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Aggregation in the ARB/RIBA wording for architects' PI

Architects in the UK are regulated by the Architects Registration Board (ARB) and many are also members of the Royal Institute of British Architects (RIBA). Neither body prescribes specific PI wording in the way the SRA does for solicitors. Instead, the ARB Code requires "adequate and appropriate" PI cover, and the market produces a range of policy wordings each with its own aggregation language. The result is that architects' PI aggregation is more wording-dependent than solicitors' but tends to follow a fairly consistent project-level pattern.

This article is Spoke 6 of the Apex hub on aggregation and series clauses in PI insurance.

Plain English explanation

An architect's work is usually organised by project. One commission, one project, one fee, one set of drawings, one Practical Completion date. The PI wording reflects that: aggregation typically aggregates claims arising from a single project as a single matter. So multiple errors on one project (a structural design error, a fire safety error, a procurement error) usually aggregate into one claim. Multiple errors on different projects usually do not aggregate, even if they are similar in character.

The picture changes for architects working on framework agreements, on large multi-phase developments, on residential schemes for repeat developer clients, and on cladding/fire safety reviews of multiple buildings. Each of these structures can drive aggregation across what otherwise look like separate projects.

The regulatory framework

ARB's Architects Code, paragraph 8.1, requires architects to "have adequate and appropriate insurance cover for you, your practice and your employees". The Code does not specify a minimum sum or particular wording. ARB publishes guidance setting out factors to consider (size of practice, type of work, contract values, retention period) and pointing to RIBA's standard appointment documents which include indemnity expectations.

RIBA's Standard Professional Services Contract for architectural services (current edition: RIBA Professional Services Contract 2020 as updated) includes default PI provisions in the appointment that the architect agrees to maintain — typically a minimum aggregate limit of £2 million or higher depending on project value, with run-off cover for a fixed period (usually six or twelve years).

Neither ARB nor RIBA prescribes the aggregation wording. The market wording does.

The typical market wording

A standard architect PI wording in 2026 will include some version of the following aggregation clause:

"All Claims against the Insured arising out of the same act, error or omission shall be considered one Claim. All Claims arising out of any one series of related acts, errors or omissions shall be considered one Claim. All Claims arising from any one Project shall, in addition, be considered one Claim."

The third rung — "all Claims arising from any one Project" — is distinctive to architect wordings. It expressly aggregates by project, regardless of how many separate errors or omissions are made within the project. The Project is usually defined elsewhere in the policy, often as "the design and supervision services provided by the Insured in respect of a single development or building".

The combined effect of the three rungs is that:

Multiple errors on one project = one claim (the "project" rung).
Multiple errors on multiple projects with a connecting series feature = one claim (the "series" rung).
Independent errors on independent projects = multiple claims.

How Woodman and Spire apply

The "series of related acts, errors or omissions" rung tracks the AIG v Woodman test from Spoke 1. Are the errors a series, and are they connected in a real way? The "project" rung aggregates by reference to the unit of work, not by reference to a cause or a relationship between matters. There is no need to invoke Woodman or Spire; the project is itself the aggregating unit.

The interaction matters for multi-phase developments and framework agreements:

Multi-phase developments. A single development built in five phases over five years may be one Project (if defined that way in the appointment) or five Projects. If one, all phases aggregate. If five, each phase is its own claim and aggregation between phases requires the "series" rung.

Framework agreements. An architect on a framework agreement with a developer might design 20 buildings on 20 sites over five years. Each building is usually its own Project (different site, different planning permission, different contract value, different end-user). Aggregation across the 20 buildings requires the "series" rung, which under Woodman requires the buildings to be related in a real way (common framework, common developer client, common design template — these usually suffice).

Phase-based design / build commissions. Particularly common in residential development. Architect designs Phase 1 of an estate, then Phase 2, then Phase 3. Are they three projects or one? The policy wording — and the appointment — controls.

Three high-risk aggregation patterns for architects

Cladding and fire safety review. Post-Grenfell, many architects have undertaken fire safety reviews of multiple buildings for the same client (typically a housing association, local authority or developer). If a methodological flaw runs through the reviews, the claims aggregate. The cladding remediation industry is now in the late stages of a multi-year wave of claims, and aggregation is litigated on virtually every cluster.

Modular and prefabricated design. Architects designing repeating modules used across multiple sites face systemic exposure. A flaw in the standard module reappears in every site that uses it. Aggregation across sites is almost certain under the "series" rung.

Volume residential schemes. A practice doing 200 design-and-build houses for a single developer on multiple estates faces both project-level and series-level aggregation questions. The wording controls.

Worked example with numbers

Take an architectural practice with a £2 million primary PI limit and a £25,000 excess. Three scenarios.

Scenario 1 — Single project, multiple defects. Architect designs a 50-unit residential block. Errors include a fire compartmentation defect (£1.5 million to remediate), a structural fault in the lift core (£500,000) and an inadequate balcony detail (£300,000). Total claim £2.3 million.

One Project = one claim under the project rung. £25,000 excess, £2 million limit. Insurer pays £1.975 million; architect pays £25,000 + £300,000 limit shortfall = £325,000.
Compare without project aggregation: three claims, each within the £2 million limit. Insurer pays £2.275 million net of three £25,000 excesses. Architect pays £75,000.
Project aggregation here costs the architect £250,000.

