Solar Overplanting Gets Legal Nod in Landmark High Court Ruling

In Ross v Secretary of State for Housing, Communities and Local Government and Renewable Energy Systems Ltd [2025] EWHC 1183 (Admin), Mr Justice Eyre dismissed a statutory review challenge to a planning Inspector’s decision to grant planning permission, on appeal, for a 49.9MW solar farm.

The decision provides guidance on the interpretation of National Policy Statement for Renewable Energy Infrastructure EN-3 in the context of a solar farm proposal, in particular, the practice of ‘overplanting’ solar panels.

The facts in relation to overplanting will be specific to each site and will have to be assessed on a case by case basis, however, we consider that the High Court’s judgment provides useful takeaways for solar farm developers.

• NPS EN-3 may be a material consideration for projects near the 50MW threshold.
  
  Even where the proposal falls below the NSIP threshold, NPS EN-3 may still apply as a material consideration. Mr Justice Eyre endorsed the Inspector’s approach in applying the NPS due to the scale and context of the scheme. The Court also reaffirmed that there is no impropriety in structuring a project to remain under the NSIP threshold.
  
  
• Overplanting can be justified for multiple reasons, not just panel degradation.
  
  The Court confirmed that overplanting for purposes such as STC variation and site maximisation is not inconsistent with NPS EN-3. The policy does not prescribe a closed list of acceptable reasons for overplanting. This clarification strengthens the legal footing for developers who justify overplanting based on operational performance and energy yield maximisation, provided that the export does not exceed the NSIP threshold and the impacts of the development are assessed on the basis of its full extent (including overplanting).
  
  
• Developers should provide a clear technical justification for overplanting
  
  If overplanting is proposed, a technical report or note explaining (1) the purpose of overplanting; (2) how it achieves that purpose; and (3) the expected impact on site performance and land use will assist in ensuring the proposal withstands scrutiny and can be justified.
  
  
• Define physical and technical limits clearly, even if exact panel numbers are unknown
  
  Even if an application does not fix exact panel counts, it’s essential to show the limits within which the development will operate. Explaining natural constraints (e.g. spacing, terrain) and using design principles in a technical report may protect against challenges based on project uncertainty.

Background

The case concerned the Longhedge Solar Farm, proposed by Renewable Energy Systems Ltd (RES), located on a site of approximately 94 hectares near Hawksworth and Thoroton. The proposed development had an installed capacity of around 78.54 MW (DC), but its grid export capacity was expressly limited by condition to 49.9 MW (AC) - just under the 50 MW threshold that would bring it within the Nationally Significant Infrastructure Projects (NSIP) regime under the Planning Act 2008.

The local planning authority had initially refused permission, but this decision was overturned on appeal by a Planning Inspector, who granted permission subject to conditions. A member of a local action group challenged the Inspector’s decision on six grounds, primarily arguing that the Inspector misapplied the National Policy Statement for Renewable Energy Infrastructure in allowing the development to proceed with such a significant level of overplanting.

Although the proposed development fell below the 50MW NSIP threshold under the Planning Act 2008, the Inspector treated the National Policy Statement as a material consideration, given the proximity to the threshold.

The Key issues

The case turned on three principal questions:

1. Does the proper interpretation of the provisions of National Policy Statement for Renewable Energy Infrastructure EN-3 permit overplanting of solar panels for reasons beyond panel degradation?
2. Was the Inspector required to assess the “reasonableness” of overplanting as a separate consideration?
3. Did the Inspector fail to properly assess the proposal on a worst-case basis, or fail to adequately address all the issues raised before him?

What is ‘Overplanting’

“Overplanting” refers to an arrangement whereby more solar panels are installed than would be needed for a solar farm with a particular AC capacity.

