The Tool Desk
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This guide concerns civil proceedings in England and Wales, particularly the Technology and Construction Court (TCC). The rules and 28-day deadline below are not a guide to procedure in other jurisdictions. Check the current rules, the order in your case and any applicable court directions before taking a procedural step.
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What it means to challenge an expert report
A challenge is a reasoned criticism of the expert’s evidence, not simply an assertion that the expert is wrong or that the opposing party’s report reaches a different conclusion. It should connect a material technical or procedural concern to the issue the court must decide. Examples include an opinion based on an inaccurate measurement, an unexplained assumption about the construction sequence, a calculation that cannot be followed, or a conclusion outside the expert’s field or the scope the court permitted.
Keep three different kinds of disagreement separate:
#1 Best Overall
- A factual dispute: the parties disagree about what happened, what a record shows, or which project information is accurate. The court may need to decide the fact; an expert should not silently turn a contested assumption into an established fact.
- A technical disagreement: qualified experts interpret the same or accepted facts differently, or reach different conclusions using defensible approaches. That is not automatically evidence of misconduct or a defect in the report.
- A demonstrable weakness: an error, omission, unsupported inference, unexplained method, or failure to address a material point that can be identified by reference to the report and relevant source material.
The distinction matters because the response should fit the problem. A disputed fact may need evidence or a decision from the court; an ambiguity may be clarified by a permitted question; a technical difference may be narrowed through expert discussion or tested at trial. The rules do not make every disagreement a basis to remove a report.
Who the construction expert works for—and whom they owe a duty to
An expert may be instructed and paid by one party, but the expert’s overriding duty is to help the court on matters within their expertise. CPR 35.3(2) states: “This duty overrides any obligation to the person from whom experts have received instructions or by whom they are paid.” CPR Part 35, rule 35.3 requires that distinction to be understood when assessing an expert’s evidence.
Practice Direction 35 (PD35) says an expert’s opinion should be independent, objective and unbiased. The expert should not act as an advocate, must consider material facts that detract from their opinion, and should say when an issue falls outside their expertise or the available information does not allow a definite view. These are relevant checks when a report appears to advance a party’s case rather than assist the court, leaves out an inconvenient fact, or expresses certainty the material does not support. PD35
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Start with the court’s permission and the report’s permitted scope
Before debating technical detail, read the order that allowed expert evidence. CPR 35.4 requires the court’s permission before a party may call an expert or put an expert report in evidence. Permission may specify the issues on which the expert may give evidence. Expert evidence must be limited to what is reasonably required to resolve the proceedings: CPR 35.1. CPR Part 35, rules 35.1 and 35.4
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- Find the relevant order and directions. Identify the permitted field, issues, expert or experts, and any instructions about the evidence or timetable.
- Map opinions to the permitted questions. For each material conclusion, note which issue it addresses and whether that issue falls within the permission.
- Identify scope concerns precisely. Separate an opinion that appears to go beyond the permitted issues from one that is merely unhelpful, disputed or expressed too broadly.
- Consider proportionality. Ask whether the evidence is needed to decide the dispute, taking account of the importance of the issue and the case. The TCC Guide emphasizes proportionate expert evidence from the pre-action phase through trial.
The TCC Guide describes the quality and reliability of expert evidence as depending on “(a) the experience and the technical or scientific qualifications of the expert and (b) the accuracy of the factual material that is used by the expert for his assessment.” TCC Guide, section 13.1.1 (October 2022). That guide is the judiciary-hosted October 2022 edition; check whether a later edition or case-specific direction applies before relying on guide-specific practice.
Audit how the expert got from the evidence to the opinion
Review the report as a chain of reasoning. For each important conclusion, trace the source material, factual assumptions, technical method and analysis that support it. The aim is to find an identifiable point the court can assess—not to label every difference in interpretation as an error.
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Check the expert’s relevant expertise and independence
- Do the qualifications and practical experience relate to the precise technical question, rather than construction generally?
- Does the report identify the expert’s relevant knowledge and distinguish it from work or information supplied by others?
- Does the reasoning present an independent technical opinion, or does it appear to argue the instructing party’s case?
- Does the expert acknowledge a limit in expertise, information or certainty where one affects the conclusion?
Check instructions, facts and source material
PD35 requires the report to state the substance of material instructions and facts on which the opinions are based. Check what the expert was asked, which facts were assumed or treated as established, and what records support them. Depending on the issue, relevant material may include drawings, site records, programmes, photographs, measurements, test results, correspondence or other project documents.
