Aggregation is the mechanism inside a professional indemnity policy that decides whether several linked claims count as one claim, sharing a single limit of indemnity, or as separate claims each drawing its own limit. For architects this matters commercially. A single design decision can be reproduced across dozens of plots, and a single specification error can be exposed by the Building Safety Act 2022's extended lookback. How the policy aggregates determines whether cover holds or collapses.
The Architects Registration Board sets the regulatory baseline. Standard 8 of the ARB Code of Conduct and Practice requires an architect to hold adequate and appropriate professional indemnity insurance for the work being undertaken. What the Code does not do is specify aggregation wording, minimum limits or a mandated set of terms. This is a marked contrast with the SRA Minimum Terms and Conditions that govern solicitors, where aggregation language is prescribed. Architects rely on the commercial market to word their cover, subject only to ARB's general adequacy test.
RIBA guidance on PI cover mirrors this approach. RIBA encourages members to consider the limit relative to project value and cumulative exposure, to review aggregation carefully, and to seek broker advice where the practice takes on repeat or series work. RIBA does not, and cannot, dictate wording — but it consistently flags aggregation as a clause that deserves scrutiny at renewal.
Most architect PI policies test aggregation on a "one claim" or "similar acts, errors or omissions" basis rather than the broader "originating cause" test seen in some solicitor and adviser wordings. The Supreme Court's decision in AIG Europe Ltd v Woodman [2017] UKSC 18 is the leading modern authority on the outer reach of aggregation language, examining what "a series of related matters or transactions" can capture. The earlier Lloyds TSB General Insurance Holdings Ltd v Lloyds Bank Group Insurance Co Ltd [2003] UKHL 48 remains the touchstone for the narrower "originating cause" construction. Between them, they establish that aggregation turns on the precise words used, and that the burden falls on the party seeking to aggregate to demonstrate the required connection.
Three fact patterns recur. A multi-plot residential development where the same design is repeated across, say, 20 apartments, and a defect emerges in every unit. Sequential design phases on a single project where errors in the concept phase carry through to detailed design. And the cladding-wave pattern — multiple, contractually separate projects that share a common non-compliant specification developed by the same practice.
In the multi-plot case, whether 20 apartment defects aggregate as one claim or 20 turns on the wording. Under "similar acts", a common design fault will almost certainly aggregate. The cladding-wave scenario is harder — different clients, different sites, different contracts, but a shared specification error. On a "similar acts" test the insurer's position is that the acts are plainly similar and one limit applies. That is the position architects need to plan for.
Section 135 of the Building Safety Act 2022 extended the limitation period for claims under section 1 of the Defective Premises Act 1972 to 30 years retrospectively for completed dwellings and 15 years prospectively. A design decision taken in the 1990s can now be actioned in the 2020s. Aggregation matters over that 30-year horizon in a way it did not before, because the number of projects potentially caught by a single specification error is far larger.
Worked example, illustrative only. A mid-sized architectural practice designed cladding on eight residential blocks between 2016 and 2019 using the same specification, later found non-compliant. In 2024, BSA 2022 claims total £14m across all eight blocks. The practice holds a £5m PI limit on an "any one claim" basis with "similar acts" aggregation.
Under the "similar acts" test, the eight claims aggregate to a single claim and the £5m limit is the ceiling on the insurer's exposure — the practice faces a £9m gap plus defence costs. Under a hypothetical "one claim per project" wording, each block draws its own £5m limit, giving up to £40m of theoretical cover and comfortably absorbing the £14m exposure. The wording is the difference between a solvent outcome and an insolvent one.
Apex approaches architect PI renewals with aggregation as a first-order question. The steps are: identify series and repeat work in the practice's portfolio; ask the insurer to confirm the aggregation test in writing, not just by reference to the policy schedule; negotiate for the most architect-favourable language available in the market for the risk profile; consider excess-of-loss layers where cumulative exposure exceeds the primary limit; and record the client's fair-presentation disclosure of series and repeat exposures at inception and each renewal.
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.