Acting as an expert witness at a planning appeal

An expert witness at a planning inquiry owes a duty to the inquiry that comes before the duty to the client paying the fee. That is not a sentiment. It governs what goes into a proof of evidence, what must be disclosed even when unhelpful, and what happens to the appellant's costs when a witness defends a position they cannot support. Understanding expert witness planning appeal duties before accepting the instruction is what keeps a consultant out of difficulty.

The practical expression of the duty is simple to state and hard to follow: say the same thing in the inquiry room that you said in the office.

Duty to the inquiry against duty to the client

The client instructs you, pays you and has a legitimate interest in the case being put at its strongest. None of that entitles them to shape your opinion.

The distinction worth holding onto is between the case and the evidence. The case is the argument, and it belongs to the advocate and the client. The evidence is your professional opinion on the matters within your expertise, and it belongs to you. An advocate may ask you to emphasise a point, to answer a question more clearly, or to address an issue you had not covered. They may not ask you to change a conclusion.

Where the client's instructions and your opinion diverge irreconcilably, the options are to narrow your scope so that you are not giving evidence on the disputed matter, or to withdraw. Both are better than giving evidence you do not hold.

A related practical point: if your opinion changes during the preparation of the case, say so immediately and in writing. An opinion that shifts the week before the inquiry is manageable. One that shifts under cross-examination is not.

What a proof of evidence must contain

A proof is your evidence in chief, written. It should stand on its own, be readable by someone who has not followed the case, and contain nothing you cannot defend line by line.

The structure that works:

1. Your qualifications, experience and the basis of your expertise, including institute membership and any relevant chartered status.
2. Your instructions and the scope of your evidence, stating what you were asked to address and what you were not.
3. A declaration of the nature of your involvement, including any previous involvement with the site or the parties.
4. The factual background, cross-referred to the statement of common ground rather than repeated.
5. Your analysis of the matters within your expertise, with the methodology stated and the data sourced.
6. Your conclusions, expressed as opinion and distinguished from fact.
7. Appendices containing the material you rely on, paginated and referenced from the text.

Two disciplines separate a good proof from a weak one. Every number has a source the inspector can check. And every opinion is framed as an opinion, so that a reader can see where fact ends and judgement begins.

Length is a consideration. A proof that repeats the statement of case, the officer report and the technical appendices is longer and less useful than one that addresses the matters in dispute. Where a summary proof is required, write it as a standalone document rather than extracting paragraphs.

Statements of common ground and their discipline

The statement of common ground imposes a useful discipline on expert evidence, because it forces agreement on facts before opinions are exchanged.

Use it. Where the experts have agreed the measured background sound level, the trip rates or the biodiversity baseline, your proof can proceed to the matter in dispute instead of defending arithmetic. Where the other side will not agree a figure, the statement records that, and the inspector knows the dispute is live.

Signing a statement of common ground commits you. A witness who agrees a figure and then qualifies it in evidence has damaged their own credibility and the client's case. Read every line before signing, and where your agreement is conditional, state the condition in the statement rather than reserving it for the inquiry.

Cross-examination and conceding a point

Cross-examination tests whether your evidence holds. It is not a personal contest, though it can feel like one.

Four habits help. Answer the question asked, not the question you would prefer. Where the answer is yes, say yes. Where you do not know, say you do not know, and offer to find out if the inspector wishes. And where a proposition put to you is correct, accept it.

Conceding a point does not lose a case. Refusing to concede an obvious point loses credibility on every other point, and inspectors say so in decision letters. The witness who accepts that a tree will be lost, that the site is in the Green Belt, or that a figure in their appendix contains an error, and then explains why the conclusion survives, is more persuasive than the witness who defends everything.

Where the concession is substantial, say what follows from it. "I accept that figure. It does not change my conclusion because the assessment is sensitive to the other variable, for the reasons at paragraph 4.12" is a complete answer.

One thing to avoid: do not answer on matters outside your expertise. A transport witness asked for a heritage opinion should decline, politely and clearly.

Separating advocacy from evidence

The line between the two is sharper at an inquiry than in the rest of planning practice, where consultants routinely advocate for their clients.

In the inquiry room, the advocate argues and the witness gives evidence. A witness whose proof reads like a submission, with rhetorical questions and attacks on the authority's motives, invites the inspector to treat it as argument rather than expert opinion, which reduces its weight.

The signs to edit out of a draft proof are familiar: characterising the council's position rather than analysing it, adjectives doing the work of analysis, selective quotation, and conclusions that reach beyond the evidence presented. The question on reading back is whether a reasonable professional instructed by the other side could have written any of these sentences. If not, the sentence is probably advocacy.

Where the appeal turns on technical reports, the author of each report should ordinarily give the evidence on it. A planning witness adopting a specialist's conclusions without the competence to defend them is exposed, and the article on where liability sits with sub-consultants covers the chain of responsibility behind that.

Declaring interests and independence

Disclose anything a reasonable person might regard as affecting your independence. Previous work for the appellant on the same site. A financial interest in the outcome. A fee arrangement contingent on success, which is incompatible with giving expert evidence. A past professional relationship with the authority's witness. Previous evidence you have given that is inconsistent with the position you now take.

The last of these catches people out. Inquiry evidence is public, and a witness who argued the opposite position two years ago should address it rather than hope nobody finds it. Explaining why the facts or the policy differ is straightforward. Being confronted with your own earlier words is not.

Independence also has a practical dimension around competence. Your institute's code will require you to work within your competence and to maintain it, and the point of checking a professional institute register applies to your own entry as much as to anyone else's.

Costs risk from unreasonable evidence

The costs regime at appeals allows an award where a party has behaved unreasonably and caused another party wasted expense. Evidence is one of the routes to that finding.

The behaviours that attract costs applications include pursuing a ground of appeal with no reasonable prospect of success, introducing substantial new evidence late without explanation, failing to produce evidence the inspector directed, withdrawing a case at the last moment after the other side has prepared, and persisting with a position that collapses under cross-examination.

For a consultant, the exposure is reputational and sometimes contractual rather than direct, since the award falls on the party. But a costs decision naming the inadequacy of the expert evidence is a document that circulates, and the piece on professional indemnity insurance for a planning consultant sets out where the risk sits.

The protection is unglamorous. Advise the client honestly on prospects before the appeal is lodged, keep a written record of that advice, and decline to give evidence you cannot support. A declined instruction costs one fee. A costs award costs more than that.

If you are assembling the technical evidence base behind an appeal, the 66 report types by category index shows where each document sits, and the professional directory is one route to a reviewer with the right competence. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.