Cumulative Impact: The Blind Spot in Consultation Law

In February 2026, the Court of Session quashed a wind farm’s planning permission because its grid connection had been assessed as a separate project. The same blind spot shows up in NHS reconfigurations and net zero delivery. Here’s why cumulative impact needs its own place in consultation practice, and what the case law now requires.

Why does splitting a decision into parts change what gets consulted on?

In February 2026, the Inner House of the Court of Session quashed planning permission for an eight-turbine wind farm because the reporter, a Scottish Government official appointed to decide the case on behalf of Scottish Ministers, had never properly asked whether the turbines and their grid connection were, in law, one project. Consulted and assessed separately, each part looked proportionate. Together, they were the development. The court noted that the same separation had gone unchallenged in 28 other UK onshore wind farm cases (Raeshaw Farms Ltd v Scottish Ministers [2026]).

This is not a quirk of one Scottish case. It is the predictable result of consulting on decisions the way organisations are structured to make them: one scheme, one business case, one committee report, one consultation. The question a resident or an objector actually needs answered (what is this when taken as a whole?) rarely maps onto that structure.

What does the law say about assessing linked decisions together?

English planning law reached a similar conclusion by a different route. In R (Ashchurch Rural Parish Council) v Tewkesbury Borough Council [2023], the Court of Appeal quashed planning permission for a road bridge because the council had assessed it apart from the housing development it existed to serve. Looked at alone, the bridge’s environmental effects were negligible. Looked at as part of what it actually enabled, they were not. The court’s term for this practice, salami slicing, has stuck for a reason: each slice looks thin enough to swallow, and the whole is never served.

Both cases apply the same underlying principle: an environmental impact assessment must address the project a decision-maker is actually approving, not a narrower version of it. The same logic reaches into consultation more widely. The Gunning principles, the common law test for fair consultation, require conscientious consideration of what consultees actually said, and a decision-maker cannot conscientiously consider a cumulative impact that no consultation ever asked about.

Where else does this pattern show up beyond infrastructure?

NHS service change frequently proceeds by a sequence of individually defensible decisions: a reduction in one service, a change of site for another, a review of a third, each consulted on to the required standard, each considered apart from what has already changed in the same area. No single consultation asks the question a patient is actually living through, what does healthcare in this place look like now, after all of it.

Net zero delivery shows the same shape. A heat network, a grid reinforcement, an electric vehicle (EV) charging rollout can each be consulted on as a stand-alone scheme, on the same street, within a few years of each other, without any single consultation asking what the street has actually been asked to carry across all three. The Principles for Navigating the Social Aspects of Grid Transformation name cumulative tracking as a distinct requirement for exactly this reason.

What should practitioners do to consult on cumulative impact properly?

1. Define the real project or programme before designing the consultation, not the version that is administratively convenient. Ask whether a reasonable objector would see the parts as one thing.
2. Track burden at the level of the place or population affected, not the level of the individual decision, so a second or third related scheme is visible to the decision-maker as part of a pattern.
3. State explicitly, in the consultation material itself, what has and has not been bundled together, and why, so the boundary is a reasoned choice rather than an unexamined default.
4. Where a related decision genuinely cannot yet be assessed, a grid connection route not yet fixed, a future phase not yet designed, say so, and explain how the cumulative effect will be revisited once it can be, rather than treating the gap as closed.

Courts are not asking organisations to consult on everything at once. They are asking them to be honest about what the real decision is, and to make sure that decision, not a narrower version of it, is what consultees were actually asked about.

How tCI Can Help

Advice and Guidance
A tCI faculty member will work alongside you to support the development of your decisions and engagement approach. We provide independent, constructive advice at critical stages, helping you strengthen stakeholder mapping, test communication strategies, and plan robust post-decision engagement. Our role is to act as a critical friend, offering practical recommendations grounded in consultation law and good practice that build confidence in your process.

Risk Assessment
Early identification of legal, political or reputational risks in your engagement approach. Using tCI’s five-risk methodology, we spot gaps before challenge arises, helping you strengthen stakeholder communication and demonstrate procedural fairness from the outset.

Executive Briefings
Concise updates for senior leaders on consultation law, engagement duties and post-decision risks. Helps boards and leadership teams make confident, defensible decisions when under pressure, with clear guidance on what good engagement looks like after difficult choices are made.

Whether you’re preparing for a high stakes service change or building defensible evidence for complex decisions, we can help.

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