Where cost is really decided in subsidence and underground claims
By Steven Coxon, Tim Branfield - 17th September 2026
Cost control in subsidence and underground claims is usually discussed in terms of reserve and repair. In practice those are the more stable elements, because the category of damage does not change once it is established and the repair reserve tends to stay where it was set. The variable that moves is the cost of establishing cause, and in claims involving the ground and drainage beneath a property, where subsidence, escape of water and damage to drains frequently share a cause, that is rarely a quick question to answer.
Why cause is harder to establish than it looks
The real cause often begins somewhere other than where it initially appears to. Steven Coxon, Head of Subsidence at Claims Consortium Group, estimates that roughly 20 to 25 per cent of valid subsidence claims are caused by an escape of water, with the majority caused by vegetation and a very small proportion arising from other factors such as mining, peat or consolidation.
Two routes account for this:
- Defects within existing underground drainage which leaks and washes away the fines (fine silt and clay particles).
- Rainwater discharges straight to ground from a downpipe rather than into a drain, washing away soil and fines in the same way without any drainage failure involved at all.
Both routes generally need rain to express themselves, which is why a peril most people associate with summer can become visible during wetter periods of the year, once the ground starts taking water again.
Establishing which applies takes more than one input, and each contributes something different:
- A CCTV survey confirms whether the drains are damaged and identifies the affected area and runs.
- A site investigation determines soil conditions, strength and type as well as foundation depths and whether the ground is desiccated.
- Monitoring establishes the pattern of movement over time.
Subsidence Claims Specialist Tim Branfield explains that a technical review will then be undertaken, reviewing such things as foundation depths, cohesive and incohesive soils, soil strength, ground moisture content and the CCTV drainage survey, to confirm the likelihood that the drain has influenced the cause of the damage or has itself been damaged as a result of ground movement.
Where ground movement has caused displaced joints and leaks, this is normally a sub-issue rather than the direct cause of the subsidence, which matters when a CCTV survey comes back showing damage and the temptation is to treat that as the answer.
What the investigation actually costs
A tree-related claim will typically absorb more in investigation costs and monitoring than a drainage claim will, so the exposure created by uncertainty over cause is concentrated there rather than in the reserve. It accumulates quietly across a portfolio instead of showing up as visible movement on individual claims, which is what makes it easy to miss.
Repairs are also sometimes instructed where it has not been possible to confirm that the damage is contributing to the reported issue, as a process of elimination, with the property then monitored further. Mark Grisdale, drainage claims specialist in the underground services team at Claims Consortium Group, describes this as a normal part of resolving cause rather than an exception, which means an element of repair spend can sit inside the investigation phase rather than after it.
The decisions that control it
Season does a good deal of the interpretive work and is available before anything is commissioned. Damage presenting through the winter months is unlikely to have been caused by vegetation, and where monitoring shows downward movement during the wetter winter months the cause is more likely to be drains or poor supporting ground. In simple terms – a claim received October to December reporting recent crack damage with no signs of vegetation nearby is likely to be drains, while a claim received in July or August with a large oak next to the area of damage is likely to be vegetation and clay shrinkage subsidence.
Knowing where damage is commonly found narrows the search. From observation, it most often sits at gullies attached to the building, at the rest bend from a soil vent pipe and the pipework immediately around it, or where a drain runs parallel to the property rather than away from it, such as along a drive at the side or front where the pipework passes close to the building on its way to a main carrier line.
Cause also determines how the claim is routed. Tim notes that where there is damage to drains within the area of damage, it is likely to be covered under the subsidence policy, either because it is identified as the cause of the movement or because the drain has been damaged as a consequence of it, and that this is a matter for technical review rather than a fixed distance. Where damage sits remote from the area of damage, the customer is advised to log a separate accidental damage to drains claim. That distinction determines which policy responds, which specialism handles the work and how the claim is recorded.
What this means for the months ahead
Notifications arriving now carry a seasonal signal that was not available in July and is worth reading before investigation is commissioned, a meaningful proportion of what presents as one peril will have started as another, and the cost of resolving cause is concentrated in investigation, monitoring and sometimes repair, which is where the control sits.
