Lessons for Construction Cost Experts from Guest v Smith (2025)

In Guest v Smith [2025] VCC (Bennett JR) a straightforward property damages case gave one of the clearest messages this year on how courts assess expert evidence. 

Although the dispute involved property valuation, the lessons apply directly to construction cost experts. 

The Case

Each side called a valuer. Their opinions differed by three hundred thousand dollars. Both were qualified and experienced. The Court accepted the expert who explained his reasoning step by step and linked his figures to real evidence.  

The other expert was competent but less transparent. He had not brought his working notes, relied heavily on his own judgement and made a few speculative suggestions that were not supported by evidence. 

The judge made an important point. The Court is not there to act as a “third valuer.” It will prefer the opinion that is reasoned, evidence based and easy to follow. 

What this means for construction cost experts 

• Show your workings 

  Set out how each rate or adjustment is derived. The Court must be able to trace the number back to the source. 

• Keep your notes 

  Bring your spreadsheets, calculations and supporting material. If you cannot produce them, your credibility is weakened. 

• Base your opinion on real evidence 

  Use actual project data wherever possible. Rates, claims and valuations that can be tied to records carry far more weight than estimates based on experience alone. 

• Be open and balanced 

  If new information would change your view, say so. The Court respects experts who are candid about how their opinion might move when new facts appear. 

• Avoid speculation 

  Only include assumptions that can be supported by evidence or accepted practice. Unsupported theories will quickly be dismissed. 

• Write for the Court 

  Use clear, direct language. Explain how you arrived at your figures and why your approach is reliable. The goal is to make the reasoning obvious, not to impress with complexity. 

Key Takeaway

Guest v Smith shows that what persuades a Court is not the figure itself but the path that leads to it. 

In construction cost disputes, the expert who can demonstrate transparent reasoning and a clear connection between evidence and conclusion will always be the one who is believed. 

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