Renewable energy on a commercial roof: planning and grid

Commercial roof solar planning work has two gates, and only one of them is the council. Part 14 of Schedule 2 to the GPDO 2015 permits solar equipment on non-domestic buildings within stated limits, and since the 2023 amendment the former megawatt cap on non-domestic rooftop installations in England no longer applies. The gate that stops more schemes is the distribution network operator, because an export connection on a constrained network can cost more than the panels.

Settle the grid position before the planning position. It changes the scheme more.

When permitted development covers it

Class J of Part 14 covers the installation, alteration or replacement of solar photovoltaic and solar thermal equipment on a non-domestic building. The limits that matter in practice:

• equipment must not protrude more than 0.2 metres beyond the plane of the roof slope or the wall surface
• on a pitched roof it must not exceed the highest part of the roof, excluding any chimney
• on a flat roof it must not be within one metre of the external edge
• on article 2(3) land, which means conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads, installation on a wall fronting a highway is excluded
• nothing is permitted on a listed building or within the curtilage of a scheduled monument

Class J also carries conditions requiring siting to minimise the effect on the external appearance of the building and on the amenity of the area, and removal when no longer needed for generating electricity. Read the current text before relying on it, because Part 14 has been amended repeatedly and summaries age quickly.

The 0.2 metre projection limit is the one that catches flat roof schemes. A ballasted frame tilted at fifteen degrees on a flat roof will exceed it, so a flat roof installation of any useful tilt is normally outside Class J and needs an application. Panels laid flat or at a very shallow angle can stay within the limit, at a cost in yield.

Prior approval and what gets assessed

For larger installations on non-domestic buildings, Part 14 requires an application to the council for prior approval of the design and external appearance of the equipment. The scope of what the council may consider is narrower than a full application, which is the advantage.

Three points on handling it. The council has a set period to determine the prior approval, and the consequences of non-determination are stated in the Order, so a complete submission is worth more than an early one. The matters the council may consider are limited, so an objection about loss of parking or about construction traffic is not a matter it can refuse on. And a prior approval decision is not a planning permission, so conditions imposed on it attach to the right conferred by the Order rather than to a permission.

Where the building is listed or the installation sits outside Class J, a full application follows and the whole planning balance opens up. On a listed building the usual answer is panels on a flat roof behind a parapet, invisible from the street, or on an adjacent modern building. Listed building consent is needed for the fixings in either case.

Structural loading and fixing method

Roof structures on commercial buildings are designed close to their limits, and a solar array adds three different loads.

Dead load from the panels, frames and any ballast, which is modest for a mechanically fixed system and substantial for a ballasted one. Wind uplift, which is the load that actually governs the design and which is calculated under the Eurocode wind standard with the building's exposure, height and roof zones. And in some locations an altered snow load, because frames trap drift.

The fixing decision follows from the roof build-up:

1. Mechanically fixed through the membrane, which is the lighter option and the one that requires the membrane manufacturer's detail and warranty position to be checked
2. Ballasted, which avoids penetrations and adds weight, often at the roof zones where uplift is highest and ballast therefore heaviest
3. Adhered or bonded systems, where the roofing manufacturer offers a proprietary solution

Three things to resolve before the application. The structural capacity from the original design drawings, verified by a structural engineer rather than assumed. The remaining life of the waterproofing, because installing an array over a membrane with five years left means removing it to recover the roof. And the roof warranty, since penetrations by a third party routinely void it.

Where the building is leased, the roof is often retained by the landlord, and a licence to alter runs alongside everything else here.

Glint and glare near an aerodrome

Solar panels reflect, and the reflection matters in three directions: towards aircraft on approach, towards road and rail drivers, and towards nearby windows.

The aerodrome case is the one with a formal process. Aerodromes have safeguarding arrangements, and where a site falls within a safeguarded area the operator is consulted and may ask for a glint and glare assessment modelling reflections against approach and departure paths and against the control tower. Check whether the site is safeguarded at the outset, because the assessment has a lead time and the modelling can require a design change.

For highways and railways the question is whether reflection could dazzle a driver at a specific point, and it is usually answered by geometry rather than by modelling. For neighbouring windows the same applies.

Two practical notes. Modern panels have anti-reflective coatings and reflect considerably less than older glass, and an assessment should use the actual panel specification rather than a generic figure. And a reflection that occurs for twenty minutes on six mornings a year is a different proposition from one that occurs daily, so the modelling output should be expressed as duration and frequency rather than as a yes or no.

Grid connection and export limitation

This is the gate that closes schemes. Three routes exist depending on size and intention.

A small installation connecting under the simpler notification process can be registered with the network operator after installation. Anything larger needs an application to the network operator, an offer, and acceptance with payment before work starts.

Where the local network has no spare export capacity, the offer will carry a reinforcement cost that can run well beyond the cost of the array. Two ways round it. An export limitation device, which caps what the installation can push onto the network and lets the scheme size itself to on-site consumption instead. Or a flexible connection, where the operator can curtail export at times of constraint in exchange for a lower connection cost.

For a commercial occupier with high daytime demand, export limitation is often the better answer anyway, because the value of a unit consumed on site exceeds the value of a unit exported. Size the array to the site's daytime baseload, check the half-hourly consumption data rather than the annual total, and the business case usually improves.

Apply to the network operator before finalising the layout. A connection offer that arrives after the planning application has been submitted frequently changes the number of panels, and a revised layout means a new application or an amendment.

Fire service access on the roof

Fire and rescue services need to work on a roof with an array on it, and three requirements shape the layout.

Access routes across the roof, kept clear of panels, so a firefighter can reach any part of the roof and any roof opening. Clear zones over compartment walls, so an array does not provide a path for fire across a compartment line. And isolation, since the direct current side of a photovoltaic system remains live in daylight whatever the main switch is doing, which is why labelled isolation points and a clear schematic at the panel matter.

Set the access routes out on the roof plan at the design stage. Retro-fitting a clear route means removing panels, and the loss of yield is always larger than the loss from designing the route in.

The same coordination problem arises with other rooftop energy plant, and our note on energy centre flue heights and air quality covers the equivalent issue where combustion plant is involved.

Documents to submit

For a prior approval or full application, the pack that gets determined without further requests:

• roof plan showing the array layout, the clear access routes, the compartment line setbacks and the distance from the roof edge
• elevations and a section showing the panel height above the roof plane, dimensioned
• panel and frame specification, including the anti-reflective coating
• a structural note confirming capacity and the fixing method
• a glint and glare assessment where the site is safeguarded or where a specific receptor is identified
• the network operator's correspondence, which is not required but reassures the council the scheme is deliverable

Where the building is part of a wider development or a planning policy requires an energy strategy, the array belongs in the Energy Statement as part of the hierarchy rather than as a bolt-on, with the Sustainability Statement covering the policy compliance. Where a heat network is in prospect in the area, the roof plant layout should leave room for a future connection, as our note on safeguarding a future heat network connection explains.

Browse the report types to see which energy documents a commercial scheme usually carries.

This is general guidance, not legal advice. Structural, electrical and glint and glare assessments should be prepared by suitably qualified professionals. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.