Roof mounted solar on agricultural buildings

For most working farms, solar panels agricultural building planning questions are answered by Part 14 of the General Permitted Development Order rather than by an application. Class J of Part 14 permits the installation of solar photovoltaic or solar thermal equipment on non-domestic premises, which covers a steel-framed barn, a grain store or a livestock building, subject to limits on how far the panels project and how high they sit. The December 2023 amendment to Part 14 removed the previous one megawatt capacity limit on rooftop solar on non-domestic buildings, which changed the economics of farm-scale arrays considerably.

The planning question is usually the easy part. The roof, the grid and the bats are where schemes stall.

Where permitted development applies to a farm roof

Class J applies to non-domestic premises, so an agricultural building qualifies. Two exclusions remove it entirely: the equipment cannot be installed on a listed building or on a scheduled monument. Within a conservation area or a World Heritage Site, installation on a wall fronting a highway is also excluded, which affects wall-mounted equipment rather than roof arrays.

Part 14 also carries general conditions. The equipment has to be sited, so far as practicable, to minimise its effect on the external appearance of the building and on the amenity of the area, and it must be removed as soon as reasonably practicable when no longer needed for microgeneration. Those are enforceable conditions rather than aspirations.

Read the current consolidated text of Class J on legislation.gov.uk rather than a summary. Part 14 has been amended several times, including in 2023, and the version in a three-year-old guidance note will be wrong.

A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful. Where the array is large, where the building's status is arguable, or where a lender or a funder wants certainty, that certificate costs far less than an enforcement dispute.

Size, projection and siting limits

The dimensional controls in Class J are short and they are tested in practice by the mounting system rather than by the panels.

The equipment must not protrude more than 0.2 metres beyond the plane of the roof slope, measured perpendicular to the external surface. On a profiled sheet roof that measurement starts from the sheet surface, so a rail-and-clamp system on deep profile cladding uses up a meaningful part of the allowance before the panel thickness is counted. And the highest part of the equipment must not sit higher than the highest part of the roof, excluding any chimney, which rules out tilting panels up above the ridge on a shallow-pitched barn.

Those two limits push farm installations towards in-plane mounting on the existing slope. South-facing slopes are preferred for yield, but an east-west split across both slopes of a barn often produces a flatter generation profile that matches farm demand better, and it uses roof area that would otherwise be wasted.

Our note on farm wind turbines and the permitted development limits covers the neighbouring classes in Part 14, which work on a different and much tighter set of thresholds.

Prior approval requirements and what they cover

The 2023 amendment introduced a prior approval step for specified cases under Class J, directed at the design and external appearance of the installation and at the effect of glare on the occupiers of neighbouring land. Whether it applies to a particular building turns on the current wording, so check the text before assuming either way.

Where prior approval is required, the application is narrow. The authority is considering the matters the class specifies, not the principle of the development, and it cannot refuse on grounds outside that scope. Submit a site plan, roof plan, elevations showing the array extent, a section showing the projection dimension, and a short note on glare towards the nearest residential windows and any road.

Allow for the determination period in the programme, and do not start work before it expires. Beginning development during a prior approval period loses the benefit of the class.

Structural capacity of an existing roof

This defeats more farm solar schemes than planning does.

Agricultural roofs are designed for the loads they were expected to carry, which on a 1980s portal frame means sheeting, wind uplift and snow, with nothing spare for 12 kilograms per square metre of panels and rails plus the altered wind loading. Purlin spacing, purlin section, sheet fixings and the frame itself all need checking against the current loading standards, and the answer is sometimes that purlins must be doubled or the frame strengthened.

Roof covering is the second issue. Cement sheet roofs installed before 2000 may contain asbestos, which brings the Control of Asbestos Regulations 2012 into play, and such roofs are fragile within the meaning of the working at height guidance. Where the roof has to come off anyway, replacing the covering and installing the array in one operation is usually the better route, and it also resolves the fixing detail.

Commission a structural survey of the specific building rather than a generic appraisal of the farmstead. The output you need is a loading verdict for the array you intend to install, with the strengthening specified if it is needed.

Grid connection and export constraints

The grid application runs in parallel with everything else and takes longer than any of it.

For an array above the small-scale threshold, connection is applied for under the relevant engineering recommendation with the distribution network operator, and the offer that comes back may be unrestricted, restricted by an export limit, or dependent on reinforcement at a cost that kills the project. A flexible or curtailable connection is often offered where firm capacity is unavailable, which trades a lower connection cost for curtailment during constrained periods.

Three practical points. Apply early, because the lead time governs the programme. Size the array against on-site demand first, since generation used on the farm is worth more than generation exported. And where the offer carries reinforcement costs, ask whether a smaller array fits within the existing capacity, because the step between 300 and 500 kilowatts can be the step between no reinforcement and a substantial contribution.

A dual-purpose roof is worth considering at the same time, and our piece on biosolar roofs covering energy, drainage and biodiversity together sets out where that works on a flat-roofed building.

Glint and glare near an aerodrome

Where the building lies within an aerodrome safeguarding zone, glint and glare becomes a real constraint rather than a formality. Civil Aviation Authority guidance on aerodrome safeguarding covers the assessment of reflective surfaces, and Ministry of Defence safeguarding applies around military airfields and some technical sites.

The assessment models the sun's path against the panel orientation and the approach paths, and it reports whether and when a reflection could reach a pilot's eye or a control tower. Mitigation is usually a change of tilt or azimuth, anti-reflective coating, or screening, and all three are easier to accommodate before the layout is fixed.

Railways and highways raise a smaller version of the same question. A driver on a road running directly north of an east-west array is the usual concern, and the answer is normally a short assessment rather than a change of design.

Landscape effects in a designated area

Within a National Landscape or a National Park, the determining question is the building, not the panels. A modern profiled-sheet barn with a dark roof absorbs an array with very little visual change. A traditional stone barn with a natural slate roof does not, and in those cases the array belongs on an adjacent modern building or on the ground elsewhere on the holding.

Where the development needs an application rather than falling within Class J, and the site is in a designated landscape or within sight of one, a proportionate landscape and visual impact assessment following the recognised methodology is what the officer will expect. Keep it proportionate: a 400 square metre roof array does not warrant the visualisation package of a solar farm.

One ecology point to settle before anything is fixed to the roof. Agricultural buildings commonly support bat roosts and barn owls, and works to a roof can disturb or destroy them. A protected species survey and any mitigation licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017. This is a legal requirement, not a recommendation, and PlanScope's draft is a starting point for that ecologist rather than a substitute.

Enter your postcode on PlanScope to see which documents a farm renewable energy scheme is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority 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.