Expert witness duties: lessons every QS can apply

Maybe without realising is, most quantity surveyors spend their careers in advocacy. You support you employer, who might be a contractor, subcontractor or employer, building cases, defending claims, and negotiating outcomes. We do this with our combination of legal, finance and engineering knowledge and it is one of the primary reasons the role exists.

When you step into the role or an expert witness, the rules change. The skills are very similar in that you understand, analyse, value and communicate, but the duties are very different. That is because your overriding duty is to the court or tribunal, and not to the party paying you.

In this article, we’ll explore the duties of an expert witness, how they differ from everyday QS practice, and what practical lessons all QSs can take away, whether or not they ever give evidence in a dispute.

Advocate v Expert

The first and most important distinction is your duty.

• In your day job as a QS, your duty is to your client. You are expected to take their side, put forward their strongest case, and defend their position to achieve the optimum commercial outcomes.
• As an expert witness, your duty is to the court. Independence and objectivity are not optional; they are essential to your credibility as an expert witness, and your credibility is your most valuable asset.

This distinction arises in the Civil Procedure Rules (CPR), specifically Part 35, which governs expert evidence in civil proceedings. The rules require that expert evidence must be unbiased, within scope, and transparent about assumptions and limitations.

It is not enough to be technically correct. The court needs to be able to trust that your opinion is based on evidence, not loyalty to the party who pays your invoice. The court are reliant on experts to assist the court in arising at a decision.

What an Expert Report Must Contain

The requirements of an expert report are detailed, but they boil down to three things: clarity, transparency, and accountability.

A compliant report should:

1. State the instructions received, so it is clear what you were asked to consider and what you were not.
2. Set out your qualifications and briefly justify why you are competent to give an opinion on the issue.
3. Identify facts and assumptions to show what evidence you relied upon and where you had to use your experience to plug gaps.
4. Present a range of opinion if it exists so the court can see where there is genuine professional debate.
5. Include a signed statement of truth confirming you understand your duty to the court and have complied with it.

These things are not just a bureaucratic hurdle; they are the cornerstones of your evidence. The discipline of spelling out assumptions, qualifications, and instructions ensures clarity for everyone and allows those reliant on your evidence (i.e. the court) to follow your reasoning and understand how you arrived at your conclusion.

Why this matters for QS’s

You may not step into a witness box, or even intend to step into one, but the principles of expert witness duties can help improve your performance.

1. Objectivity adds credibility

When preparing valuations, variations, or claims, evidence-based reasoning is always stronger than advocacy. Making baseless claims will not get stronger, no matter how loudly you protest, until they are supported with evidence. If your position relies on clear data and transparent assumptions, it is harder to ignore or dismiss.

2. State your assumptions

Too often in project QS work, assumptions can sometimes be silent, whereas expert work forces us to do the opposite and make them explicit. Doing so reduces disputes because it is easier to identify what you agree upon and what the points of difference are.

3. Be ready to justify your methods

If challenged, could you explain why you priced a variation the way you did? Could you show how you derived a production rate or why you selected one benchmark cost over another? Building this discipline into everyday practice strengthens your work and your professional reputation.

4. Independence is a long-term asset

Yes, as a QS you are paid to advocate for your client. But if you build a reputation for objectivity and fairness, clients, lawyers, and contractors will trust you more. In an industry built on relationships, that trust is invaluable.

A Practical Example

Imagine you are preparing a valuation on a live project.

• As an advocate QS, you might feel pressured to maximise recovery for your client, perhaps by stretching assumptions or downplaying weaknesses in the data.
• As an expert, you would set out the assumptions openly, highlight the limitations, and still provide a reasoned valuation.

Which approach is more persuasive in the long run? The second. Not only does it protect you from challenge, but it also allows the decision-maker, whether a client, adjudicator, or tribunal, to place weight on your analysis.

That discipline is what makes expert reports robust. And it is exactly the same discipline that makes day-to-day QS valuations more reliable.

Final Reflections

The duties of an expert witness may feel a world away from the site office, but the underlying principles, independence, objectivity, and clarity will always serve you and your career.

Whether you are pricing a variation, preparing a payment application, or giving evidence in court, the same rule applies:

• Build your position on evidence.

• Make your assumptions transparent.

• Be ready for scrutiny.

Do that consistently and you will develop a reputation for being a reliable and credible profession. And in today’s construction industry, credibility is one of the most valuable assets a QS can hold.

Next week: we’ll look at how to approach valuations when information is limited, exploring why a first principles approach can often be the most robust and defensible method.

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