Grain stores and drying plant: noise and the neighbours

A grain store is usually agricultural permitted development. The dryer inside it often is not the issue either. What generates complaints, and occasionally an abatement notice, is a continuous fan running from late July into October within earshot of a dwelling that was converted from a barn on the same farm twenty years ago. Grain dryer planning noise cases turn on background sound levels in quiet countryside, and on the fact that the plant runs at night when those levels are lowest.

Deal with it at design stage and it is a siting and specification problem. Deal with it after the first harvest and it is a dispute.

Whether the building is permitted development

Part 6 of the General Permitted Development Order 2015 permits buildings reasonably necessary for the purposes of agriculture on units of five hectares or more, within limits on ground area, height and proximity to roads and to protected buildings. A grain store for crops grown on the unit will usually be within those limits, subject to the prior approval procedure for siting, design and external appearance where the conditions require it.

Three points commonly take a store outside the right:

• The unit is under five hectares, or the building is not reasonably necessary for the agriculture of that unit.
• The store will handle a substantial volume of grain grown elsewhere, which moves the use towards commercial storage rather than agriculture.
• The height or the ground area exceeds the limits in the class, which taller drying towers and larger modern stores often do.

Where the right applies, the prior approval process looks at siting and appearance. It does not look at noise. That gap is the origin of most of the trouble.

Why drying plant is assessed separately

Drying plant can be a separate operation from the building that houses it. Where the store is permitted development but the plant is installed later, or where the plant sits outside the building as a continuous flow dryer with external fans and a cyclone, the authority may treat it as development in its own right.

More often the question arrives as a complaint rather than as an application. Noise from industrial and commercial premises is subject to statutory nuisance controls under the Environmental Protection Act 1990, and a local authority satisfied that a statutory nuisance exists must serve an abatement notice. Agricultural operations carry no blanket exemption.

So the practical risk is a notice requiring the plant to be abated during the only weeks of the year it is needed.

There is a second route to the same place. Where a dwelling nearby has a condition or a planning history that recognises the farm's operations, an officer will weigh that. Where the nearest house was sold off the holding without any such record, the new owner arrives with a reasonable expectation of quiet nights and no knowledge of what August sounds like. Selling a farm cottage is a planning decision as much as a financial one.

How seasonal noise is assessed

The standard method for industrial and commercial sound affecting residential receptors is the British Standard on rating and assessing such sound, which compares a rating level for the specific sound against the background sound level, and considers context.

Three features of a grain dryer make the assessment unusual.

1. Operation is seasonal and intense. The plant may run continuously for days and then not at all for ten months.
2. It runs at night, when background sound in open countryside can be very low indeed.
3. The character of the sound includes a steady fan tone, which attracts a penalty in the rating because tonal sound is more noticeable.

The seasonal element cuts both ways. An acoustician can argue that the duration is limited and the operation is a normal part of agriculture in a farmed landscape. An environmental health officer can reply that the harvest coincides with warm nights when windows are open. Both are right, which is why the assessment has to be site specific.

Background sound in quiet rural areas

This is where rural sites differ from industrial estates. A background sound level in a village at 2am can be extremely low, and the lower the background, the smaller the margin before a plant becomes a problem.

Measure the background properly. That means a survey at the nearest noise sensitive receptors, at the times the plant will run, over enough nights to characterise the variation, and outside a period when the harvest itself is contributing to the measured level. A survey carried out in daytime, or during a week of high wind in the trees, will overstate the background and produce an assessment nobody can rely on.

A Noise Impact Assessment prepared on that basis is what an environmental health officer expects, and the work needs an acoustician with the relevant competence.

Mitigation that works

In rough order of effectiveness and cost:

• Siting. Distance costs least of all, and putting the plant on the far side of an existing building from the nearest dwelling costs nothing at design stage.
• Screening with the store itself, an earth bund or a stack of bales, which breaks the line of sight from the fan to the receptor.
• Attenuators on fan inlets and discharges, specified by sound power level rather than by a general claim of quietness.
• Enclosure of the plant, with attention to the openings, because an acoustic enclosure with an unlined air path achieves little.
• Speed control and staged operation, running fewer fans for longer during the night and more during the day.
• Replacing an older dryer, since modern units are often quieter at the same throughput.

Specify the sound power level of every item of plant in the application. A condition limiting noise at the boundary is unworkable without knowing what the plant emits.

Conditions to expect

Where permission is needed, likely conditions cover hours of operation, a limit on the rating level at the nearest noise sensitive facade, plant specification by reference to the submitted sound data, and a requirement for a post-installation verification survey.

Read the hours condition carefully before accepting it. A restriction to daytime operation can be unworkable for a dryer that has to run through the night to clear a crop coming off the field wet, and the time to argue that is during determination rather than after.

What to submit when permission is needed

A proportionate application contains the planning statement, a site plan showing the plant, the nearest dwellings and the distances, manufacturer sound data, the noise assessment with background survey, and a short operational description covering the season, the hours and the throughput.

Where the store is part of a wider diversification of the holding, note that residential conversions elsewhere on the farm can create new receptors that constrain the farm's own operations later. Anyone looking at converting a rural building should map the noise sources on the holding before choosing which building to convert.

Work out the document list before commissioning survey work: enter your postcode on PlanScope to see which reports the site is likely to need.

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