Solar on a commercial roof: the Class J limits

Solar panels on a commercial building are usually permitted development under Class J of Part 14 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015. The limits that decide it are geometric rather than policy based. On a pitched roof the equipment must not protrude more than 0.2 m beyond the plane of the roof slope, measured perpendicular to the external surface. On a flat roof nothing may rise more than 1 m above the highest part of the roof, excluding a chimney, and nothing may sit within 1 m of the external edge.

Get those three dimensions wrong and the whole array is unauthorised development rather than only the part that breaches. That is the risk worth managing, because a retrofit on an occupied warehouse is not easy to unwind.

What Class J permits

Class J covers the installation, alteration or replacement of solar photovoltaic or solar thermal equipment on a building other than a dwellinghouse or a block of flats. Offices, warehouses, retail units, factories, schools and agricultural buildings all sit inside it, which is why the class gets used far more than its domestic equivalents.

The right is withheld in a series of situations. The ones that bite most often are:

• Installation on a listed building, or within the curtilage of a listed building.
• Installation on a scheduled monument or within its site.
• Installation in a conservation area or World Heritage Site where the equipment would go on a wall or roof slope that forms the principal elevation and fronts a highway.
• Any protrusion beyond the limits set out above.

Two conditions then attach to anything installed under Class J. The equipment must, so far as practicable, be sited 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. Both are enforceable, and the siting condition is the one an environmental health or planning officer reaches for when glint and glare complaints arrive from a neighbouring office.

The 0.2 m measurement in practice

The projection limit is measured from the external surface of the roof slope, perpendicular to it. For an in-plane mounting system on a profiled metal roof, that leaves very little room once you account for the rail, the clamp and the module frame.

Standing-seam clamps and trapezoidal brackets typically keep a system inside the limit. Tilted frames on a pitched roof do not. Where an installer proposes to tilt modules on a shallow pitch to improve yield, check the total build-up before assuming the class applies, and measure to the highest point of the module rather than to the rail.

Capacity limits and why the number keeps moving

Class J has carried a generating capacity limit for most of its life, and that is the provision most often amended.

Until amendments made in 2023 the class was capped at 1 megawatt of installed capacity, which put a ceiling on rooftop arrays on large distribution sheds. The 2023 changes relaxed the position for rooftop installations as part of a wider push on renewables, with additional procedure for larger schemes. Because the drafting has been revisited more than once and may be again, read the current text of Part 14 on legislation.gov.uk before you design to a figure. Older technical guidance and older trade articles still quote the previous cap.

The number that constrains most real projects is not in the Order at all. It is the capacity the distribution network operator will accept. A connection application for a generator of any size goes to the network operator, and the answer may be a lower export limit, a requirement for export limitation equipment, or a reinforcement cost that reshapes the business case. Run that enquiry in parallel with the planning question, not after it.

Flat roofs, ballast and parapets

On a flat roof the 1 m height limit and the 1 m edge set-back work together, and the edge set-back is the one people forget.

A parapet does not change the height measurement, because the test is height above the highest part of the roof. It does help with visual effect, and on a scheme in a sensitive setting a parapet-concealed array is much easier to defend under the siting condition.

Ballasted systems raise a separate issue. Adding 15 to 25 kg per square metre across a roof deck designed in the 1980s is a structural question before it is a planning one, and the wind uplift on an unfixed system at the roof perimeter is exactly why the 1 m set-back is useful anyway.

Designated land, listed buildings and the application route

Where Class J does not apply, the route is a planning application, and in a conservation area that is usually straightforward if the array is not on the principal elevation.

For a listed building the position is different. Listed building consent is required for works affecting the special architectural or historic interest of the building, and an array on a slope visible in long views will need a proper assessment of harm. A Heritage Statement setting out significance, the degree of harm and the public benefit of the carbon saving is the document that carries the case. Reversibility matters: fixings into mortar joints rather than through historic fabric, and a stated removal commitment.

Agricultural buildings converted to dwellings under Class Q sit outside Part 1 roof rights, and the same logic about building history applies to solar schemes on converted structures. Our note on Class Q structural works and what is reasonably necessary covers how far a building can be altered under a prior approval class without needing a full application.

An illustrative example

This example is illustrative. A 4,000 m2 distribution unit built in 1994 has a profiled metal roof at 6 degrees. The installer proposes 10 degree tilted frames to lift winter yield. The build-up measures 0.42 m perpendicular to the roof plane at the rear of each row, which exceeds the 0.2 m limit, so the scheme as drawn is not permitted development. Redesigning to in-plane mounting brings it to 0.14 m and keeps the array within Class J. Yield falls by a single-figure percentage. The owner accepts that rather than making a planning application and waiting eight weeks.

Structural capacity as the real constraint

The limiting factor on most commercial retrofits is the roof, not the Order.

A structural appraisal should look at the existing purlin and rafter capacity, the condition and remaining life of the sheeting, the fixing method and the wind loading on the modified roof form. Older industrial roofs may be asbestos cement, which is fragile, and work on or near it falls under the Control of Asbestos Regulations 2012 and the Health and Safety Executive guidance on fragile roofs. Access, edge protection and fall arrest all have to be designed in. A Structural Survey is the normal way to record the appraisal, and it is also what an insurer will ask for.

Where the array forms part of a larger scheme requiring permission, the carbon and energy case belongs in an Energy Statement rather than being asserted in a covering letter, and the planning case itself in a Planning Statement.

Read the current Part 14 text, measure the build-up, and get the network operator's answer before ordering modules. Enter your postcode on PlanScope to see which reports a scheme on your site 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, and remember that a lawful development certificate is the only formal way to confirm that works fall within permitted development. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.