Free range poultry units and ammonia assessment

A poultry unit ammonia assessment decides more free-range applications than landscape or odour ever will. Ammonia released from housing ventilation and from the range deposits as nitrogen on land downwind, and where that land is a designated habitat the deposition is measured against a published critical load. Exceed it, or add materially to an existing exceedance, and the application stalls whatever the building looks like. The assessment therefore drives the siting of the shed, the position of the pop-holes and the shape of the range, and it has to be done before the layout is fixed.

Why ammonia drives poultry decisions

Ammonia is emitted from litter inside the building and from droppings on the range, with the highest concentrations close to the pop-holes where birds congregate. It travels, deposits, and acts as a fertiliser on habitats that depend on low nutrient availability.

Sensitive habitats lose their characteristic species quickly under added nitrogen. Lichen and bryophyte communities on ancient woodland trees, heathland, unimproved acid grassland and raised bog are all vulnerable at concentrations that would do nothing to a grass ley.

Two measures are used. The critical level is a concentration of ammonia in air, conventionally 1 microgram per cubic metre where lichens and bryophytes are an interest feature and 3 micrograms per cubic metre for higher plants. The critical load is a rate of nitrogen deposition, expressed in kilograms of nitrogen per hectare per year and set for each habitat type. Both are site-specific in application, and the background is often already at or above the load before your unit contributes anything.

That background position is what makes these applications hard. You are frequently arguing about a small addition to an existing exceedance, which is a different conversation from causing one.

Screening distances to designated habitats

Screening establishes whether detailed work is needed at all. The Environment Agency's air emissions risk assessment guidance sets screening distances for intensive farming, measured from the installation to the nearest point of the designated site, with a longer distance applied to European sites than to nationally designated ones. The published distances change between sectors and between revisions, so read the current guidance rather than working from a figure quoted in an older report.

Identify everything within those distances: special areas of conservation, special protection areas and Ramsar sites, sites of special scientific interest, ancient woodland, local wildlife sites and any priority habitat mapped nearby.

Where nothing falls within the relevant distance, say so with a plan and a distance schedule, and the assessment can be short. Where something does, screening moves to the next stage, in which the predicted process contribution is compared against the critical level and load. The Agency screens out effects where that contribution is a small percentage of the relevant standard, with a tighter percentage for European sites than for sites of special scientific interest. Take the current percentages from the published guidance.

Detailed modelling and the critical load concept

Beyond screening, the assessment is a dispersion modelling exercise. An established atmospheric dispersion model is run using emission rates for the housing and the range, stack or ridge outlet parameters, several years of local meteorological data, terrain, and receptor points placed at the designated features rather than at the site boundary.

The outputs are a predicted ammonia concentration and a nitrogen deposition rate at each receptor. Those are added to the mapped background to give a predicted environmental concentration, and the result is compared with the critical level and the habitat-specific critical load.

Emission rates matter more than the model. Use rates appropriate to the system, the bird type and the litter management proposed, state their source, and show the sensitivity of the result to them. An assessment that produces a comfortable answer from an optimistic emission factor will be unpicked by Natural England.

Where the unit is large enough to fall within Schedule 1 to the Town and Country Planning (Environmental Impact Assessment) Regulations 2017, or is screened in under Schedule 2 for intensive livestock installations with new floorspace above the stated threshold, the work sits inside a formal Environmental Impact Assessment. Those Regulations require the Environmental Statement to be prepared by competent experts, so an AI-assisted draft is a starting point for that team and never a substitute for it.

Cumulative assessment with neighbouring units

In-combination effects are where applications fail. Natural England will ask what else contributes, and a unit that screens out on its own can screen in once the neighbouring sheds are counted.

Build the cumulative list from permitted installations on the public register, from planning permissions granted and not yet implemented within the screening distance, and from applications currently undetermined. Model them together where the data allows, or at minimum present the contributions additively with the assumptions stated.

Where a European site is affected, the local planning authority as competent authority must carry out an appropriate assessment under the Conservation of Habitats and Species Regulations 2017 unless likely effects can be ruled out. That is a legal test applied to the in-combination position, and it cannot be discharged by a condition requiring further work later.

Mitigation available at the building and the range

Mitigation at source is more persuasive than mitigation at the receptor.

• Keep the litter dry. Drinker design and management, adequate ventilation and prompt attention to wet patches reduce emission more reliably than anything bolted on afterwards.
• Reduce dietary crude protein within nutritional limits, which lowers nitrogen excreted.
• Design the ventilation so discharge is vertical and unimpeded, since capped or horizontal outlets increase deposition close to the building.
• Manage the range. Free-range laying flocks are limited by marketing standards to a maximum stocking density on the range, and tree and shrub planting across the range encourages birds to disperse rather than concentrate near the pop-holes, which spreads the nitrogen load.
• Hard-surface and regularly scrape the apron immediately outside the pop-holes, where droppings are heaviest.
• Consider shelterbelt planting downwind of the outlets. Tree belts do capture ammonia, and any capture efficiency claimed in the assessment needs evidencing rather than asserting, because Natural England will ask for the basis of the figure.

Abatement equipment such as scrubbing is normally a housed-unit measure associated with a permit, and it rarely fits a free-range laying system.

Environmental permitting thresholds

An installation for the intensive rearing of poultry above the number of bird places set in Schedule 1 to the Environmental Permitting (England and Wales) Regulations 2016 needs a bespoke permit from the Environment Agency, with best available techniques, an ammonia assessment and monitoring conditions attached.

Below that threshold the unit is regulated through planning alone, plus the general farming rules for water and the storage rules where a nitrate vulnerable zone applies.

An illustrative example shows how the thresholds interact. A free-range laying unit of around 32,000 birds in a new building of roughly 3,000 square metres sits below the Schedule 1 environmental impact assessment threshold for hens and below the permitting threshold for poultry places, but comfortably above the Schedule 2 floorspace threshold for intensive livestock installations. It therefore needs a screening opinion, will probably need an ammonia assessment because of a woodland within the screening distance, and will not need a permit. Those figures are illustrative and the real thresholds should be read from the Regulations.

What a consultee response usually asks for

Natural England's response to a poultry application follows a pattern. Expect requests for the screening distances and what lies within them, the emission rates used and their source, the model and meteorological data, receptor locations at the designated features, the background deposition figures used and their date, the process contribution as a percentage of the critical level and load, the in-combination assessment, and the mitigation with its evidence.

Answer all of it in the Air Quality Assessment or the ammonia chapter rather than in a covering letter, and cross-refer from the Planning Statement so the case officer can find it. Councils expect this work from a suitably qualified author and professional review of any draft is strongly advised.

Odour and scale raise a separate set of questions, covered in our note on poultry units, scale and the screening question. The two assessments use different methods and different receptors, and running them together saves a round of consultation.

To see which supporting documents a poultry scheme on your holding is likely to need, enter your postcode on PlanScope and run the constraint check before the shed is positioned.

This is general guidance, not legal advice. Check the position with your local planning authority, Natural England, the Environment Agency or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.