PI wording · Defence costs deep-dive
PI defence cost erosion — what the worst case actually looks like
Reviewed by Matthew Bartlett, Director, Apex Insurance Brokers Limited · Published 14 July 2026
Most UK PI policies include defence costs within the limit of indemnity. In a well-defended claim, legal fees can erode a substantial portion of the limit before any damages settlement. This page sets out the worst-case picture — and how a specialist broker structures cover to protect against it.
The mechanics of erosion
- Policy limit is the total available for damages plus defence costs (if inclusive).
- Defence costs typically drawn on the limit as they are incurred.
- Complex claims can generate defence costs of £100k-£1m+ before any damages are settled.
- Where the eventual settlement or judgment approaches the limit, defence-cost erosion determines the effective damages ceiling.
- Multiple aggregating claims compound the erosion rate.
What a worst case looks like
- Scenario: Small firm with £2m limit, defence costs inclusive. Client alleges £1.8m loss from professional negligence.
- Two years of defence. Expert-witness fees, counsel fees, discovery, mediation attempts. Defence spend £350k.
- Trial. Additional legal fees £200k. Total defence £550k.
- Judgment. Client wins £1.6m damages.
- Policy response. £2m limit less £550k defence = £1.45m available for damages. Firm bears the £150k shortfall personally.
This is not an extreme scenario — it's the routine mathematics of a defended mid-range PI claim.
How to protect against erosion
- Higher policy limit. Simple but expensive. £3m instead of £2m gives £1m more headroom after defence costs.
- Defence costs additional to limit. Where available, insurer separately funds defence up to a defined additional limit. Premium loading typical.
- Aggregate reinstatement. Where policy reinstates after first aggregate exhaustion, additional claims have fresh cover. Not always available.
- Sub-limits for high-defence-cost activities. Ring-fence specific activity where defence expenditure is likely high.
- Cost-containment protocols. Insurer panel solicitors with fee caps, mediated resolution timelines, budget-monitoring at insurer level.
Aggregation and multiple-claim erosion
- Related claims aggregating under one policy limit multiply defence exposure.
- Each defended matter draws separately on the aggregate limit.
- Systemic issue affecting 5-10 clients can trigger 5-10 parallel defence processes.
- Aggregate exhaustion happens faster than most firms plan for.
- Reinstatement provisions matter more here than in single-claim scenarios.
What underwriters see
- Firms understanding defence-cost erosion signal risk-management maturity.
- Cover limits sized against the plausible worst case (damages + defence) are underwriting-positive.
- Sub-limits on high-defence-cost activities show sophisticated approach.
- Firms holding £5m aggregate with plausible £5m damages exposure and defence-cost consumption of £500k+ face limit inadequacy — underwriter flag.
Specialist broker approach
- Calculate plausible defence-cost worst-case for each firm profile.
- Size cover limit above the plausible worst-case combination.
- Test defence-costs-additional structure where market permits.
- Review aggregate exhaustion scenarios and reinstatement options.
- Update cover-sizing as firm risk profile evolves.
Frequently asked
How much can defence costs erode my PI policy limit?
For a moderately defended mid-range claim, £150k-£500k. For a heavily-defended complex claim, £1m+. Multi-year litigation with multiple experts and appeals can reach £2m+ in defence costs alone.
Is defence-costs-inclusive the market norm in 2026?
Yes for most UK professional PI. SRA MTC solicitors' mandatory layer, standard architects', accountants', surveyors' and engineers' PI wordings all typically include defence costs inclusive of the limit.
Can I get defence-costs-additional cover?
Sometimes. Available in some markets, particularly on higher-limit layered programmes and specialist-market placements. Premium loading applies. Specialist broker tests the market.
Does defence-cost erosion apply to regulatory investigation costs too?
Depends on the wording. Regulatory investigation cover may be sub-limited or included within the main limit. Some newer wordings provide regulatory-cost cover in addition to the main limit.
What is aggregate reinstatement?
A policy provision that restores the aggregate limit after the first aggregation event exhausts it, subject to specific conditions. Some policies include one automatic reinstatement; some offer paid reinstatement.
How does defence-cost erosion affect multi-claim scenarios?
Where multiple related claims arise under one aggregate limit, each claim generates its own defence costs. Aggregate limit is drawn on for damages plus defence across all matters. Exhaustion happens faster than in single-claim scenarios.
What if my limit is exhausted mid-defence?
The insurer's defence obligation ends when the limit is exhausted. The firm bears further legal costs personally. Cover-sizing should anticipate this — setting the limit above the plausible worst case avoids the scenario.
Should firms with strong risk management hold lower cover because claims are unlikely?
No — the point is not claim likelihood but claim consequence. A firm with strong risk management still faces the same worst-case cost profile when a claim does arise. Cover sizes to the tail, not the mean.