Look for a missing document, a disputed version of a drawing, an inaccurate date or measurement, or an assumption about sequence or scope that materially changes the analysis. Identify the source and the consequence: for example, explain how the disputed measurement affects the calculation or why a different programme assumption changes the causal analysis. A contested input is not automatically proof of expert error; state whether the problem is that the fact is unresolved, the source was misread, or the expert has drawn an unsupported inference from it.
Check methods, tests and reasoning
The report should identify relevant literature and other material relied on, describe examinations, measurements, tests or experiments, and say who carried them out. Follow the method far enough to see whether another reader can understand what was done and how it supports the conclusion. Check the calculations, units, sample or measurement basis where applicable, and whether an inference follows from the stated results.
Rank #3
Ask whether the report addresses alternative causes, scopes or interpretations when they are material to the issue. A report need not discuss every conceivable possibility; the useful criticism is that it omits or dismisses a plausible alternative that could affect the conclusion and is supported by the case material.
Check contrary evidence, uncertainty and the range of opinion
PD35 requires the expert to consider material facts that detract from their opinion. Look for contrary evidence that is cited but not addressed, or a material fact that appears to have been overlooked. The report should summarize any range of opinion and explain the expert’s view. Consider whether it acknowledges genuine uncertainty, explains why an alternative view is rejected, and distinguishes what the evidence establishes from what it cannot establish.
A weakness is strongest when it is concrete: identify the paragraph, assumption, source or calculation; explain the problem; and connect it to the permitted issue. Avoid broad allegations such as “biased” or “unreliable” unless the evidence gives a sound basis for them.
Check the report’s formal requirements
PD35 sets requirements for expert reports, including identifying the material facts and instructions underlying the opinions, stating what is within the expert’s own knowledge, identifying relied-on literature and other material, and describing relevant examinations, measurements, tests or experiments and who performed them. It also addresses the expert’s duty to the court and the report’s statement of truth. PD35
Use those requirements as a structured review, not as a checklist for seeking exclusion over every imperfection. Record what is absent or unclear, why it matters, and whether a proportionate clarification could address it. For example, a missing explanation of who performed a material test may be significant if the reliability or interpretation of that test is disputed; a formal omission with no bearing on the issue may call for a different response. The applicable order and circumstances determine what remedy is appropriate.
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CPR 35.6 normally allows one set of written questions to an opposing expert or a single joint expert. The questions must be proportionate, asked within 28 days after service of the report, and directed to clarification. More questions or a broader purpose require the court’s permission or the other party’s agreement. Under PD35, copy the other parties at the same time. Answers form part of the report. CPR Part 35, rule 35.6 and PD35
- Calculate the deadline from service. Work from the date the report was served, and check the current rule, any applicable directions and the case order. The ordinary CPR 35.6 period is 28 days; do not assume it applies outside England and Wales or overrides a case-specific direction.
- Select the points that need clarification. Prioritize material ambiguities, assumptions, sources, calculations, tests or unexplained reasoning. Avoid using questions to repeat your expert’s argument or make submissions.
- Ask a focused, neutral question. Identify the relevant point and request the information or explanation needed to understand the opinion. Do not build an accusation or a preferred answer into the wording.
- Serve the questions and copy the other parties. Follow the rule and any directions for service. Keep a record of when the questions were served.
- Review the answers as part of the report. Determine whether they resolve the ambiguity, narrow the disagreement, reveal a further issue or leave a material question unanswered.
A useful question isolates one point: “Which drawing revision did you use for the measurement in paragraph X, and where in your report is the measurement recorded?” A calculation question might ask the expert to identify the inputs and method used for a stated result. A test question might seek clarification of who conducted it and what observations or results underpin the conclusion. The question should seek an explanation, not tell the expert what conclusion to reach.
