Expert witness duties at a planning inquiry

An expert witness at a planning inquiry owes their primary duty to the inquiry, not to the party paying the fee. That single rule governs everything else: what goes in the proof of evidence, what you concede in cross-examination, whether you can act at all where you designed the scheme, and how you handle a client who wants a conclusion your data will not support. Inspectors are practised at spotting a witness who has forgotten it, and the consequences run from evidence given no weight to an award of costs against the party who called you.

Where the duty comes from

Planning inquiries are not courts, and witnesses are not normally on oath. The obligation is nonetheless well settled. It comes from the general law on expert evidence, from the Planning Inspectorate's own procedural guidance, and from the codes of the professional institutes.

The Royal Institution of Chartered Surveyors operates a mandatory practice statement for members acting as expert witnesses, which sets out the duty to the tribunal, the required declarations and the treatment of conflicts. The Royal Town Planning Institute's code of professional conduct requires members to act with competence, honesty and integrity, and not to make statements they know to be misleading. Ecologists bound by CIEEM, acousticians by the Institute of Acoustics and fire engineers by the Institution of Fire Engineers work to equivalent obligations.

The practical effect is the same across all of them. You give the opinion your evidence supports, you say where it is uncertain, and you say so in writing before anyone asks you in the room.

The documents you will produce

An inquiry generates a sequence of documents, and each has a different job.

The statement of case sets out the full particulars of the case a party intends to put, and it is filed early. The proof of evidence is your own signed evidence on your own subject area. Where a proof runs to more than 1,500 words, a summary is required, and the summary is what you will read out. Appendices carry the survey data, the modelling outputs and the policy extracts. Rebuttal proofs answer the other side's evidence and are permitted on a tight timetable, usually only with the inspector's agreement.

Proofs of evidence and their summaries are normally due no later than four weeks before the inquiry opens. The Procedural Guide for planning appeals in England sets the current timetable and should be read at the start of every instruction, because the arrangements have been revised repeatedly since the Rosewell review reshaped inquiry procedure.

What belongs in a proof, and what does not

A proof states your qualifications and experience, the instructions you received, the facts you relied on, the method you applied, the results, your opinion and the reasons for it. It ends with a declaration that the opinions expressed are your true and professional opinions.

Three things do not belong in it.

The first is advocacy. Arguing the planning balance is the advocate's job, and a witness who does it invites the inspector to treat the whole proof as partisan. The second is evidence outside your field, however tempting: an acoustician commenting on heritage significance is easy to dismantle. The third is a conclusion that your data does not reach. If the survey shows a moderate adverse effect, the proof says moderate adverse, whatever the client would prefer.

Say what you were not asked to do as well. A proof that records the limitations of the survey, the assumptions in the model and the areas where further work would reduce uncertainty is stronger, not weaker, in the room.

Statements of common ground

The statement of common ground is agreed between the parties and filed before the inquiry. Its purpose is to strip out the matters nobody disputes so that inquiry time goes on the real disagreement.

Take it seriously. A well-drafted statement of common ground on your subject can remove your appearance entirely, which saves the client several days of fees. Where agreement is partial, record precisely what is agreed and what is not, in numbered paragraphs, rather than producing a bland document that leaves the inspector to work out where the fight is.

Do not agree a fact you have not checked because agreement feels cooperative. You will be cross-examined on it.

Under cross-examination

The advocate's aim is to narrow your evidence until it no longer supports the case that relies on it. The counter is simple to describe and hard to do.

Answer the question that was asked. Concede the point that must be conceded, immediately and without qualification, then explain why it does not change your conclusion. Do not speculate outside your expertise, and say so when a question crosses that line. Keep your answers short, because long answers invite follow-up questions in territory you did not choose.

When you are shown a document you have not seen, ask for time to read it. Inspectors expect that and advocates dislike it, which is a reasonable indication that it is the right move.

Conflicts, fees and prior involvement

Prior involvement in the scheme is not an automatic bar, but it has to be declared and it changes how your evidence is read. A consultant who designed the drainage strategy and then gives expert evidence on flood risk is giving evidence about their own work, and the proof should say so plainly at the front.

Conditional or success-related fees are not acceptable for expert evidence. An expert whose payment depends on the outcome cannot give independent opinion, and confirming that no such arrangement exists is a standard part of the declaration.

The same discipline applies to reviewing and signing off documents you did not write. Where a professional reviews an AI-assisted draft through the PlanScope professional directory or any other route, the sign-off is a professional act carrying the reviewer's own liability, and it should be preceded by the same checks you would apply to a junior colleague's work: the data, the method, the site visit and the conclusions.

Costs risk and what triggers it

Either party can apply for an award of costs against the other for unreasonable behaviour, and the Planning Practice Guidance on appeals sets out what counts. Evidence is squarely within scope.

Producing expert evidence that is not supported by the underlying data, failing to substantiate a reason for refusal at inquiry, introducing new material late, or withdrawing a case at the last moment can all attract an award. A local planning authority that defends a refusal on ecology grounds with no ecological evidence is at risk. So is an appellant whose noise witness relies on measurements taken in the wrong conditions.

That risk is the reason to be careful at the instruction stage rather than the inquiry stage. Where the evidence will not support the case, the useful thing an expert does is say so in the first week, while the client can still choose a different route. A Planning Statement and the supporting technical documents assembled properly at application stage prevent a good many inquiries from happening at all, and it is worth checking which of the 66 report types should have been in the original submission.

This is general guidance rather than legal advice. Check the current procedural rules and your institute's requirements before accepting an instruction as an expert witness.

Enter a postcode on PlanScope to see which reports a site is likely to need before an application is made. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.