Part 14 Class J: solar on non-domestic roofs

Commercial rooftop solar planning usually runs through Class J of Part 14 of the General Permitted Development Order 2015 rather than a full application. Class J covers solar equipment on the roof of a building that is not a dwellinghouse or a block of flats, and it requires a prior approval application dealing with design and external appearance, with particular attention to glare affecting neighbouring occupiers.

That prior approval is not a formality on a large warehouse roof beside a road or an airfield. It is the whole of the planning risk, and a glare study is what removes it.

What Class J covers

The right applies to the installation, alteration or replacement of solar photovoltaic or solar thermal equipment on the roof of a building other than a dwellinghouse or a block of flats, and on a building within the curtilage of such a building.

That is broad. Warehouses, factories, offices, schools, hospitals, retail sheds, agricultural buildings and car park canopies attached to a building all sit within it. Ground-mounted arrays do not. A field array is a full application, and above certain capacities it may fall to a different consenting route altogether.

The building must exist and the array must go on the roof. Erecting a new frame that reads as a structure in its own right takes the proposal outside Class J.

The capacity limit and where it went

Class J historically carried a capacity ceiling of one megawatt, which ruled out permitted development on the largest distribution roofs. That ceiling has been removed by amendment to the Order, so a substantially larger rooftop installation can now qualify.

Part 14 has been amended more than once, and the consolidated text on legislation.gov.uk is what governs. Read it before you commit to a route, particularly if you are working from a guidance note written before the change.

Two practical consequences follow from the removal of the cap. Roofs that were previously split into sub-megawatt phases can be developed in one go. And the design and appearance question becomes more prominent, because the arrays that benefit most from the change are the most visible ones.

Dimensional limits on a flat commercial roof

The dimensional limits mirror the domestic classes and they are the ones that catch installers.

The equipment must not protrude more than 200 millimetres beyond the plane of the wall or the roof slope, measured perpendicular to that surface. And it must not result in the highest part of the equipment being higher than the highest part of the roof, excluding any chimney.

On a pitched industrial roof with in-plane panels, neither limit causes difficulty. On a flat roof, a ballasted tilted frame at 10 or 15 degrees can breach both. Where the roof has a parapet, measure from the roof surface rather than the top of the parapet, because the parapet is not the roof.

That single point sends more schemes into a full application than any other. Where the frame will exceed the limit, either lower the tilt, accept the yield penalty, or apply for planning permission and argue the case on its merits.

Prior approval and the glare question

Class J is a prior approval right. The developer applies to the local planning authority for a determination as to whether prior approval is required for the design or external appearance of the development, and in particular the impact of glare on the occupiers of neighbouring land.

Glare is the reason the condition exists, and it is the issue that generates refusals. Reflected sunlight from a large array can affect three types of receptor.

1. Residential occupiers, where reflection into habitable rooms at particular times of year is the complaint.
2. Drivers on a nearby road, especially where the road runs east to west and the array faces it.
3. Aircraft and air traffic control, where the site lies under an approach path or near an aerodrome. The Civil Aviation Authority publishes guidance on assessing solar glare in the vicinity of aerodromes, and the aerodrome operator is the body to consult early.

A glare assessment models the sun position through the year, the array geometry and the receptor location, and reports when and for how long reflection reaches each receptor. Where the result is nil or negligible, say so plainly and attach the output. Where it is not, the mitigation is usually a change in tilt or azimuth, anti-reflective coating, or screening.

Determination periods for prior approval are set by the Order and development may proceed where the authority does not notify a decision within the period. Confirm the current figure from the Order text rather than assuming, and do not start work on the strength of a diary calculation alone.

Listed buildings, monuments and Article 4

Class J does not permit installation on a listed building or within the curtilage of one, and it does not permit installation on a building within a site designated as a scheduled monument.

For a listed commercial building the route is listed building consent alongside planning permission, supported by a Heritage Statement addressing reversibility, fixing methods, the visibility of the array from public viewpoints and the effect on the roofscape. Conservation officers are often more receptive than applicants expect where the roof is a later replacement and the panels are not visible from the street.

An Article 4 direction can withdraw Class J for a defined area. Check the council's register, because directions made for townscape reasons in commercial conservation areas do exist.

Where the building sits in a conservation area but is not listed, read the current text of Class J carefully rather than assuming the domestic restrictions apply. The classes are drafted differently and the restrictions are not the same.

Grid connection, inverters and the equipment on the ground

Grid connection is a contractual and regulatory matter between you and the distribution network operator. It is not a planning consideration and the council has no role in it.

What the council does have a role in is anything you build on the ground to make the connection work. A customer substation, a switchgear housing, an inverter cabin or a battery container standing in the yard is a building operation. Permitted development rights for electricity undertakers under Part 15 cover the network operator's own equipment, not yours, so budget for a planning application for the ground-level kit even where the roof array itself is permitted development.

That application is small, and it is best submitted at the same time as the prior approval so the two are considered together. A short Planning Statement linking the two consents keeps the case officer from treating the substation as an unexplained industrial box.

What to submit with the prior approval application

Keep it tight and complete. The application should carry:

• A site location plan and a roof plan showing panel layout, setbacks from the eaves and any maintenance access.
• Elevations and a roof section showing the mounting height against the ridge or parapet, with the 200 millimetre dimension marked.
• A short design and appearance statement covering colour, frame finish and visibility from public vantage points.
• A glare assessment for any receptor within a meaningful distance, including a statement of the aerodrome position where one is nearby.
• Confirmation of the capacity and the equipment schedule.

Where the array is part of a wider decarbonisation project, an Energy Statement is useful context for any associated planning application, though it is not required for the prior approval itself.

Browse all 66 PlanScope report types to see which documents a rooftop solar scheme and its ground-level equipment are 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, and remember that a lawful development certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.