Anaerobic digestion plants on farms: the planning case

Anaerobic digestion planning permission is nearly always a full application, not an agricultural notification. The moment a plant takes feedstock from outside the holding, or exports gas or electricity as its main output, it stops being reasonably necessary for the purposes of agriculture and drops out of Part 6 of the General Permitted Development Order. That single point decides how the rest of the project is scoped, because a full application brings odour, noise, traffic, landscape and permitting evidence with it.

Small plants that digest only the holding's own slurry and spread the digestate on the holding's own land sit in a narrower space, and even those usually need permission once the tanks and the combined heat and power housing are counted.

Where the plant sits in planning terms

There is no use class for anaerobic digestion. It is sui generis, which means any material change of use to it needs permission and there is no equivalent movement between classes to rely on.

The practical test a case officer applies is whether the plant is ancillary to the farming operation or a standalone energy or waste business that happens to be on a farm. Imported feedstock, gate fees, a grid connection sized for export and a digestate product sold off the holding all point to the second.

Part 6 Class A still matters for the ancillary elements. On an agricultural unit of five hectares or more it permits buildings reasonably necessary for agriculture up to 1,000 square metres, with a 12 metre height limit, and prior approval for siting, design and external appearance is needed where the ground area of the works exceeds 465 square metres. Digester tanks are rarely accepted as agricultural buildings, so do not plan the scheme around that route.

A Planning Statement has to deal with the development plan's renewable energy policy, its countryside and landscape policies, and the agricultural justification if one is being made. Say plainly what the plant is. Applications that describe an export scale plant as farm infrastructure lose officer trust early and never get it back.

Scale, siting and landscape

Digester tanks are tall, circular and industrial, and they read as industrial from a long way off in open countryside. Feedstock clamps, a gas upgrading skid, a flare stack and a digestate lagoon add horizontal spread around them.

Site the plant against existing farm buildings wherever the topography allows. Grouping the new structures with the existing yard shortens the visual reach of the development and gives the landscape officer something to work with.

Where the site is within or close to a National Landscape, a National Park, a registered park or a designated heritage asset's setting, a Landscape and Visual Impact Assessment prepared to GLVIA3 is the expected evidence. Include winter views. Screen planting that works in July is doing nothing in January, and inspectors know it.

Green Belt sites are harder again. Renewable energy development in the Green Belt is generally inappropriate, and the applicant has to show very special circumstances that outweigh the harm by definition plus any other harm.

Feedstock, traffic and the highway case

The transport case usually turns on the annual tonnage and the harvest peak rather than the average day. Maize and wholecrop arrive in a compressed window, and a plant that averages six loads a day can see thirty in late September.

Set out the movements honestly in a transport statement: vehicle types, routes, peak week, and the junction that the loaded vehicles have to turn out of. Where the access is onto a rural road with poor visibility splays, expect the highway authority to ask for improvement works before anything else is discussed.

Feedstock sourcing also affects the planning judgement on whether the proposal is rural diversification at all. A plant importing food waste from thirty miles away is a waste management facility in policy terms, and the sequential and locational policies for waste development in the local waste plan will then apply.

Odour and the assessment councils expect

Odour is the issue that generates objections and the issue that most often produces a refusal. Sources are the feedstock reception area, the clamp face, digestate storage, and any point where digestate is loaded or pumped.

An Odour Assessment for a plant of any size should identify the sources, characterise them, and then either model dispersion to sensitive receptors or use a risk-based approach where modelling is disproportionate. The Institute of Air Quality Management's odour guidance sets out how assessors are expected to structure that judgement, and environmental health officers will read the report against it.

Mitigation that carries weight is physical: enclosed reception, negative pressure buildings with biofilter treatment, covered digestate stores, and a management plan with recorded checks. Mitigation that carries little weight is a promise to manage odour carefully.

Separation distance is not a policy standard in England, and there is no national number that makes a plant acceptable. Distance to the nearest dwelling in other ownership is still the first thing an officer measures.

Digestate storage and spreading

Storage capacity is where farm-scale schemes come unstuck. The plant produces digestate continuously, and there are periods when spreading is not permitted, so the store has to carry several months of output.

Work out the capacity from the annual digestate volume and the closed spreading periods that apply in a Nitrate Vulnerable Zone, then add freeboard and rainfall on any open store. The Farming Rules for Water apply to the spreading itself, including the requirement to plan applications to soil and crop need rather than to storage convenience.

Where the digestate meets PAS 110 and the associated quality protocol it can cease to be waste, which changes how it is handled and moved. That is a permitting question rather than a planning one, but planning officers ask about it because the answer determines the traffic movements for export.

The store's location matters for odour, for landscape and for water. Keep it away from field drains and watercourses, and show the separation on the drawings rather than describing it in the statement.

Environmental permitting alongside planning

Planning permission and an environmental permit are separate consents with separate tests, and getting one does not commit the other. An anaerobic digestion plant treating waste normally needs a bespoke or standard rules permit from the Environment Agency, and plants below certain thresholds may fall within an exemption.

Screening under the Town and Country Planning (Environmental Impact Assessment) Regulations 2017 runs in parallel. Schedule 2 categories covering energy industry installations and waste disposal installations bring plants over half a hectare of site area into screening, and the local planning authority decides whether the effects are likely to be significant.

Ask for a screening opinion before you spend money on the full evidence base. A negative screening opinion in writing is worth more than an assumption, and a positive one changes the programme by months.

The combined heat and power engine adds a noise source that runs continuously. A Noise Impact Assessment to BS 4142:2014+A1:2019 should measure the existing background at the nearest dwellings at night, not during the working day, because the rating level comparison at night is what sets the acoustic design.

Conditions and the community

Expect conditions on feedstock type and origin, annual tonnage, vehicle movements, hours of delivery, odour management, noise limits at named receptor positions, and landscape planting with a replacement clause. Read the tonnage condition carefully, because a cap set on the basis of the transport assessment becomes the operating ceiling for the life of the plant.

Talk to the parish council and the nearest neighbours before submission. Rural objections to digesters are rarely about renewable energy in principle and almost always about smell, mud on the road and the noise of vehicles at six in the morning. Those are all answerable, and they are much harder to answer for the first time at committee.

Enter your postcode on PlanScope to see which of the 66 report types a farm-scale energy scheme is likely to need before you commission consultants.

This is general guidance rather than legal advice. Check the position with your local planning authority, 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.