horizontal runs, provide safe access to the roof termination, and specify the cleaning frequency in the maintenance schedule. Industry guidance on grease removal from ventilation systems sets cleaning intervals based on hours of use, and insurers increasingly ask for evidence that it has been followed.

Councils frequently condition a maintenance regime. Offering one with the application is better than receiving one written by someone who has not seen your kitchen.

The evidence pack for the application

Assemble these before submission rather than in response to a validation query.

1. Existing and proposed elevations showing the duct route, with the termination height dimensioned against the ridge or eaves.
2. A section through the building showing the route, any riser and the discharge point.
3. A plan showing the canopy, the extract rate calculation and the duct sizing.
4. The odour risk assessment and the abatement specification that follows from it.
5. Plant noise data with the proposed mitigation and the resulting level at the nearest receptor.
6. A maintenance and cleaning schedule with responsibilities named.

A Ventilation and Extraction Statement pulls those strands into one document, which is what most local validation lists ask for on a hot food use.

Where the unit is listed or in a conservation area, add the heritage case for the external works, because the duct will be assessed against the fabric and appearance of the building as well as against its performance.

Enter your postcode on PlanScope to see which documents a restaurant change of use is likely to need.

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