Overplanting is referenced in the National Policy Statement for Renewable Energy Infrastructure (EN-3) at paragraph 2.10.55 and footnote 92, as follows:

2.10.55: “The installed generating capacity of a solar farm will decline over time in correlation with the reduction in panel array efficiency. There is a range of sources of degradation that developers need to consider when deciding on a solar panel technology to be used. Applicants may account for this by overplanting solar panel arrays. 92”

92: “Overplanting” refers to the situation in which the installed generating capacity or nameplate capacity of the facility is larger than the generator’s grid connection. This allows developers to take account of degradation in panel array efficiency over time, thereby enabling the grid connection to be maximised across the lifetime of the site. Such reasonable overplanting should be considered acceptable in a planning context so long as it can be justified and the electricity export does not exceed the relevant NSIP installed capacity threshold throughout the operational lifetime of the site and the proposed development and its impacts are assessed through the planning process on the basis of its full extent, including any overplanting.”

The developer had explained that the overplanting was to address three factors, namely:

1. The fact that solar panels degrade over time with the consequence that the electricity generated from a bank of a fixed number of panels will reduce over time (“module degradation”).

2. The fact that the maximum output of a solar panel is determined in laboratory conditions (“the STC rating”) but the actual output of any given panel in the field will be less than in laboratory conditions.

3. The combined effect of configuration of the site and of fluctuations in the level of sunlight over the course of the day and of the year which meant that more panels would be needed to ensure an export capacity of 49.9MW for a greater part of the day and of the year (“site maximisation”).

The Court’s Decision

The principal argument was that the Inspector had erred in interpreting NPS EN-3 paragraph. 2.10.55 and footnote 92 as allowing overplanting for reasons beyond panel degradation.

It is worth noting that the Claimant’s planning consultant had previously written to the Secretary of State for clarity on policy EN-3 and the Claimant had sought to rely on a ministerial response stating that “In the Energy Policy Statement EN-3 guidance, overplanting is countenanced where reasonable, to address panel degradation. Unreasonable overplanting, or overplanting for any other reason, would not be supported. It will be a matter of planning judgement for the decision maker in any case to decide what the purpose of the overplanting is and whether it is reasonable…”

The Court rejected this argument, affirming that the interpretation of planning policy is an objective legal exercise, not dictated by subjective statements from ministers or departments. The Court reaffirmed the well-established legal principles in relation to interpretation i.e. policies should be read as a whole, having mind to the objectives and exercising great restraint before implying any terms.

The Court found that EN-3’s wording did not limit overplanting to degradation purposes alone. The policy did not expressly purport to address all forms of overplanting. Rather, the policy focuses on the impacts of the proposal. Just because one justification is mentioned expressly does not imply that others are impermissible. That implication is neither natural nor necessary.

Mr Justice Eyre concluded that overplanting to accommodate STC performance variations and to maximise generation across daylight hours is consistent with EN-3’s objectives. It was held that overplanting could be acceptable as long as it can be justified, the export capacity remains below the NSIP threshold, and the full impacts of the development, including overplanting, are assessed through the planning process.

Further, the Claimant argued that overplanting required a standalone “reasonableness” test prior to applying the planning balance. Mr Justice Eyre disagreed, finding that EN-3 requires a justification, not a two-stage reasonableness analysis. He concluded that it is sufficient that the decision-maker considers whether the overplanting is justified in light of its purpose and impacts.

In this case, the Inspector had proactively sought and received a technical note from RES explaining the rationale, extent, and impact of the proposed overplanting. That level of scrutiny satisfied the Court that the justification had been properly considered.

The Claimant alleged that the Inspector failed to assess the scheme on a worst-case basis due to the absence of an exact panel count and information outlining the panel density. The Court rejected this, noting that while precise numbers were not fixed, there were sufficient physical and planning constraints in place, including:

• conditions limiting the build area;
• maximum panel heights and clearances;
• landscape and biodiversity mitigation strategies; and
• operational constraints related to shadowing and maintenance.

The Court acknowledged that not all information is always available at the point of decision and a flexible but evidenced approach is acceptable.

Mr Justice Eyre held that the potential loss of energy as a consequence of clipping was not an obviously material consideration which the Inspector was required to take into account as part of the planning balance.