Picture opposing counsel working through a quantity surveyor’s expert report line by line. Where did this quantity come from? Who told you to assume that? Why this rate and not the contractor’s quote? A report that can’t answer those questions on its own page is in trouble, however experienced its author.
If you’re still deciding whether you need a QS expert at all, start with our earlier article on why and when to engage a quantity surveyor as an expert witness. This one goes a step further: what makes the resulting evidence able to withstand scrutiny.
The short answer: a QS’s expert evidence is more likely to hold up when the expert has relevant specialised knowledge, identifies the facts and assumptions relied on, uses a transparent and suitable method, explains the reasoning behind each opinion, stays within their field, and complies with the applicable expert witness rules. Qualifications alone don’t make an opinion admissible or persuasive.
This article is general information, not legal advice. Whether particular evidence is admitted, and what weight it receives, is always for the court.
What Does “Hold Up in Court” Actually Mean?
It helps to separate four ideas that are often run together:
- Admissibility: whether the court can receive the evidence at all under the rules of evidence.
- Evidentiary foundation: whether the opinion is properly grounded in facts, assumptions, material and specialised knowledge.
- Weight: how much the court relies on the evidence once it is admitted.
- Credibility: whether the expert’s reasoning, method and conduct survive testing.
Admissibility and weight are not the same thing. A report can be admitted yet given little weight because its reasoning is thin. A report can also be technically impressive but face admissibility problems if its foundation isn’t shown.
Why Qualifications Alone Are Not Enough
In NSW, the starting point is that opinion evidence is generally inadmissible. Section 79(1) of the Evidence Act 1995 (NSW) creates an exception: where a person has specialised knowledge based on their training, study or experience, the opinion rule doesn’t apply to their opinion if it is wholly or substantially based on that knowledge.
That test has two limbs, as the High Court confirmed in Dasreef Pty Ltd v Hawchar [2011] HCA 21. The expert must have specialised knowledge, and the particular opinion must be wholly or substantially based on it. Years in practice satisfy, at most, the first limb. The second depends on the report itself.
The High Court has since stressed the same point from another angle. In Honeysett v The Queen [2014] HCA 29 the Court looked closely at whether an opinion was in fact based on the expert’s specialised knowledge. In Lang v The Queen [2023] HCA 29, a Queensland case decided at common law, it reaffirmed that an expert’s opinion must be shown to be based on specialised knowledge beyond ordinary experience.
The Link Between Specialised Knowledge and Opinion
The most quoted guidance comes from the NSW Court of Appeal in Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705. Heydon JA said the expert’s evidence must explain how their field of specialised knowledge applies to the facts assumed or observed so as to produce the opinion. If the steps aren’t made explicit, the court can’t tell whether the opinion really rests on expertise.
For a QS, that bridge looks like a chain:
- Specialised knowledge: measurement, construction costs, pricing, procurement
- Facts and documents: drawings, contracts, inspection records, claims
- Assumptions: what the expert has been told or has had to assume
- Methodology: how quantities and costs are established
- Analysis: the calculations
- Opinion: the figure and the reasons for it
If any link is weak, the opinion becomes vulnerable. A sentence like “In my opinion, the reasonable rectification cost is $850,000” is a conclusion, not evidence of how it was reached. The report needs to show the route: scope, quantities, trades, rates and where they came from, access, demolition, preliminaries, constraints, and any escalation, contingency, exclusions and assumptions that apply. How much of that is needed depends on the size and complexity of the dispute.
Start With a Defensible Factual Foundation
An expert opinion is only as good as the material under it. Depending on the dispute, a QS might rely on drawings and specifications, engineering and defect reports, the contract and variations, progress claims and invoices, quotations, tender documents, photographs, site inspection records, correspondence, programmes and cost reports.
The report should list what was relied on, and it should keep three things apart:
- Fact: something established by the evidence or material, such as “the contract drawings show 1,240 m² of external cladding”.
- Assumption: something the expert has been instructed or has had to assume, such as “I am instructed that the cladding must be fully replaced”.
- Opinion: the conclusion the expert reaches using specialised knowledge, such as “the reasonable cost of that replacement is…”.
When these are blended together, a cross-examiner can pull them apart, and the opinion may fall with the weakest of them.
Make Every Important Assumption Visible
Assumptions can make or break a cost opinion. They should be explicit, relevant, reasonable, traceable to the instructions or evidence, and tested where that’s practical.
Compare two ways of dealing with demolition:
- Weak: “The existing structure can be demolished at normal market rates.”
- Stronger: the report states what was assumed about access and site constraints, hazardous materials, permitted working hours, disposal, temporary works and propping, sequencing, and the extent of demolition, and where each assumption came from.
The court decides which assumptions are established. But an opinion built on hidden or unrealistic assumptions is easy to undermine, because once an assumption fails, the figure built on it has nothing left to stand on.