If an opposing expert fails to answer a question properly made under CPR 35.6, the court may prevent the instructing party from relying on that expert or limit recovery of the expert’s fees from another party. That is a possible consequence for the court to consider, not an automatic outcome. Questions outside the rule’s ordinary scope should not be treated as permitted merely because they are sent in writing.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use expert discussions to narrow technical disagreements
The court may direct experts to discuss issues and prepare a statement identifying what they agree and disagree about, with reasons. PD35 makes clear that the aim is to narrow issues, not to settle the case. Each expert must give their own opinion; experts can sign a joint statement without authority from the parties to agree. If an expert significantly changes an opinion during the discussion, the joint statement must explain the change. CPR Part 35 and PD35
Prepare a focused agenda tied to the real technical issues. The experts should be able to identify the question, the material facts or assumptions each relies on, the point of agreement or disagreement, and the reasons for any difference. A useful outcome may be a narrower list of disputed issues rather than a single agreed conclusion. The parties should not use the discussion to pressure experts into agreement or turn it into an unstructured negotiation about the claim.
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Compare competing reports on the same basis
Where both parties have reports, compare their answers to the same court-permitted question and, as far as possible, the same factual record. A side-by-side review makes it easier to see whether the real difference concerns expertise, an input, a method, or the inference drawn from shared material.
| Comparison point | What to check |
|---|---|
| Qualifications and experience | How directly the expert’s experience relates to the technical issue in dispute. |
| Duty and independence | Whether the report recognizes the duty to the court and gives a reasoned opinion rather than advocacy. |
| Facts and assumptions | Whether each expert uses complete, accurate source material and identifies contested assumptions. |
| Methods and evidence | Whether data, literature, calculations, measurements, tests and their provenance are disclosed and explained. |
| Contrary evidence and alternatives | Whether each report addresses material facts that cut against its opinion and relevant alternative explanations. |
| Reasoning and uncertainty | Whether the conclusion follows from the stated method and evidence, and whether limits or a range of opinion are explained. |
| Scope and proportionality | Whether the opinion stays within the permitted issue and whether the evidence is reasonably needed for the case. |
Do not treat the mere existence of opposing conclusions as proof that one report is defective. Pinpoint the step at which the experts’ reasoning diverges. If they rely on different facts, identify that difference; if they accept the same facts but use different methods, isolate the method and its effect; if they agree on method but not outcome, test the inputs and calculations.
What to do if clarification does not resolve the problem
If a material issue remains after questions or expert discussion, raise it through the case-management process and seek directions that address the actual problem. The court controls expert evidence and may define its scope, direct expert discussions and manage how evidence is given. The TCC Guide indicates that unresolved points in opinion evidence from professionals involved in the underlying events may be dealt with by application or at the pre-trial review. The appropriate step depends on the order, procedural stage, evidence and nature of the issue.
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Be specific about the concern and the outcome sought. A request might concern clarification of a defined point, the permitted scope of evidence, a further discussion, or how a remaining issue will be tested. Explain why the point is material, what has already been done to narrow it, and why the proposed direction is proportionate. The court’s response is not fixed by the label attached to the complaint; the defect, case directions and interests of justice matter.
At trial, questioning may test the correctness of an expert’s view, clarify it or address an issue omitted from the report. The court may direct concurrent evidence—experts giving evidence together—or evidence organized issue by issue. These are case-management options, not a party’s automatic entitlement; the court controls whether and how they are used. PD35
Can the court exclude the expert report?
The court controls expert evidence, but challenging a report does not automatically exclude it. CPR 35.1 limits evidence to what is reasonably required, and CPR 35.4 gives the court control over permission to call an expert or rely on a report and the issues the expert may address. The rules also provide for consequences in particular situations, including a failure to answer permitted written questions. They do not say that every methodological disagreement or disputed conclusion removes a report. CPR Part 35
Match the request to the demonstrated problem. A clarification may address an ambiguity; a direction may help define or test a disputed issue; a more serious procedural consequence requires the court to consider the circumstances and applicable rules. Do not assume that an alleged breach guarantees exclusion, or that asking for exclusion is the only effective way to challenge evidence. A focused challenge can affect how much weight the court gives an opinion without removing it from the case.
Quick Recap
A practical checklist before you act
- Confirm the jurisdiction, current procedural rules, court order and relevant deadlines.
- Identify the precise expert issue the court permitted and the conclusion you dispute.
- Separate disputed facts, genuine technical disagreement and demonstrable report weaknesses.
- Trace the important opinion through its sources, assumptions, method, analysis and conclusion.
- Check expertise, independence, contrary evidence, uncertainty and relevant report requirements.
- Use the CPR 35.6 question procedure only within its scope and ordinary time limit unless an exception applies.
- Use expert discussions to narrow genuine technical issues, not to demand settlement or agreement.
- If a material issue remains, explain it and seek proportionate case-management directions suited to the procedural stage.
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