Walkthrough: a solar canopy over a supermarket car park

Solar canopy car park planning applications almost never turn on the energy case. They turn on drainage and trees. The walkthrough below is illustrative, assembled from the issues these schemes raise rather than from any one real application, and it follows a photovoltaic canopy scheme over an existing supermarket car park from first sketch to decision.

The lesson at the end of it is simple. The carbon arithmetic took an afternoon. The column grid took four months.

The site and the energy case

An out of centre food store with a surface car park of roughly 180 spaces, built in the late 1990s, bitmac throughout with eleven mature trees in planted islands and perimeter lighting on 8 m columns. The operator wanted canopies over about two thirds of the parking, with the generation feeding the store directly and surplus exported.

The commercial logic was straightforward. Day load in a food store peaks when the sun does, chillers and lighting run constantly, and a canopy also gives shaded parking and a place to hang electric vehicle chargers.

None of that is a material consideration in its own right beyond the general policy support for renewable energy. Planning Practice Guidance on renewable and low carbon energy expects the environmental benefits to be given weight, and the current National Planning Policy Framework carries a clear direction of travel on low carbon energy. But support in principle does not answer where the columns go.

Whether the canopies were permitted development

The first question the agent was asked, and the answer was no.

Part 14 of Schedule 2 to the GPDO 2015 deals with renewable energy. Class J covers solar equipment installed on a non-domestic building. A freestanding canopy is not equipment on a building. Class K covers standalone solar on non-domestic land, and its limits are modest: a single installation within the curtilage, a ground area measured in single figures of square metres, and tight height and dimension caps. A canopy array over 120 parking spaces is several orders of magnitude outside that.

So a full planning application was needed. The site area also pushed the proposal over the 0.5 hectare threshold, which made it major development with a 13 week determination period and a lead local flood authority consultation.

Worth checking the current text of the Order before you advise on any of this, because Part 14 has been amended more than once and further change to the rooftop classes has been under discussion.

The street frontage and what the canopies look like from outside

The car park fronted a distributor road with a low brick wall and a hedge. A continuous 3 m high canopy deck behind that boundary reads very differently from open tarmac, particularly in winter when the hedge thins.

Three design moves settled it. The canopy was pulled back from the frontage by one parking bay depth so the boundary planting still read as the edge of the site. The deck was broken into bays following the aisle pattern rather than running as one plane. And the structure was specified in a dark recessive finish rather than galvanised steel, which flares badly in low sun.

The design and access statement had to explain the second of those properly, because an unbroken 90 m deck was the least expensive structural solution and the officer had seen the earlier drawings.

Surface water, and why a canopy changes the drainage

Here is where the scheme nearly failed, and it surprises people every time.

The car park was already impermeable, so there was no increase in impermeable area. The objection was about pathway and concentration. Rain that previously sheeted across the bitmac to a line of gullies now lands on a sealed deck, runs to a gutter, and arrives at a small number of downpipes as a concentrated flow at a much higher rate. The existing gully grid was never designed to take point discharges.

The lead local flood authority wanted three things:

1. A model of the revised flow routes showing the existing drainage can take the concentrated discharge, or the attenuation that makes it possible.
2. Treatment of the runoff, because the deck surface itself is clean but the discharge joins a car park system carrying hydrocarbons.
3. Confirmation that the exceedance route in a storm beyond the design event does not send water towards the store entrance.

The answer was downpipes discharging to a series of linear channels at the base of each column line, feeding geocellular attenuation beneath two parking bays per aisle, with a flow control before the existing connection. The drainage strategy set out the design event, the climate change allowance applied and the maintenance responsibilities, and it referenced the Defra non-statutory technical standards for sustainable drainage.

The incidental loss was the tree pits. Four of the eleven trees sat in soft landscaped islands that had been quietly taking runoff from the surrounding bays for twenty five years. Roof them over and that stops.

The car park trees and the column grid

A BS 5837:2012 survey came first, and it changed the layout more than any other single document.

Two trees were category A, six category B, three category C. Root protection areas, calculated from stem diameter under the standard, overlapped the preferred column positions in five places. Driven piles or pad foundations within a root protection area were not acceptable to the council's tree officer on the category A and B trees.

The resolution was unglamorous. Three column positions moved to the next bay line, which cost two bays of array. One canopy bay was omitted entirely over the largest tree, leaving a gap in the deck that the design statement had to justify as deliberate. Two category C trees were removed and replaced at a higher ratio than one for one.

The arboricultural impact assessment carried the tree protection plan, the construction exclusion fencing and a method statement for the two foundations that remained within a root protection area using a no dig approach. That document is the one the officer read twice.

Shading cuts both ways here, incidentally. Mature canopies over the array reduce output, and the operator had to accept lower generation in the two aisles nearest the retained trees rather than argue for removal. That trade is easier to make at design stage than at committee.

Lighting, signage and the grid connection

The existing 8 m columns no longer worked under a 3 m deck, so lighting was rebuilt as under canopy luminaires with the columns retained only on the uncovered aisles. That needed a lighting assessment showing illuminance within the car park, spill at the boundary and upward light, assessed against the Institution of Lighting Professionals guidance on obtrusive light and the environmental zone for the location. The under canopy arrangement actually reduced sky glow, which was a useful point to make in the planning statement.

Signage on the canopy fascia needed separate advertisement consent under the Control of Advertisements Regulations 2007. It was left out of the planning application entirely to avoid confusing the scope.

The grid connection brought an inverter and switchgear compound at the service yard edge, about 12 m², plus a cable route under two aisles. Where the distribution network operator carries out works as a statutory undertaker, different parts of the Order apply to it, so the application covered the customer side equipment and the housing around it. Electric vehicle charging points were added separately, since Part 2 of Schedule 2 permits much of that equipment in a non-domestic car park within stated limits.

The pack that went in

A planning statement, a design and access statement, the drainage strategy, the arboricultural impact assessment, the lighting assessment, a short transport statement confirming no change in parking capacity or servicing, and a landscape and visual appraisal of views from the distributor road.

Permission was granted with conditions on materials and finish, the tree protection method statement, a drainage verification requirement before first use, and a lighting specification to be approved. A condition also required the omitted canopy bay over the retained tree to stay omitted, which is a sensible piece of drafting that people forget to anticipate.

Compare this with the quite different set of questions a ground mounted array raises on farmland, where agricultural land classification does the work that drainage and trees did here.

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.

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