Error Type
Unverified Assumption
Court
England & Wales CoA
Liability
All Defects
The Scenario
In Large v Hart (2021), the England and Wales Court of Appeal examined a surveyor's liability when conducting a property inspection for a prospective buyer.
The surveyor inspected a property and noted that the rendered external walls appeared in good condition. Based on this visual observation alone, the surveyor assumed that adequate damp-proof membranes (DPMs) were present behind the render.
Critically, the surveyor did not recommend that the buyer obtain a Professional Consultant Certificate (PCC) from a specialist to verify the hidden damp-proofing system.
The Technical Error
Mechanism of Failure:
Assumption Without Evidence
Visible Render = Assumed DPM (Unverified)
The surveyor's error was treating a visual assumption as equivalent to verified fact. The rendered walls hid the true condition of the damp-proof membranes. Without invasive testing or specialist consultation, there was no way to confirm their presence or effectiveness.
This parallels coordinate/elevation errors in surveying: just because a measurement looks right on a plan doesn't mean the underlying data (datum, transformation, or physical monument) is correct. The surveyor failed to apply the principle: "If you can't see it, you can't certify it."
⚠️ Construction Elevation Critical Risk
FEMA flood certificates demand strict adherence to NAVD88. Failing to use VERTCON 3.0 to adjust old NGVD29 plans routinely results in catastrophic base flood elevation (BFE) design failures.
Calculate Vertical Elevation Error Risk →The Outcome
After purchase, the buyer discovered extensive damp problems. The property required significant remedial work, far beyond what a simple DPM failure would suggest.
The Court of Appeal ruled:
- Surveyor Liable for All Defects: The surveyor was held responsible not just for the visible issues, but for all property defects that a proper investigation would have revealed.
- Inadequate Suspicion Trail: The court found that the surveyor should have been suspicious of hidden defects and recommended further investigation (PCC).
- Diminution in Value: Damages were calculated based on the full reduction in property value, not just the cost of the specific damp-proofing repair.
Professional Lesson
Assumptions Are Liabilities.
🛡️ Professional Lesson
Document What You Cannot Verify—And Recommend Verification.
For surveyors and engineers, this case is a stark warning about the scope of professional liability:
- Never assume hidden conditions: If you can't see it (buried utilities, subsurface soil, datum monuments), you can't certify it.
- Recommend specialist verification: When critical data is inaccessible (e.g., original survey monuments, coordinate transformation parameters), advise the client to obtain independent verification.
- Liability extends beyond the immediate error: If your assumption leads to a flawed conclusion, you may be liable for all consequential damages, not just the direct cost of the error.
In coordinate work: if you're transforming data between datums and haven't verified the transformation parameters against known control points, you're making an "assumption without evidence." That's a Large v Hart scenario waiting to happen.
Source: England & Wales Court of Appeal / HJA Legal Commentary
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