Show Your Methodology
Another suitably qualified QS should be able to follow how the expert moved from the source material to the conclusion. For cost evidence, that usually means explaining:
- Quantities: measured from drawings, taken off site, or taken from another document?
- Rates: where did they come from, and why are they appropriate?
- Market evidence: which quotations, benchmarks or project data were considered?
- Adjustments: why were rates changed, and by how much?
- Project conditions: how were site-specific constraints priced?
- Time: what date are costs expressed at, and was escalation applied?
- Scope and exclusions: exactly what work is in, and what is deliberately left out?
- Sensitivity: would the opinion change materially if a key assumption changed?
Not every report needs a formal sensitivity analysis. The depth of analysis should match the complexity of the issue and how much turns on it.
Stay Within Your Area of Expertise
A QS may have specialised knowledge in construction costs, quantities and measurement, cost planning, valuing construction work, rectification costs, variations, progress claims, cost to complete, and the cost consequences of delay where appropriately qualified and instructed.
Being a QS doesn’t make someone an expert in structural engineering, architecture, building pathology, compliance, programming or contract interpretation. Drifting into those areas is “expertise creep”, and it is one of the quickest ways to weaken a report. An opinion outside the expert’s field may fail the first limb of section 79 for that issue.
For example, a QS can give an opinion on the reasonable cost of replacing failed balcony tiling. The opinion that the failure was caused by a defective membrane or substrate movement belongs to a waterproofing specialist, building consultant or engineer. In construction disputes, cause, defect, compliance, cost, delay and contract meaning often need different experts. A good QS report says where its expertise stops and what it has relied on from others.
Independence: The Expert Is Not an Advocate
Under the NSW Expert Witness Code of Conduct, an expert witness has an overriding duty to assist the court impartially on matters within their expertise. The paramount duty is to the court, not to the party who engaged them, and an expert witness is not an advocate for a party.
That is the line between an expert witness and a client’s consultant, commercial adviser or advocate. A cost consultant advising a developer works toward the developer’s objectives. An expert witness gives the same answer whoever is paying.
An expert may reach a conclusion that doesn’t help the party who engaged them. That doesn’t necessarily weaken the report. It is often what makes it credible. Shifting an opinion to suit a client’s case, by contrast, tends to come out in cross-examination or in a joint conference with the other side’s expert.
The Expert Witness Code of Conduct
In NSW courts, rule 31.23 of the Uniform Civil Procedure Rules 2005 requires an expert witness to comply with the code in Schedule 7. Unless the court orders otherwise, an expert’s report or oral evidence may not be received unless the expert acknowledges reading the code and agreeing to be bound by it. NCAT has its own expert evidence rules in NCAT Procedural Direction 3 (Expert Evidence), based on the same principles. Many strata and home building cost disputes are heard in NCAT, so the same discipline applies there.
The code covers:
- General duty: to assist the court impartially; paramount duty to the court; not an advocate
- Content of the report: including the expert’s qualifications, the facts and assumptions relied on, the reasons for each opinion, and any qualification where the opinion may be incomplete or inaccurate
- Limits of expertise: stating where an issue falls outside the expert’s field
- Change of opinion: if the expert changes their opinion on a material matter, they must promptly provide a supplementary report
- Court directions: complying with directions, including conferring with other experts and preparing joint reports
The code isn’t a box to tick on the cover page. Most of what makes QS evidence strong (visible assumptions, stated reasons, acknowledged limits) is the code’s requirements put into practice.
What Weakens QS Expert Evidence?
These issues can make an expert’s evidence more vulnerable to challenge, or reduce the weight a court gives it:
- an opinion with no stated reasoning
- cost figures without supporting calculations
- an unclear scope of what was assessed
- hidden or undisclosed assumptions
- reliance on incomplete documents without saying so
- blurring fact, assumption and opinion
- rates without a stated source
- unexplained adjustments
- opinions outside the QS’s expertise
- advocacy instead of independent analysis
- ignoring contrary evidence, such as the other side’s quotes
- overstating certainty
- no stated limitations
- not issuing a supplementary report after a material change of opinion
- poor document referencing
- calculation errors or inconsistencies
- presenting a client’s instructions as established fact
How QS Evidence Works in Construction Disputes
QS expert evidence is most useful where the question is “what is it worth?” or “what will it cost?”:
- Builder payment disputes: was the claimed work done, and what was it reasonably worth?
- Variations: what extra work was performed, and what was its reasonable cost?
- Defect rectification: what is the reasonable cost of fixing identified defects?
- Cost to complete: what will it cost to finish the remaining work?
- Delay-related costs: where within the expert’s expertise, what cost consequences can be quantified?
What the contract means, and who is liable, remain questions for the lawyers and the court. The QS quantifies; it doesn’t interpret.
Archi-QS also carries out inspections under the NSW Strata Building Bond and Inspections Scheme, so defect evidence and rectification costs are part of our everyday work.
