Flat roof photovoltaics and permitted development limits
Rooftop solar rights sit in Part 14 of the GPDO, and the class depends on the building rather than the panels. Here are the limits, the prior approval step and when an application is safer.
Flat roof solar permitted development sits in Part 14 of Schedule 2 to the General Permitted Development Order 2015, and the class you need depends on the building rather than on the panels. Class A covers solar equipment on a dwellinghouse or a block of flats. Class J covers solar equipment on a non-domestic building. Classes B and K cover stand-alone installations on the ground.
Part 14 has been amended more than once, including changes in December 2023 that removed the capacity cap previously applied to rooftop solar on non-domestic buildings. Read the current consolidated text on legislation.gov.uk before you rely on any figure, including the ones below. That is not a disclaimer for its own sake. Installers quote limits from memory and the memory is often three amendments out of date.
The conditions attached to the solar classes
Every class in Part 14 comes with conditions, and they are conditions rather than guidance. Breach one and the installation is not permitted development at all, which means it is unauthorised rather than merely non-compliant.
The conditions that apply across the solar classes run along these lines. The equipment must be sited, so far as practicable, to minimise its effect on the external appearance of the building and on the amenity of the area. It must be removed as soon as reasonably practicable when no longer needed for microgeneration. And installations on listed buildings, scheduled monuments and parts of buildings within conservation areas or World Heritage Sites that would be visible from a highway are restricted or excluded.
For non-domestic buildings, Class J also brings in a prior approval step covering the design and external appearance of the equipment, including the effect of glare on the occupiers of neighbouring land. That is an application to the authority, with a determination period, and it needs to be in the programme rather than discovered a week before installation.
Projection limits above the roof plane
On a pitched roof the long standing limit is that equipment must not protrude more than 0.2 metres beyond the plane of the roof slope, measured perpendicular to the external surface. That is the figure most people know, and it is the reason panels on pitched roofs sit flush rather than tilted.
Flat roofs are treated differently, because a flush array on a flat roof faces the sky rather than the sun. The allowance is expressed as a height above the highest part of the roof, excluding any chimney, rather than as a perpendicular projection. Check the current figure in Part 14 for the class you are using, because this is exactly the provision that has been amended.
The practical consequence for a landlord is that a 10 or 15 degree tilt on a flat roof is usually achievable within the right, while a steeper tilt designed for winter yield may not be. Ask the installer to show the proposed array height on a section drawing against the stated limit before ordering frames.
Edge setbacks and the visual impact test
Two things keep an array off the roof edge. The Order's own conditions restrict installation within a stated distance of the external edge of the roof, and the siting condition requires the effect on external appearance to be minimised so far as practicable.
That second condition is the one that gets argued about. It is not a numerical test, so an enforcement officer looking at an array visible above a parapet from the street has room to form a view. The defensive answer is a set of photographs taken from the obvious viewpoints, showing that the array is not visible or is barely visible, produced before installation rather than after a complaint.
On a flat roof with a parapet, keeping the array below the parapet line solves the appearance question entirely and usually solves the wind loading question too. Where the roof has no parapet, a setback of a metre or more from every edge is the sensible starting point.
Arrays above a stated capacity
The 1 megawatt cap that used to apply to rooftop solar on non-domestic buildings was removed by amendment in December 2023, which changed the calculation for larger commercial roofs. Verify the current position for the class you are using.
Capacity matters for a separate reason that has nothing to do with planning. Connecting generation to the distribution network is governed by the Engineering Recommendations that the network operators apply, with smaller installations following a connect and notify route and larger ones requiring an application and an offer before connection. Grid capacity, not planning, is the usual reason a commercial rooftop scheme stalls.
Apply to the distribution network operator early. The planning position can be resolved in weeks and a constrained network connection can take far longer.
