Hot-Tubbing: Concurrent Expert Evidence in Property Disputes

Hot-tubbing expert evidence, formally called concurrent expert evidence, is a trial procedure in which the experts for each side give their evidence together, in the witness box at the same time, rather than one after the other. The judge leads a structured discussion around the points still in dispute, the experts can question each other, and counsel cross-examine afterwards. It is permitted in England and Wales under Practice Direction 35 and is increasingly directed in construction, dilapidations, valuation and boundary cases where a chartered surveyor gives evidence. This guide explains how hot-tubbing works, when a court is likely to order it, and how an expert witness surveyor should prepare for it.

What is hot-tubbing in expert evidence?

In a conventional trial, each party's expert is called in turn: examination-in-chief (usually just confirming the written report), cross-examination by the other side, and re-examination. With concurrent evidence the experts are sworn in together and sit side by side. Working from an agenda of disputed issues, the judge asks each expert for their view on the first issue, invites them to comment on each other's answers, and then moves to the next. Counsel for each party may then put questions, and the judge may ask further questions of their own.

The procedure originated in Australian courts and tribunals and was adopted in England and Wales in 2013 as part of the Jackson civil justice reforms. Its purpose is to let the judge hear the experts test each other's reasoning directly, point by point, which is usually faster and more revealing than two separate cross-examinations.

Where does the rule come from? PD35 and the joint statement

The power to order concurrent evidence sits in paragraph 11 of Practice Direction 35, which supplements CPR Part 35. The court may direct at any stage in the proceedings that some or all of the evidence of experts from like disciplines be given concurrently. The direction is normally made at the case management conference or pre-trial review, and the court sets or approves an agenda based on the areas of disagreement recorded in the experts' joint statement under CPR 35.12.

That link with the joint statement matters. The joint expert statement is where the experts narrow the issues in writing; the hot tub is where the judge explores whatever remains. An expert who has recorded vague disagreements without reasons will find the session uncomfortable, because the judge will ask for exactly the reasoning that is missing. Written Part 35 questions answered earlier in the case are also likely to be put back to the expert during the discussion.

A Civil Justice Council review in 2016 found that judges and practitioners who had used it generally considered that it saved trial time and improved the evidence, while noting it is not suitable for every case. The practice direction was later expanded so that the court can hear experts issue by issue, back to back or in a combined format, tailored to the dispute.

When will a court order concurrent expert evidence?

Hot-tubbing is a discretionary case-management tool, not a default. Courts tend to direct it where:

• The experts are from the same discipline and address the same technical questions, for example two building surveyors on the cause of a defect, or two valuers on the market value of a property at a given date.
• The joint statement leaves a manageable number of clearly defined disagreements, so an agenda can be drawn up with real questions to put to both experts.
• The dispute is technical rather than factual, so the judge benefits from hearing the experts' reasoning tested against each other rather than from a credibility contest.
• Trial time is limited, particularly in the Technology and Construction Court and in the Business and Property Courts, where the procedure is most familiar.

It is rare on the small claims and fast tracks, where expert evidence is usually written only, and rarely appropriate where an expert's credibility is itself in issue or where the case turns on facts the experts cannot resolve. Either party can invite the court to order it, and a party who thinks it would disadvantage them should say so at the case management stage.

What the hot tub looks like in a property dispute

Consider a construction defects claim about water ingress through a flat roof. The two building surveyors have agreed that the roof leaks and that the membrane is defective, but disagree on whether the cause is poor workmanship or a design fault, and on whether full replacement or localised repair is the appropriate remedy. The judge's agenda has two items: causation and remedy. On each, the experts set out their view in a few minutes, the judge probes why the alternative explanation is excluded, invites the other expert to respond, and then hears from counsel. The exchange takes an hour rather than a day of cross-examination. The same format is used in dilapidations disputes over the cost of repair and in valuation disputes where two valuers disagree on comparables or yield.

The atmosphere is noticeably different from cross-examination. The tone is collaborative, and an expert who concedes a point where the other side's reasoning is better is seen as helpful rather than weak, consistent with the overriding duty to the court under CPR 35.3.

How an expert witness surveyor should prepare

Concurrent evidence rewards experts who know their own report, the other side's report and the joint statement in detail, and who can explain their reasoning in plain English without notes. Practical preparation usually involves:

• Re-reading the joint statement against the agenda and making sure every disagreement has a short, reasoned answer ready.
• Anticipating the strongest version of the other expert's argument and deciding in advance which points can properly be conceded and which cannot.
• Preparing to speak to the judge, not to counsel. Answers should be direct and technical, with references to the evidence, photographs or standards relied on in the report.
• Keeping to the discipline. A building surveyor asked about structural loading or a valuation figure outside their expertise should say so, as the RICS practice statement on surveyors acting as expert witnesses requires.

Solicitors expecting hot-tubbing should choose an expert with courtroom experience and allow for a preparation meeting shortly before trial. Time spent in the hot tub is charged at the same rate as other court attendance, typically £1,000 to £2,000 per day on top of report fees of £5,000 to £15,000 for a straightforward matter, but the overall trial cost is often lower because the expert phase is shorter.

Advantages and drawbacks of hot-tubbing

The main advantages are speed, clarity and quality of evidence. Judges hear both experts on the same point within minutes, experts are held to their reasoning rather than to a scripted set of answers, and partisan opinions tend to be exposed quickly. The drawbacks are that counsel lose some control over presentation, a confident expert may dominate a more reserved one, and the format depends on a good agenda. Where the joint statement has done its job, the hot tub usually works well; where it has not, the session can become an unstructured argument. That is another reason to instruct an expert who takes the written stages of the case seriously, as explained in what an expert witness actually does.

Expert witness surveyors from Survey Merchant

Survey Merchant provides RICS chartered surveyors as expert witnesses for construction, dilapidations, valuation, boundary and party wall disputes across England, Wales and beyond. The panel's experts prepare CPR Part 35-compliant reports, take part in joint discussions and are used to giving evidence in court, including concurrently with the other side's expert where the judge directs it. Solicitors and litigants can request a matched expert CV and a fixed-fee quote through the expert witness services page or by contacting the team on 0204 579 8270.