2025 was a busy year for me professionally, with two more cross-examinations following on from three in 2024, which took me over 100 instructions (I am definitely starting to show my age). In 2025, judgements were handed down in three of those cases:

1. February 2025 – (1) Iya Patarkatsishvili & (2) Yevhen Hunyak v. William Woodward-Smith [2025] EWHC 265 (Ch) (the widely publicised Horbury Villa moth case)
2. May 2025 – Matière SAS v. ABM Precast Solutions Limited [2025] EWHC 1434 (TCC), a case concerning a dispute between two former JV partners on an HS2 package under an NEC4 Option C subcontract with complex amendments
3. October 2025 – Ioannis Mallas v. (1) Persimmon Homes Limited & (2) Simpson Associates Consulting Engineers LLP [2025] EWHC 2581 (TCC)

This article discusses only Horbury Villa and Mallas v. Persimmon, both of which concerned private homeowner disputes but with very different backgrounds. Horbury Villa was, in fact, a case where fraudulent misrepresentation was the primary case pleaded, with an alternative pleaded for remedial works further described below. Horbury Villa was heard in the Property, Trusts and Probate Court, whereas Mallas was a more conventional construction dispute relating to alleged defective foundations and was heard in the Technology and Construction Court.

In this article, we'll also explore the approaches taken by the courts in relation to assessing the likely cost of the remedial works required and consider how practitioners may wish to factor in the relative uncertainties in works of this nature.

The facts

In 2012, the Defendant bought a property in Notting Hill, which he later renovated and substantially extended by c. 200%, and is known as Horbury Villa.

In 2019, the Claimants bought Horbury Villa from the defendant for £32.5m. Shortly after moving in, they noticed a presence of moths. After enquiries with the Defendant, the Claimants carried out a number of investigations and established that prior to the sale of Horbury Villa, there were multiple visits by pest control companies carried out on behalf of the Defendant due to this issue, who reported the problem back to the Claimants. Critically, this was not disclosed to the Claimants during the sale of the property.

These reports identified that the wool-based insulation used extensively throughout Horbury Villa as part of the renovation works (called Thermafleece Ecoroll and/or Soundblocker) was the likely source of the moth problem and would need to be removed and replaced with a synthetic substitute to ensure eradication of the infestation. That work, however, was not carried out by the Defendant before the sale of Horbury Villa, and the Claimants found themselves living in a home infested with moths.

In 2020, the Claimants carried out investigatory works of their own with a builder and removed what appeared to be the infested insulation and left the remaining insulation in place. Whilst these works reduced the infestation, they did not eradicate it, and the problem persisted, albeit to a lesser degree.

The Claimants then commenced an action against the Defendant claiming the sale was made on the basis of a fraudulent misrepresentation and claimed rescission of the contract of sale. As an alternative remedy, they claimed the cost of remedial works required to ensure complete eradication of the infestation. This is why I was instructed to provide independent quantum expert evidence.

Judgment

The court decided in favour of the Claimants and awarded damages, including the full purchase price of Horbury Villa (£32.5m), stamp duty (£3.7m), other associated purchase costs and further resultant damages, with credit given for time spent living there.

In addition, the court also considered the alternatively pleaded position in relation to the likely remedial costs to ensure full eradication of the infestation [279]–[310].

In doing so, the court exercised discretion in arriving at its assessment and used my analysis of the remedial scheme as the starting point for its judgement. In paragraph [307], the court made the following adjustments:

“For the increased initial scope of works, I will add 50% to the totals for the ceilings and walls. For the preliminaries I will add 25%, on the basis that there will be an increase in some of the elements of the assessed preliminaries but not all. The OHP is calculated at 20%, which will be applied to the new total. I will then add a contingency item at 20% of the aggregate cost of the works (which is double what would be a standard allowance for designed works, on account of the increased risk of further works being needed), and there will be VAT at 20%.”

In paragraph [309], the court went on to explain these adjustments as follows:

“While acknowledging that this is a broad brush exercise, it is similar to the kind of assessment that an intending purchaser would have to make, absent the opportunity to carry out exploratory works to establish exactly what is required. A purchaser intending to bid for the house would not have the benefit of a fully and accurately costed scheme of works. It therefore does not seem to me to be unjust to assess the difference in value of the house in this way.”

