When a Judge Rejects the Only Surveyor Expert: Pembrokeshire Damp and Mould Appeal Orders a Fresh Hearing — Lessons for Expert Witnesses, November 2026

A tenant family lost a damp and mould claim, were ordered to pay £8,000 in costs, and then won their appeal because the trial judge never explained why the only expert in the case was wrong. That is the short story of a surveyor expert witness damp and mould appeal reported on 12 November 2026, and it carries a clear message for every surveyor who writes a causation report and every solicitor who relies on one.

Key Takeaways

• A circuit judge in the County Court at Swansea allowed the tenants' appeal against Pembrokeshire County Council and ordered a rehearing before a different judge.
• The tenants' surveyor was the only expert on causation. The council called no expert of its own.
• The first judge preferred lay findings about clothes drying, ventilation and clutter, but did not explain how those findings displaced the expert's analysis.
• A court may disagree with an expert, but it must show its reasoning. An expert report that deals openly with occupier behaviour is far harder to set aside.
• The RICS expert witness standard, 5th edition, takes effect on 30 November 2026 and raises the bar on assumptions and alternative conclusions.

What happened in the Pembrokeshire case

The decision was reported by Mark Smulian for Local Government Lawyer, and the judgment has been published on BAILII. The tenants rented their home from Pembrokeshire County Council and brought a claim about damp, condensation and mould.

At a hearing in January 2026, a deputy district judge, DDJ Williams, dismissed the claim and ordered the tenants to pay £8,000 in costs.

The tenants had relied on a surveyor. He was the only expert witness on what caused the damp and mould, and his report dealt with technical matters such as thermal bridging and insulation. The council did not call a competing expert.

The deputy district judge rejected the surveyor's conclusions. He relied instead on factual findings about how the home was used: clothes dried indoors, ventilation, airflow in a study room, and clutter.

The grounds of appeal

The tenants argued four points:

• The judge rejected the only causation expert without a sufficient evidential or analytical basis.
• The judge used a disrepair framework, asking whether works were repairs or improvements, instead of the statutory test of fitness for human habitation.
• The judge did not properly apply section 96 of the Renting Homes (Wales) Act 2016, which asks whether the tenant's acts or omissions were the whole or main cause of the unfitness.
• The judge gave too little weight to the fitness for human habitation regulations and Welsh Government guidance on damp and mould.

Why the surveyor expert witness damp and mould appeal succeeded

HHJ Beard allowed the appeal. His concern was not that the first judge reached a different view from the expert.

"The issue is not that the deputy district judge disagreed with [the surveyor]."

The problem was the missing reasoning. The judgment set out the factual findings and set out the expert's conclusions, but it did not explain how one displaced the other. HHJ Beard said the "analytical route by which the former displaced the latter" could not be identified.

Two gaps stood out:

• The judgment did not ask whether the surveyor had already taken ventilation and occupation into account when reaching his opinion.
• It did not explain why the lifestyle findings should be treated as overriding causes rather than merely contributing ones.

Where expert evidence is rejected on matters within its expertise, HHJ Beard said, "particular care was required to explain why".

He also found that the claim had been approached through disrepair and improvement concepts, rather than the inquiry into the condition of the dwelling that section 91 of the 2016 Act requires. And the section 96 exercise had not been carried out: the reasoning moved straight from how the home was occupied to the conclusion that the tenants were to blame.

The case will now be heard again by a different judge.

What this means for surveyors acting as expert witnesses

The surveyor in this case was not criticised. But the appeal shows where damp and mould reports are attacked, and how to write one that holds up.

In practical terms, a surveyor instructed on causation should:

• Inspect and record the building fabric: insulation, cold bridges, ventilation provision, heating.
• Ask about and record occupier behaviour, then say plainly whether it is a main cause, a contributing cause, or not material.
• Explain why a home used in an ordinary domestic way should or should not suffer mould.
• Identify the assumptions made and what would change the opinion.

That last point is about to become a formal duty. The RICS professional standard "Surveyors acting as expert witnesses", 5th edition, takes effect on 30 November 2026. It requires experts to identify their assumptions, set out alternative conclusions, and disclose any use of AI in preparing evidence.

Expert evidence: the rules behind the decision

Under CPR Part 35 and Practice Direction 35, an expert's overriding duty is to the court, not to the party paying the fee. The report must be independent, reasoned, and confined to the expert's own field.

A judge is never bound to accept an expert's opinion, even an unchallenged one. But this surveyor expert witness damp and mould appeal confirms the other half of that rule: if the court departs from expert evidence on a technical question, the judgment must show the route.

For landlords, the tactical lesson is direct. Relying on cross-examination and lay evidence alone, with no expert in reply, is a risk. The options are:

• Call a competing expert, so the court has a technical basis for preferring a different cause.
• Agree a single joint expert, which is common in lower-value housing claims and gives both sides one independent report.
• Put written questions to the other side's expert under Part 35, so that the points on occupation and ventilation are answered before trial.

Wales and England: the same question

The case was decided under the Renting Homes (Wales) Act 2016. In England, the equivalent duty sits in section 9A of the Landlord and Tenant Act 1985, with a similar exception where unfitness is caused by the tenant's own conduct.

In both countries the same factual question decides most damp and mould claims: is the cause the building, the way it is lived in, or both? That is a question for expert evidence, and this decision shows how seriously appeal courts treat it.

Frequently Asked Questions

Can a judge reject the only expert in a case?

Yes. But the judge must explain why, with particular care where the point lies within the expert's field. A bare preference for lay evidence is open to appeal.

Does this mean tenants always win if the landlord has no expert?

No. The appeal did not decide who was right about the cause. It decided the reasoning was inadequate, and sent the case back to be heard again.

What should a damp and mould expert report cover?

The building fabric, heating and ventilation, the way the home is occupied, the readings and method used, and a reasoned view on the main cause and any contributing causes.

When does the new RICS expert witness standard apply?

The 5th edition takes effect on 30 November 2026.

Conclusion

The Pembrokeshire decision is a reminder that expert evidence on causation cannot be brushed aside by common-sense impressions of how a family lives. Surveyors should deal with occupier behaviour head-on in their reports. Landlords should decide early whether to instruct their own expert or agree a joint one. Solicitors reviewing a lost case should check whether the judgment explains how the expert was displaced.

Survey Merchant connects solicitors, landlords and tenants with RICS-regulated surveyors who act as expert witnesses in housing condition, defect and property disputes. Compare surveyors and request quotes before the next directions deadline.