Buildings excluded from the right
| Situation | Position |
|---|---|
| Listed building | Listed building consent needed for works affecting character, and permitted development is restricted |
| Scheduled monument | Excluded |
| Conservation area or World Heritage Site | Restricted where the equipment would be on a wall or roof slope fronting and visible from a highway |
| Article 4 direction in force | The right may be removed entirely |
| Flats formed by conversion | Class A applies to a block of flats, but check the building's status and any conditions |
| Higher-risk building | Building control approval through the Building Safety Regulator applies to relevant work |
Leasehold and freehold consents sit alongside all of this. Permitted development is a planning concept and it gives no right to alter a building you do not own outright, so check the lease, the mortgagee's requirements and the buildings insurance before work starts.
A lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that an installation would be lawful. On a commercial roof where the array is large and the tenant is paying for it, that certificate is worth its fee.
Structural and fire considerations
Ballasted flat roof systems add load and change wind behaviour. The existing roof structure has to take the dead load of frames and ballast plus the uplift forces, so get a structural appraisal rather than a reassurance, particularly on older warehouse and retail roofs where the original design had no allowance for it.
Fire matters too. Array layout should leave access routes across the roof, keep clear of smoke vents and roof lights, and respect separation from boundaries. DC isolation, cable routing and labelling affect how the fire service can safely tackle a roof fire, and insurers increasingly ask about it. Get the fire engineer's view where the building has residential accommodation above a commercial floor.
Where a green roof is already in place or proposed, the two systems can be combined rather than competing for the same area, which we have looked at in biosolar roofs.
When an application is the safer route
Apply for planning permission, rather than relying on the right, in four situations. The array will be visible above a parapet from a public viewpoint. The building is in a conservation area and the roof is visible from a highway. The tilt or height is close to the stated limit. Or the installation forms part of a wider fabric upgrade that needs permission anyway, in the way that external wall insulation in a conservation area usually does.
An application also lets you present the carbon case. Where the array supports a policy compliant position on emissions, the figures belong in an Energy Statement or, for a wider scheme, a Sustainability Statement.
Illustrative example. A landlord adds a 200 panel array to the flat roof of a three storey converted office building. The installer proposes a 25 degree tilt on ballasted frames. A section drawing shows the rear rank standing above the parapet, so the array is redesigned at 10 degrees in east west orientation, sitting below the parapet on all elevations. A certificate of lawfulness is obtained and the installation proceeds without an application. The example is illustrative.
This is general guidance rather than 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. Enter your postcode on PlanScope to see which documents a rooftop energy scheme is likely to need.
Frequently asked questions
Which permitted development class covers rooftop solar?
Part 14 of Schedule 2 to the General Permitted Development Order 2015. Class A covers solar equipment on a dwellinghouse or a block of flats, Class J covers non-domestic buildings, and Classes B and K cover stand-alone installations on the ground. Part 14 has been amended several times, so check the current consolidated text.
How high can panels sit above a flat roof?
Flat roofs are treated by reference to height above the highest part of the roof, excluding chimneys, rather than by the 0.2 metre perpendicular projection limit that applies to a pitched roof slope. This provision has been amended, so confirm the current figure in Part 14 for the class you are relying on before ordering mounting frames.
Does a commercial rooftop array need prior approval?
Class J brings in a prior approval step covering the design and external appearance of the equipment, including the effect of glare on the occupiers of neighbouring land. That is an application to the local planning authority with its own determination period, so allow for it in the programme rather than discovering it late.
Is there still a one megawatt limit on rooftop solar?
The capacity cap that previously applied to rooftop solar on non-domestic buildings was removed by amendment in December 2023. Verify the current position for the class you are using. Capacity still matters for the network connection, which follows the network operators' engineering recommendations and is often the real programme constraint.
What about listed buildings and conservation areas?
Listed building consent is needed for works affecting the character of a listed building, and permitted development is restricted. On conservation areas and World Heritage Sites the right is restricted where equipment would be on a wall or roof slope fronting and visible from a highway. An Article 4 direction can remove the right entirely.
How do I prove an installation is lawful?
Apply for a certificate of lawfulness for a proposed use or development under section 192 of the Town and Country Planning Act 1990. The council decides it on the facts and the law rather than on planning merits, and the certificate protects exactly what the application describes, so submit accurate sections showing array height and setbacks.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.