Flue height for a restaurant below residential flats

A restaurant in the ground floor of a building with flats above has to take its kitchen extract to a termination above the roof of that building, which in practice means a duct running the full height of the elevation or through an internal riser, discharging vertically about a metre above the ridge or eaves. Everything else about the application follows from whether that route exists. If the only available route stops at second floor level beside a bedroom window, the kitchen cannot support heavy cooking whatever abatement is fitted.

The flats are the complication. Receptors are not across the street. They are three metres above the canopy, with openable windows on the same elevation as the duct.

Why flue height is the central issue

Odour that leaves a duct at low level is caught in the pressure field around the building and drawn back towards openings on the floors above. Height and vertical discharge velocity are what get the plume clear of that zone, and no filter removes odour completely.

Abatement reduces the load. Dispersion decides where what is left ends up. An environmental health officer reading your application looks at the section drawing before the equipment schedule, because a system with three treatment stages discharging at first floor level will still generate complaints from the flat above.

There is also a practical reason the height question comes first. Agreeing the route with the freeholder and the upper leaseholders takes longer than specifying equipment, so it should start on day one.

Termination height above the roof or opening windows

The working expectation in most authorities is termination at least one metre above the eaves or the ridge of the building, and above the roofline of anything immediately adjoining, with the discharge pointing upward and no cowl that turns the plume down. Separation from openable windows, air intakes and other flues on the same elevation is assessed alongside it.

Where an adjoining building is taller, its roof sets the relevant level. And where the building sits in a courtyard, a narrow light well or a steep street, local air movement can make a technically adequate height perform badly, which is the point at which a dispersion assessment earns its fee rather than a rule of thumb.

Routing a flue up a rear elevation

An external duct on the rear elevation is the normal answer and it is usually acceptable in appearance terms, powder-coated to match the brickwork, kept tight to the wall, with brackets detailed on the drawing. Three constraints shape the route.

The windows of the flats above come first. A duct passing within a short distance of an openable bedroom window creates both an odour and a noise path, so the route usually hugs a blank section of wall, a stair core or a corner.

Then the legal position. The elevation is almost certainly not demised to you. You need the freeholder's consent for fixings, and where the duct passes a demised window or crosses a party structure, the leaseholders' position and the Party Wall etc. Act 1996 both come into play. Planning permission does not override any of that.

Last, the alternative of an internal riser. Where the building has a redundant chimney or a service shaft, routing inside avoids the appearance question entirely, at the cost of a fire-rated enclosure through every compartment floor. On a listed building that internal route is usually the only acceptable one, and the intervention still needs listed building consent.

Abatement stages and when each is needed

Match the abatement to the assessed odour risk, following the Defra guidance on the control of odour and noise from commercial kitchen exhaust systems. Grease filtration at the canopy is universal, both for odour and because grease accumulation in a duct above a residential building is a fire risk.

Beyond that, electrostatic precipitation deals with particulate and some odour, activated carbon deals with odour and has a finite service life, and ultraviolet or ozone systems oxidise odour compounds and need lamp replacement. A restaurant with charcoal grilling or heavy frying under flats will normally need more than one stage, and the Odour Assessment should state the design duty, the stages, the expected removal performance from the manufacturer's data and the maintenance intervals.

Write the risk assessment first and select equipment from it. A supplier's package quoted without reference to the receptors above is the commonest weakness in these applications.

Fan noise and anti vibration mountings

Fan noise reaches the flats two ways: through the air from the termination and the duct wall, and through the structure from the fan mountings. The airborne path is assessed as commercial and industrial sound against the measured background at the nearest window, and the structure-borne path is the one that generates complaints at midnight.

The mitigation is standard and should be shown on the drawings rather than promised in a paragraph: anti-vibration mounts under the fan, flexible connections at both ends, a duct attenuator, resilient brackets where the duct touches the structure, a larger duct running at lower velocity, and a fan sited away from habitable rooms. Mounting a fan directly onto a floor slab shared with a flat above is the single most reliable way to generate a statutory nuisance complaint.

The Noise Impact Assessment needs manufacturer sound power data, a background survey covering late evening and night where the restaurant will trade late, and calculations at the nearest residential window. Plant serving food premises raises the same questions as other building services in mixed-use blocks, and the approach set out in cold stores and refrigeration plant noise applies directly to extract plant and condensers on the same elevation.

Access for cleaning and maintenance

A grease duct serving a restaurant needs cleaning on a schedule, and a duct that cannot be reached will not be cleaned. Access panels at intervals along the run, at every change of direction and at the fan, are a design requirement rather than an extra, and industry guidance on grease duct cleaning sets the expectation that councils and insurers both rely on.

Where the duct is external, maintenance access means a safe route: a fixed ladder with a cage, a roof hatch with edge protection, or a cradle arrangement on a taller building. Where it is internal, it means access panels inside the fire-rated shaft at each floor, which has to be agreed with the freeholder because the shaft usually sits in common parts.

An illustrative example. A four-storey building with a restaurant unit at ground floor and three flats above. The duct runs externally up a blank section of the rear elevation beside the stair core, terminating one metre above the ridge, powder-coated to match the brick. Grease filters, electrostatic precipitation and carbon, an attenuated fan on anti-vibration mounts inside the kitchen rather than at roof level, access panels at each floor reached from a fixed ladder, and a signed maintenance contract named in the submission. That case is approvable. The same restaurant with a wall grille into the rear yard is not.

Conditions and verification

Expect conditions requiring the system to be installed and commissioned in accordance with the approved details before the use starts, a verification report from a competent person confirming the installed performance, maintenance records kept and produced on request, a noise limit expressed against background at the nearest residential window, and fixed opening hours.

Agree the verification mechanism before permission is issued if you can. A condition that requires a report "to the satisfaction of the local planning authority" with no stated method is the one that delays an opening. The Ventilation and Extraction Statement is the right place to propose the verification method yourself.

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