Rooftop solar on commercial buildings and the grid limit
Planning is usually the easy part of a commercial rooftop solar scheme. The Part 14 limits, the safeguarding questions and the connection offer that sets array size.
On most commercial buildings the planning route for rooftop solar is straightforward and the grid connection is not. Commercial rooftop solar planning usually proceeds under Part 14 of Schedule 2 to the General Permitted Development Order 2015 rather than through a full application, so the array size ends up being set by the distribution network operator's export limit and by the roof structure, not by the planning officer.
The permitted development route for roof-mounted panels
Class J of Part 14 allows the installation, alteration or replacement of solar photovoltaic or solar thermal equipment on a non-domestic building, subject to limits and conditions. A 2023 amendment to the Order removed the capacity cap that previously applied to these installations, so the megawatt figure that used to drive scheme design no longer bites. Confirm the current wording before you rely on it, because Part 14 has been amended several times.
The route is closed in three situations. A listed building needs listed building consent for the works and planning permission for the installation. A scheduled monument is excluded. And in a conservation area or a World Heritage Site, equipment on a wall or roof slope forming the principal elevation and fronting a highway falls outside the class.
Class J also carries a prior approval step covering design and external appearance, so the council has to be approached before work starts even where the limits are met. Treat it as a short application rather than a formality: a prior approval refused on appearance is harder to recover from than a well-presented submission in the first place.
Height, projection and area limits
The limits in Class J are dimensional and they are the reason flat-roof schemes get redesigned.
| Constraint | Typical limit in Class J |
|---|---|
| Projection beyond a roof slope | 200mm measured perpendicular to the plane of the slope |
| Height above the highest part of a flat roof | 1 metre, excluding any chimney |
| Distance from the external edge of the roof | 1 metre set back from the edge |
| Equipment no longer needed | Removed as soon as reasonably practicable |
Check those figures against the current text of the Order, since they have been adjusted. The practical consequence is that a tilted array on a flat roof has to sit within the height allowance, which for a 10 or 15 degree frame restricts panel row length, and the one metre edge set-back removes a surprising amount of a small roof.
Where the building is agricultural rather than commercial, different classes apply and the reasoning differs, as set out in roof-mounted solar on agricultural buildings.
Glint and glare near aerodromes
Reflection from panels is the issue that most often turns a permitted development scheme into a full application, and it arrives through safeguarding rather than through planning policy.
Where the building sits inside an aerodrome safeguarding zone, the airport operator is consulted and will normally want a glint and glare assessment covering the approach paths and the control tower, using an accepted modelling methodology. Civil Aviation Authority guidance sets the framework, and the operator's own safeguarding team sets the acceptance criteria. Ministry of Defence safeguarding applies around military aerodromes and some technical sites.
Two mitigations work. Changing the tilt or azimuth of affected panel rows, and using anti-reflective coated modules with the manufacturer's reflectivity data submitted. Removing panels from the affected part of the roof is the fallback and it is the one that hurts the business case.
Road and rail receptors come up as well. Assessment against a driver's or signaller's sight line is a narrower exercise than the aviation case, and it rarely changes a rooftop scheme.
Structural capacity of the existing roof
A ballasted flat-roof array adds dead load that many older industrial roofs were never designed for, and a frame-mounted array adds wind uplift at the perimeter, which is where the one metre set-back earns its place.
Commission a structural appraisal of the existing roof before the array layout is fixed. The appraisal needs the actual purlin and sheeting specification rather than the original design intent, because reroofing and overcladding during the building's life change the capacity. A Structural Survey of the existing building is the usual starting document, and on a leased building the landlord will want to see it before giving consent.
Two findings recur. The perimeter zones cannot take the uplift without additional fixings, and the purlin capacity limits the ballast, which pushes the design towards a mechanically fixed system and a warranty conversation with the roof manufacturer.
Export capacity and the connection offer
This is the constraint that actually sizes most commercial arrays.
Generation above the thresholds for the smaller connection standard needs an application to the distribution network operator, which responds with a connection offer stating the export capacity available and the cost of any reinforcement. In a constrained part of the network the offer may permit a fraction of the array's nameplate output, or come with a reinforcement cost that exceeds the panels.
Four responses are available, and most schemes use more than one.
- Size the array to on-site demand and apply for an export limitation scheme, so the inverters curtail rather than export.
- Accept a flexible or managed connection, which allows export when the network has headroom.
- Pay for reinforcement where the payback supports it.
- Add storage to shift generation into periods of on-site demand.
Apply to the network operator before you commit to a panel count. The planning route is quick and the connection process is not, and a prior approval obtained for an array the network will not accept is wasted work.
Battery storage alongside the array
Electricity storage in England was taken out of the national infrastructure consenting regime under the Planning Act 2008, other than pumped hydro, so even large battery schemes are determined by the local planning authority. For a rooftop solar project the battery is usually a containerised unit at ground level, and it is the battery rather than the panels that generates the planning issues.
Expect questions on fire separation distances and emergency access, on noise from inverters and cooling, on the containers' appearance, and on decommissioning. Fire and rescue consultation is routine. The sizing logic and the consenting issues are covered in battery storage alongside a solar scheme.
A battery at ground level will not normally be permitted development, so a scheme with panels under Part 14 and a battery needing permission produces two parallel processes on one site. Programme them together.
Documents for a full application
Where the permitted development route is unavailable, because of listing, a conservation area frontage or a scheme that exceeds the limits, the submission is modest by the standards of most commercial applications.
- Site and roof plans with the array layout, showing the edge set-back and panel heights.
- Elevations and a section through the array showing the projection above the roof.
- A design and access statement, short, covering visibility from public vantage points.
- A glint and glare assessment where safeguarding applies.
- A structural appraisal of the roof.
- A heritage statement where the building is listed or in a conservation area.
Where the array contributes to the carbon case for a wider development, the generation and the carbon saving belong in the Energy Statement for that scheme rather than in a standalone report, with the export limitation stated so the figures reconcile.
This is general guidance, not legal advice. Check the current wording of the General Permitted Development Order and take advice from a qualified professional and your local planning authority 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.
Frequently asked questions
Is rooftop solar on a commercial building permitted development?
Usually yes, under Class J of Part 14 of Schedule 2 to the General Permitted Development Order 2015, subject to dimensional limits and conditions. Listed buildings and scheduled monuments are excluded, and in a conservation area equipment on a principal elevation fronting a highway falls outside the class. Class J also includes a prior approval step on design and external appearance.
Is there still a capacity cap on rooftop solar?
The capacity cap that previously applied to non-domestic rooftop installations was removed by a 2023 amendment to the Order, so the limiting factors are now the dimensional conditions, the roof structure and the grid connection. Part 14 has been amended several times, so confirm the current wording before relying on it for a particular scheme.
When is a glint and glare assessment needed?
Where the building sits within an aerodrome safeguarding zone, the airport operator is consulted and will normally require an assessment covering approach paths and the control tower. Ministry of Defence safeguarding applies around military aerodromes. Road and rail receptors are occasionally raised but rarely change a rooftop scheme, unlike a ground-mounted one.
Why does the grid connection limit the array size?
Because the distribution network operator's connection offer states how much export capacity is available at that point on the network, and in a constrained area that may be well below the array's nameplate output. The options are export limitation so the inverters curtail, a flexible connection, paying for reinforcement, or adding storage to use generation on site.
Does a battery need separate planning permission?
Normally yes. A containerised battery at ground level is not usually permitted development, so a project can run the panels under Part 14 and the battery through a full application at the same time. Electricity storage in England sits outside the nationally significant infrastructure regime other than pumped hydro, so the local planning authority determines it.
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.