Expert Witness vs Expert Advisor: Which Does Your Dispute Need?

The difference between an expert advisor and an expert witness comes down to one question: who does the expert owe their duty to? An expert advisor (sometimes called a shadow expert or consulting expert) is retained privately to advise you and your solicitor, and everything they produce is normally protected by legal privilege. An expert witness is instructed to give independent evidence to the court under CPR Part 35, and their overriding duty is to the court rather than to the party paying their fee. Choosing expert advisor vs expert witness is therefore about what you need the surveyor to do at each stage of the dispute. This guide explains both roles and when each is appropriate.

What does an expert advisor do?

An expert advisor is a chartered surveyor, engineer or valuer engaged by one party to help them understand the technical merits of their case. Their work is advisory and confidential. Typical tasks include:

• Carrying out an early inspection and giving a frank, privileged view on whether a claim or defence has technical merit.
• Reviewing the other side's expert report and identifying weaknesses, gaps or errors in method.
• Helping the legal team draft the letter of instruction and the written questions put to the opposing expert under CPR 35.6.
• Advising on settlement figures, realistic repair costs or the likely range of value before a mediation or Part 36 offer.

Because the advisor acts for you, they can be as candid as you need. If the advice is that the claim is weak, that opinion stays within the privileged relationship with your solicitor and does not have to be disclosed. That candour is what makes the role valuable before proceedings are issued, when the decision to litigate, settle or walk away is still open.

What does an expert witness do?

An expert witness gives opinion evidence to assist the court on matters within their expertise. In England and Wales the role is governed by CPR Part 35, Practice Direction 35 and the Guidance for the Instruction of Experts in Civil Claims. For chartered surveyors, the RICS practice statement Surveyors acting as expert witnesses adds a further professional layer. The key features are:

• The expert's overriding duty is to the court (CPR 35.3), and this overrides any obligation to the instructing party.
• The report must follow the prescribed form, including a statement of truth, the substance of all material instructions, and a declaration that the expert understands their duty (PD35 paragraph 3).
• Where both sides have experts, the court will usually direct a discussion and a joint statement under CPR 35.12 setting out points of agreement and disagreement.
• The expert may be called to give oral evidence at trial and be cross-examined.

Nothing an expert witness writes for the court is privileged once served. That is why the expert witness cannot be your advocate: an expert who argues a party's case rather than giving a balanced opinion will have their evidence discounted, and the court can refuse permission to rely on it at all. For more on the role, see what an expert witness does in property disputes.

Expert advisor vs expert witness: the key differences

• Duty: the advisor owes their duty to the client; the witness owes their overriding duty to the court.
• Privilege: the advisor's work is normally privileged and need not be disclosed; the witness's report, joint statement and answers to questions are served on the other side.
• Permission: anyone can retain an advisor at any time; expert evidence can only be relied on at trial with the court's permission under CPR 35.4, and the court will restrict it to what is reasonably required.
• Format: advisory notes can take any form; a Part 35 report must comply with PD35 and carry the statement of truth and expert's declaration.
• Cost recovery: the winning party can usually recover a reasonable proportion of expert witness fees as costs; advisory fees are generally not recoverable.
• Independence: an advisor can be partisan; a witness must be, and be seen to be, independent.

The panel's experts typically charge £150–£300 per hour, so a preliminary advisory inspection and note may cost a few thousand pounds, while a full CPR Part 35 report on a straightforward matter is more often £5,000–£15,000, rising to £20,000–£50,000 or more on complex claims. Court attendance is usually charged at £1,000–£2,000 per day. Our guide to expert witness costs in the UK sets out these figures in more detail.

Can the same surveyor be both advisor and expert witness?

In principle, yes, but there are risks. A surveyor who has spent months shaping your strategy as an advisor may find it hard to demonstrate independence when they later sign a Part 35 declaration, and opposing counsel will explore that history in cross-examination. There is a further problem: once the advisor becomes the witness, earlier privileged advice can become disclosable, because the report must set out the substance of all material instructions and the court can order disclosure of the underlying documents if it doubts that summary.

For those reasons, many solicitors prefer a two-expert model on higher-value claims: one surveyor as a privileged advisor throughout, and a separate surveyor instructed formally as the Part 35 expert. On smaller claims, the usual route is to instruct one surveyor early on an advisory basis, decide on the merits, and then either convert the instruction to a formal expert witness appointment (accepting the loss of privilege from that point) or instruct a fresh expert. Either way, record the decision in the letter of instruction. Where a single joint expert is directed by the court, the picture changes again, because both parties instruct the same witness and neither has a private line to them; see what a single joint expert is.

Which does your dispute need?

• Pre-action, merits uncertain: start with an expert advisor. A privileged inspection and note on the likely cause of the defect, the realistic repair cost or the value range tells you whether the claim is worth pursuing before you commit to litigation costs.
• Pre-action, merits clear, heading to court: instruct an expert witness early enough that the report can be exchanged in line with the Pre-Action Protocol for Construction and Engineering Disputes or the relevant protocol. Early expert evidence also strengthens your position in mediation.
• Small claims and fast track: the court will normally allow only one expert per issue, often a single joint expert, and will cap recoverable fees. A privileged advisor can still help you brief that expert effectively.
• Multi-track, high value: consider both roles. The advisor tests the other side's evidence and supports counsel; the witness gives the independent evidence the court relies on.

The discipline matters as much as the role. A construction defects claim needs a building surveyor or engineer; a valuation dispute needs an RICS Registered Valuer; a boundary dispute needs a surveyor experienced in title plans and measured surveys; and a negligence claim against a surveyor needs an expert practising in the same field. For a step-by-step checklist on the formal appointment, read how to instruct an expert witness surveyor.

Arrange an expert advisor or expert witness through Survey Merchant

Survey Merchant provides RICS chartered surveyors, engineers and Registered Valuers from a national panel of 2,400+ professionals for both advisory and expert witness roles. Tell us the nature of the dispute, its stage and whether you need privileged advice, a CPR Part 35 report or both; we will match the discipline to the dispute and quote a fixed fee for the initial stage. The panel's experts prepare reports that comply with CPR Part 35, PD35 and the RICS practice statement. Visit the expert witness services page to see the dispute types covered, or contact Survey Merchant on 0204 579 8270 to discuss your case.