Aggregation of claims in UK professional indemnity insurance
Aggregation is the mechanism by which a PI wording treats multiple claims arising from a common cause. The choice between an aggregated wording and a per-claim wording can be the difference between paying a single limit and paying multiple limits — often the largest structural decision in a placement, larger even than the sticker limit itself.
What aggregation actually means
In its simplest form, aggregation is a rule the insurer uses to answer one question: are these two claims ‘the same’ for policy limit purposes?
The mechanism matters most where a professional has given the same or similar advice to many clients — a repeatable service model, a shared drafting error, a common software configuration. Where aggregation applies, the insurer treats the claims as one and pays one limit across all of them, less prior payments and defence. Where aggregation does not apply, each claim gets its own limit.
For a firm carrying £5m limit with 20 clients affected by a common error, the difference is stark: £5m of cover total under aggregation, potentially £100m in theoretical maximum exposure without.
The four main aggregation-wording types
1. Per claim (no aggregation)
Each claim carries its own limit. Most protective for the insured; costliest for insurers. Rare in modern PI wordings outside specific regulated regimes (e.g. SRA MTC minimum-terms language).
2. Aggregated by claimant
Multiple claims from the same claimant treated as one. Most common in personal-injury adjacent classes.
3. Aggregated by act, error or omission
The classic middle-ground wording. Claims arising from the same underlying professional act are aggregated. Widely used in accountants and general-professional wordings.
4. Aggregated by series — the broadest form
Claims arising from ‘the same or a related series of acts, errors, omissions or events’ are aggregated. Series-language is the strictest for the insured and is common in cyber-adjacent and technology PI.
The key case law
Lloyds TSB v Lloyds Bank Group Insurance [2003]
The House of Lords decision that set the modern ‘unifying factor’ test. Claims aggregate where they share a common originating cause — a shared professional act, decision or event.
AIG v Woodman [2017]
Supreme Court decision on the meaning of ‘the same or a related series of acts’ in the SRA MTC. Confirmed that the connection between acts must be real and material, not incidental.
Baines v Dixon Coles [2021]
Court of Appeal decision on aggregation in solicitor conveyancing claims. Clarified how conveyancing errors across multiple transactions do or do not aggregate.
How aggregation plays out by profession
Solicitors under the SRA MTC
The SRA MTC uses the ‘same or a related series of acts’ language. Interpretation follows AIG v Woodman — requires real connection between the acts, not merely coincident timing.
Architects and engineers under BSA 2022
BSA 2022 s.135 does not directly change aggregation but the extended 30-year tail on higher-risk-building work materially raises the stakes. Aggregation across a single project's multiple homeowners is now a live question for firms that worked on residential developments.
Accountants and tax advisers
R&D tax credit advisers face the sharpest aggregation risk — a shared methodology across multiple clients that HMRC subsequently challenges creates the classic series scenario.
IT consultants and software developers
Repeatable code, SaaS platforms and shared framework configurations create textbook aggregation exposure. A single library flaw affecting 50 clients aggregates.
Aggregation at renewal — what to test
- Read the aggregation clause word by word. ‘Claim’ vs ‘occurrence’ vs ‘act, error or omission’ vs ‘series’ all mean different things.
- Ask the insurer to confirm treatment of specific scenarios you can identify in your practice mix.
- Test against real portfolio: if you had one shared error affecting your five biggest clients, would this wording pay one limit or five?
- Compare wordings across insurers at market run — aggregation language varies materially even between comparable placements.
- Consider excess layers that specifically respond to aggregation events where primary is aggregated.