The facts

Mr Mallas purchased a four-bedroom timber-framed house built on conventional strip foundations from Persimmon for £649,495 in 2015 in a development called Sibley Park (the “Property”). Critically, this development was on land purchased from the University of Reading, which was previously a halls of residence surrounded by woodland. Some trees were removed before the land was purchased and were within the “zone of influence” of the Property.

Shortly after moving in, Mr Mallas discovered defects in the Property that appeared to be the result of differential movement. It became apparent that Persimmon’s foundation design had not properly considered the effect of shrinkage and heave that the removal of trees would cause, and used an unsuitable strip foundation method. It was said that a piled foundation was more suitable in the circumstances. In 2021, Mr Mallas commenced an action against Persimmon seeking the cost of any necessary remedial works.

Persimmon’s appointed designer was joined in the proceedings, although the dispute between them settled shortly before the trial.

Persimmon agreed that the strip foundations as designed and built were inadequate for the Property, given the ground conditions, and agreed that the remedial scheme would require a piled foundation solution. However, Persimmon disagreed with Mr Mallas on the extent of the overall remedial work required.

The matter progressed to trial, and based on the structural engineer's expert evidence provided, three schemes were considered:

1. A demolition and rebuild scheme with extensive piling, including piles to outside the footprint of the Property, such as the garden, retaining masonry wall and surrounding public footpath (the Claimant’s Scheme 1), together with optional prices for piles to and without the garage.
2. The same as the item above, with the retention of the existing timber frame (the Claimant’s Scheme 2), together with optional prices for piles to and without the garage.
3. A piled underpinning scheme to the existing house only (the Defendant’s scheme).

Pricing differences

As to the pricing of these schemes, my two counterparts and I applied a blend of Spon’s rates and market-tested rates, which we largely agreed on, save for the limited items which the judge considered in paragraphs [90]–[112]. At paragraph [89], the judge commented on this when saying:

“There is a considerable degree of agreement between Mr Daly and Mr Somerset as to many elements of the costings for that scheme, though there are also some points of difference that are non-trivial in financial terms.”

[Emphasis added]

A recent article in the Adjudication Society newsletter appeared to infer that Persimmon had defaulted “…to SPONS as its primary source of cost reference”.

This was not the case throughout – Persimmon relied on a blend of rates as noted above, including rates derived from Spon’s, although from reading the judgement, it is understandable how this inference might be drawn.

Further, in relation to the application of different rates addressed by the court between paragraphs [93]–[100], in the limited areas of difference between the quantum expert witnesses, the rates derived from Spon’s were one of several comparable rates provided, with their use qualified as such.

Notwithstanding this, the court did not find their inclusions as comparable rates useful.

Judgment

The court found that Scheme 3 was the most appropriate starting point and went on to consider the limited areas of disagreement between the quantum expert witnesses.

In doing so, and as with Horbury Villa (discussed above), the court showed a willingness to depart from the rates presented and make certain adjustments of its own, including an increase in contingency from 5% to 15%. This was to account for the risk to the Claimant for (i) known unknowns, (ii) the risk of latent defects and/or damage, (iii) that Spon’s might be understated in the context of a remediation scheme, (iv) damage to fitted furniture etc., and (v) inflation [106].

The court also went on to make certain allowances for professional fees, surveys and underpinning to the garage [107]–[112].

So what are the key takeaways from these two judgments? I think the judge in the Mallas case summed it up quite well:

“I am conscious that cost estimation is not an exact science, and that the rates that the Claimant ultimately has to pay will be determined by a range of factors and circumstances.” [92]

In summary:

1. A court might be willing to apply certain adjustments to a remedial scheme pricing.
2. Those adjustments might not be the same as those presented by the experts.
3. Where the experts do not agree, and Spon’s is applied, it should be regarded as a ‘starting point’ only.
4. Where departing from Spon’s, ensure the rates and reasoning are directly applicable to the matter in hand – any departure from this, even if qualified, may just be a distraction and considered inappropriate.
5. It may be appropriate to account for such “known unknowns” by way of an adjusted contingency allowance.

As to this final point, I would offer a word of caution. Whilst this approach may have merit in principle, and quantum experts may be able to reach a common view on an appropriate level of contingency adjustment, it remains vulnerable to challenge on grounds of subjectivity.

Although we agreed on appropriate contingency levels in both matters, none of the quantum expert witnesses in the Mallas case anticipated the approach ultimately adopted by the court.

Where practitioners can reach an agreement, that is clearly preferable. However, in the context of High Court litigation, this may be challenged on the basis of its inherent uncertainty.