When a professional's negligence causes loss, the client's instinct is often to sit tight and wait for the claim to resolve. English law imposes a duty on a claimant to take reasonable steps to reduce the loss, and damages are assessed on the footing that reasonable steps have been taken. A claimant who lets a bad situation get worse may find recoverable damages reduced accordingly.
The principle sits alongside but-for causation and contributory negligence: three related doctrines that shape what an insurer will actually pay.
The classic statement is in British Westinghouse Electric and Manufacturing Co Ltd v Underground Electric Railways Co of London Ltd [1912] AC 673. Viscount Haldane LC framed it as a duty on the claimant to take all reasonable steps to mitigate the loss consequent on the breach, and debarring the claimant from recovering any part of the damage due to neglect of such steps. The rule was already established — Roper v Johnson (1873) LR 8 CP 167 had applied it to a repudiated coal contract, holding the buyer to a duty to go into the market and buy replacement supplies rather than let the loss run.
It is not a duty owed to the defendant in the strict sense — the claimant cannot be sued for failing to mitigate. It is a rule about the measure of damages. The court asks what loss the claimant would have suffered had reasonable steps been taken, and awards that figure rather than the larger sum that actually accrued.
Reasonableness is judged from the claimant's position at the time, not with hindsight, and the bar is not set high. In Payzu Ltd v Saunders [1919] 2 KB 581 the Court of Appeal held that a buyer wrongfully refused credit was expected to accept the seller's alternative cash-terms offer, because a reasonable businessperson would have done so. The case also underlined the limits: a claimant is not required to take exceptional risks, put themselves in a humiliating position, or spend money they do not have.
A claimant is expected to:
A claimant is not required to spend beyond their means, borrow at penal rates, or take steps that carry a serious risk of making the loss worse.
The burden of proving a failure to mitigate rests on the defendant. It is not enough for the insurer to speculate. The insurer must plead and prove, on the balance of probabilities, that a specific reasonable step was available, that the claimant did not take it, and that taking it would have reduced the loss by a stated amount. Absent that evidence, the full loss stands.
A surveyor negligently under-values a property and misses a serious damp defect. The buyer completes on the strength of the survey. Six months in, the defect is discovered and a reasonable remediation quote comes in at £18,000. The buyer refuses the works and lets the defect run for two years, by which time the cost has risen to £42,000 because the damp has spread into structural timbers.
At trial the surveyor's insurer accepts negligence and the original £18,000. On the additional £24,000 the insurer runs a mitigation argument: the buyer was on notice, had a quote in hand, and could have funded the works. If the court accepts that a reasonable buyer would have done so, the incremental worsening is disallowed. The claimant recovers £18,000, not £42,000. That £24,000 gap is avoidable mitigation loss.
Whether mitigation was in fact reasonable in any given case turns on the buyer's financial position, the nature of the defect, the advice received, and the state of the market at the relevant dates.
For the professional facing a claim, mitigation is one of the most productive lines of defence. Document, from the moment the client is on notice, what steps the client took and when. For the client pursuing a claim, the discipline is the same in reverse: keep a contemporaneous record of steps taken, advice received, and reasons for any decision to hold rather than act. Claims prosper on evidence.
Apex Insurance Brokers Limited is authorised and regulated by the Financial Conduct Authority. Firm reference number 724952. This entry is general information, not advice on any particular policy.