Ordinary QS Report vs Expert Witness Report
| Ordinary QS report | Expert witness report |
|---|---|
| Client and project focused | Prepared for a court or tribunal proceeding |
| Supports commercial decisions | Gives an independent expert opinion |
| May recommend a course of action | Must assist the court impartially |
| Assumptions may be informal | Assumptions must be identified |
| An estimate may be enough | Reasoning and foundation are critical |
| Scope set by the client | Role defined by court rules and instructions |
The exact differences depend on the engagement and the forum. A report doesn’t become expert evidence just because “expert report” appears on the cover.
What Happens When Two QS Experts Disagree?
Competing cost opinions are common, and courts often direct the experts to confer and prepare a joint report setting out where they agree and disagree. The question for the court isn’t simply “which QS has more experience?”
The court may look at the relevance of each expert’s expertise to the issue, the factual foundation, the assumptions, the methodology and depth of analysis, internal consistency, how each expert responded to contrary evidence, the clarity of the reasoning, and how each held up under questioning. Often the gap comes down to a handful of assumptions or rates, and joint conferencing is designed to find them.
What Gets Tested in Cross-Examination?
The questions usually follow the evidence chain:
- Qualifications: “Are you qualified in this specific area?”
- Scope: “What exactly did you assess, and what didn’t you?”
- Assumptions: “Who told you that?”
- Evidence: “Where is the supporting document?”
- Rates: “Why did you use this rate?”
- Methodology: “Why did you calculate it this way?”
- Alternatives: “Did you consider the other rectification method?”
- Contrary material: “Did you see the contractor’s quotation?”
- Limitations: “What would change your opinion?”
- Independence: “Did your client ask you to reach this conclusion?”
A report that has already answered these questions on the page leaves much less room for them to do damage.
AIQS Expert Witness Requirements
The Australian Institute of Quantity Surveyors maintains a list of members who act as expert witnesses. To be eligible, its expert witness page says a QS must:
- have at least ten years as a practising quantity surveyor
- hold AIQS Member or Fellow grade for at least 12 months
- hold the Certified Quantity Surveyor (CQS) designation
- be CPD compliant for the last 12 months
- have previous experience as an expert witness
- have undertaken expert witness training
- have read the AIQS guidance note on the quantity surveyor’s role as an expert witness
These are professional eligibility criteria, not rules of evidence. Meeting them is a sound signal of experience and training. It doesn’t make any particular opinion admissible or determine its weight; that turns on section 79, the code of conduct and the report itself.
How Archi-QS Supports Construction Disputes
Archi-QS is a quantity surveying and building consultancy that works on NSW construction and strata matters. Its principal, Samuel Star FAIQS, CQS, is listed on the AIQS Expert Witness Panel. Our work in disputes covers defect rectification cost assessment, cost to complete, builder claims, variations and broader construction cost analysis, alongside our everyday quantity surveying services.
If you’re a solicitor, owners corporation, developer or builder with a cost question in dispute, contact Archi-QS to discuss the scope, the material available and the questions the expert would be asked to answer.
FAQs
Can a quantity surveyor act as an expert witness?
Yes, on matters within their specialised knowledge, such as construction costs, quantities and rectification costs, subject to the court’s rules and the requirements of section 79 of the Evidence Act.
What does a QS expert witness report contain?
Typically the expert’s qualifications, instructions, documents relied on, facts and assumptions, methodology, calculations, opinions with reasons, limitations and an acknowledgement of the code of conduct.
What makes expert evidence admissible in NSW?
Broadly, the expert must have specialised knowledge from training, study or experience, and the opinion must be wholly or substantially based on it. The court decides admissibility in each case.
Does an expert witness have to be independent?
Yes. Under the NSW code, the expert’s paramount duty is to the court, not the party who engaged them, and an expert witness is not an advocate.
Can a QS give an opinion outside their expertise?
They shouldn’t. Opinions on engineering cause, compliance or contract meaning belong to other experts or the court, and straying into them can weaken the evidence.
What happens if two QS experts disagree?
The court may direct them to confer and prepare a joint report. It then weighs each opinion on its foundation, method and reasoning, not just on seniority.
Can a QS assess defect rectification costs for court proceedings?
Yes, once the defects and the rectification method are identified. Whether something is a defect, and how it should be fixed, usually need technical experts.
What is the difference between a QS report and an expert witness report?
An ordinary QS report serves a client’s commercial needs. An expert witness report is prepared to assist a court impartially, with its assumptions and reasoning laid open to scrutiny.
What qualifications should a QS expert witness have?
Relevant specialised knowledge for the issue is the legal test. Professionally, AIQS’s expert witness criteria include CQS designation, at least ten years in practice and expert witness training.
Can an expert change their opinion after issuing a report?
Yes. If an expert changes their opinion on a material matter, the NSW code requires them to promptly provide a supplementary report.
Conclusion
A QS expert report holds up because its reasoning can be followed and tested, not because of its author’s title. That means relevant expertise, reliable evidence, explicit assumptions, a sound method, transparent calculations, a reasoned opinion and stated limits, with each link visible on the page.