Professional indemnity

Series clauses in PI aggregation explained

Category: Aggregation · Reviewed by the Apex broking team · Last reviewed 2026-08-22

In short: A series clause is the limb of an aggregation wording that gathers up a run of connected acts, omissions, matters or transactions and treats the claims arising from them as one claim. It is the limb that decides whether twenty similar files are twenty claims against twenty limits and twenty excesses, or one claim against one. The words that do the work are usually “a series of related…”, and after AIG Europe Ltd v Woodman the question is not whether the matters look alike but whether they are connected in significant ways.

Where the series limb sits

An aggregation clause normally offers the underwriter and the insured several alternative routes to treating multiple claims as one. Typical limbs group claims arising from a single act or omission; from one originating cause or source; from related acts or omissions; and from a series of related matters or transactions. The general architecture is set out in our entry on the aggregation clause, and the practical picture across the UK PI market is in aggregation of claims in PI insurance.

The series limb is the widest of the ordinary formulations short of an originating cause wording. It does not require a single act, and it does not require a single cause. It requires a set of things that can properly be called a series, and a relationship between the members of that set.

The two questions a series clause asks

First, is there a series at all? A series implies more than coincidence: a sequence or a set of matters that belong together rather than a random collection of files that happen to be open at the same time.

Second, are the members related? This is where the litigation happens, because “related” is doing almost all of the work, and it is not a term of art. The answer is a matter of construction of the particular wording applied to the particular facts.

AIG Europe Ltd v Woodman [2017] UKSC 18

The leading authority is AIG Europe Ltd v Woodman, decided by the Supreme Court on 22 March 2017. The wording was clause 2.5 of the Solicitors Regulation Authority’s minimum terms and conditions, which permitted claims to be treated as one where they arose from “similar acts or omissions in a series of related matters or transactions”.

The underlying facts involved a solicitors’ firm that held investor money in escrow for two holiday-development schemes, one in Turkey and one in Morocco. Money was released against a “cover test” intended to confirm that the security was sufficient. The test was misapplied, the developments failed, and a large number of investors sued. Whether the claims aggregated determined whether the insurer’s exposure was one limit or many.

The Court of Appeal had read the clause as requiring the transactions to be “intrinsically” related to one another, in the sense of being dependent on each other. The Supreme Court, in the judgment of Lord Toulson, rejected that gloss as neither necessary nor satisfactory. The correct approach is an objective, fact-sensitive inquiry into whether the transactions are connected in significant ways — whether, looked at as a whole, they fit together.

Applied to the facts, the claims relating to each development were aggregated: within a single scheme the transactions shared a common objective and were tied together through the trust and security structure. But the two developments could not be aggregated with each other. They were similar, and that was not enough. Apex has fuller notes at the AIG v Woodman deep dive and AIG Europe v Woodman.

“Similar” and “related” are not the same word

That distinction is the single most useful thing a professional firm can take from the case law. Twenty conveyances handled the same way by the same fee earner are similar. They become a series of related transactions only if there is something connecting them beyond the method — a common development, a common counterparty, a shared scheme, a single set of security arrangements. A firm reviewing its own exposure should ask what would link the files in the eyes of an outsider reading them together, not whether the same mistake was repeated.

The specific formulations used across the market are unpacked in series clause application, series of related matters, series of related transactions and series of related professional services. The contrast with an originating-cause style wording is drawn in originating cause versus matter.

Which way does aggregation cut?

There is no fixed answer, and this is the part most often assumed rather than analysed. Aggregation is neutral machinery whose effect depends on where the numbers land.

If a firm faces many small claims from one connected set of matters and the policy limit is generous, aggregating them into one claim means one excess rather than many — helpful. If the same connected set produces claims that together exceed the limit, aggregation caps recovery at a single limit and leaves the balance with the firm — painful. The direction of travel therefore depends on the relationship between the likely aggregate value of the connected claims, the limit structure, and the size of the excess. A firm whose work naturally produces clusters of related files should model both scenarios before deciding whether a wide or narrow aggregation clause suits it.

Where the professional body prescribes minimum terms, the firm often has no choice about the aggregation limb at all; the comparison across regulators is set out in aggregation clauses by regulator.

Excess, reinstatement and the practical mechanics

If a series clause bites, the aggregated claim usually attracts a single excess, and it usually erodes the limit once. Where the policy provides reinstatement, whether a reinstatement is triggered by an aggregated claim is a wording question and not a given. Defence costs treatment matters too: whether costs sit inside or outside the limit changes the arithmetic of an aggregated cluster considerably.

Negotiating the point at renewal

For firms outside a prescribed minimum-terms regime there is room to negotiate. The useful moves are usually about clarity rather than width: making sure the aggregation limb, the excess provision and any reinstatement provision are consistent with each other; making sure the definition of “claim” and the aggregation trigger use the same vocabulary; and pressing for a wording whose behaviour the firm has actually modelled against its own worst realistic cluster. Our practical guide is at how to negotiate PI aggregation clauses, and the broader picture at aggregation and series clauses in UK PI insurance.

Frequently asked questions

What is a series clause in professional indemnity insurance?

It is the limb of an aggregation wording that treats claims arising from a series of related acts, omissions, matters or transactions as a single claim for the purposes of the limit and the excess. It is wider than a single-act limb because it does not require one act, but narrower than an originating-cause wording because it requires the members of the series to be related to one another.

What did AIG v Woodman decide about "a series of related matters or transactions"?

The Supreme Court held on 22 March 2017 that the transactions do not have to be intrinsically related or dependent on one another, rejecting the Court of Appeal's narrower gloss. The test is an objective, fact-sensitive inquiry into whether the transactions are connected in significant ways and fit together. On the facts, claims within each of two developments aggregated, but the two developments did not aggregate with each other.

Is aggregation good or bad for the insured firm?

It depends on the numbers. Aggregating a cluster of small related claims means paying one excess instead of many, which helps. Aggregating a cluster whose total value exceeds the limit caps the recovery at a single limit, which hurts. The sensible approach is to model the firm's most likely cluster of related matters against the limit and the excess before deciding what wording to press for.

Does similar work automatically create a series of related matters?

No. Similarity of the acts or omissions is generally a separate requirement from relatedness of the matters, and on its own it does not aggregate. There has to be a real connection between the matters themselves, such as a shared scheme, transaction structure or counterparty, rather than simply the same error repeated across unconnected files.