PI aggregation wordings compared: event, related matters and original cause

Reviewed by Matthew Bartlett, Director · Last reviewed 2026-07-11

Not all aggregation clauses are equal

Aggregation is the mechanism that decides whether several professional indemnity claims are treated as one for the purposes of the limit of indemnity and the self-insured excess. The words used to trigger aggregation vary from policy to policy, and the case law shows they differ significantly in width. Understanding where your wording sits on that spectrum is central to knowing what your cover really provides.

The three main wordings, narrowest to widest

1. Event or occurrence — the narrowest

An "event" or "occurrence" wording requires something happening at a particular time, place and in a particular way. The courts apply the unities of cause, locality, time and intent. Separate professional errors made for different clients at different times will rarely aggregate. This reading comes from decisions such as Lloyds TSB v Lloyds Bank, Municipal Mutual v Sea Insurance and Scott v Copenhagen Re.

2. Series of related matters or transactions — the middle ground

The Solicitors Regulation Authority Minimum Terms use "similar acts or omissions in a series of related matters or transactions". In AIG Europe v Woodman the Supreme Court held that the acts must be similar and the underlying matters must be genuinely related — connected, fitting together — before claims aggregate. This is wider than "event" but not unlimited.

3. One source or original cause — the widest

Wordings that aggregate claims "consequent on or attributable to one source or original cause" reach furthest. As Standard Life v Ace European and Spire Healthcare v RSA show, this can gather scattered losses that share a common origin, even where the individual claims look quite different.

Why the width matters both ways

A wider wording aggregates more readily. That reduces the number of excesses payable, which helps the insured, but it also consolidates more claims against a single limit, which can hurt. A narrower wording does the reverse. Neither is automatically preferable — the right answer depends on the limit structure and the kind of error a firm is realistically exposed to.

• Check which of the three families your policy uses.
• Read the aggregation clause together with the limit and the excess, never in isolation.
• Consider how a systemic error in your work would be characterised under that wording.

Applying it to your firm

The comparison is not academic. Firms reviewing solicitors' PI insurance live with the Minimum Terms wording, while surveyors and other professions may see any of the three families depending on the insurer. Apex reads the aggregating words alongside a firm's actual work when arranging cover.

A quick self-check

Before renewal, it is worth confirming three things about your aggregation clause:

• Which of the three families it belongs to — event, related matters, or original cause.
• Whether the limit is each-and-every-claim, aggregate, or a mixture.
• Whether defence costs sit inside or in addition to the limit.

Those three points together, not the wording alone, determine what your cover pays.

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.

Apex Insurance Brokers Limited is authorised and regulated by the Financial Conduct Authority. Firm reference number 724952.