Scenario 2 — Multiple projects, common cladding error. Architect designs 8 buildings on 8 sites for the same developer client between 2017 and 2020. Each building uses the same cladding system. Post-Grenfell remediation costs are c. £400,000 per building, total £3.2 million.

"Series" rung: 8 buildings connected by common developer, common timeframe, common cladding system. Likely aggregates under Woodman. One £25,000 excess, £2 million limit. Architect pays £25,000 + £1.2 million limit shortfall = £1.225 million.
Compare without aggregation: 8 individual claims, each within £2 million. Insurer pays 8 × £375,000 = £3 million. Architect pays 8 × £25,000 = £200,000 in excesses.
Aggregation here costs the architect c. £1 million more than non-aggregation.

Scenario 3 — Independent commissions, similar errors. Architect designs 5 unrelated buildings for 5 unrelated clients on 5 different sites. Each has an inadequate stair-design (the same standard misunderstood). Each costs £100,000 to remediate. Total £500,000.

Are the 5 buildings a "series of related matters or transactions"? Probably not under Woodman: different clients, different sites, different timing, no structural connection. Each is its own matter.
"Same act, error or omission" rung: the architect's misunderstanding of the standard might be argued as a single error repeated, but the AIG v Woodman analysis suggests not — each instance is a separate omission, even if identical in character.
"Series of related acts, errors or omissions" rung: closest call. The omissions are related (common methodological flaw) but the underlying matters are not. The Court of Appeal pre-Woodman might have aggregated; post-Woodman probably not.
Likely outcome: 5 individual claims. Architect pays 5 × £25,000 = £125,000 in excesses; insurer pays 5 × £75,000 = £375,000.

These three scenarios show why architect aggregation analysis is highly project-shape dependent. A single project absorbs everything (good for excess, bad for limit). Multiple connected projects aggregate (mixed). Multiple unconnected projects do not aggregate (good for limit, bad for excess multiplication).

Sector implications within architecture

Practices doing single large commercial projects. Project aggregation is the dominant feature. Limit adequacy is everything.

Practices on framework agreements. Aggregation across the framework is plausible. Limit adequacy should be sized for the worst-case framework cluster, not the worst-case single project.

Practices doing residential developer work. High risk of aggregation across an estate, a phase or a portfolio of estates.

Practices doing post-Grenfell cladding review. High aggregation risk — see above. Many wordings now include specific cladding exclusions or sub-limits; review carefully.

Sole practitioner architects. Aggregation by project usually works in favour of the sole practitioner because excess multiplication risk is low (each project is one matter regardless). But limit adequacy is critical, particularly post-PC.

What this means for your firm

Understand how "Project" is defined in your policy. Read the definition carefully. Is it per appointment? Per planning permission? Per building? Per phase? Per development? The definition controls how multi-phase work aggregates.

Map your framework and repeat-developer exposures. If you have framework agreements or repeat-client work, those are aggregating features under Woodman. Disclose them at proposal and size limits accordingly.

Buy run-off cover sized for your worst-case post-PC cluster. Architect PI run-off is typically 6 or 12 years. The aggregating clauses apply in run-off as in live cover. See our architects' run-off cover deep dive.

Stress-test for post-Grenfell exposure. If you have done any cladding or fire safety review work, your worst-case aggregated cluster could be very large. Many policies now have sub-limits or exclusions; check yours.

Disclose at proposal. Use the cluster-disclosure approach in our architects PI proposal completion guide.

How architects' aggregation differs from solicitors' and surveyors'

Project rung is distinctive. Architects' wordings typically include an express project-level aggregation rung that solicitors' and surveyors' wordings do not. This makes aggregation analysis cleaner in the simple case (one project, one claim) but more complex in multi-phase and framework cases.

Cladding overlay is unique to architects (and contractors). The post-Grenfell wave of claims has produced bespoke aggregation language and bespoke sub-limits in architect wordings that do not exist in other professions.

Design and Build wordings can shift aggregation. D&B contractors and design-led contractors operate under a separate wording (see Spoke 12 and the D&B contractor proposal completion guide).

FAQs

Q1. Does ARB prescribe aggregation wording? No. ARB's Code requires "adequate and appropriate" cover but does not prescribe specific wording.

Q2. Does RIBA prescribe aggregation wording? No. RIBA's standard appointments include PI maintenance requirements but do not prescribe specific aggregation language.

Q3. What is a "Project" in my policy? Depends on the policy definition. Often "the design and supervision services provided in respect of a single development or building". Some wordings define it by appointment; others by site; others by planning permission. Read carefully.

Q4. Does the project rung override the series rung? The two rungs operate in parallel — claims aggregate if any rung is satisfied. The project rung aggregates within a project; the series rung aggregates across projects where the matters are related.

Q5. How do framework agreements affect aggregation? A framework agreement is a connecting feature for the Woodman series test. Multiple projects under one framework will usually aggregate as a series.

Q6. Are there specific cladding aggregation rules? Many architect wordings now include cladding exclusions, sub-limits, or bespoke aggregation provisions for cladding claims. There is no industry-standard approach; the wording varies between insurers. Review yours.

Q7. Does the practice need separate cover for each director? No. The policy covers the practice and its named insureds. Aggregation applies to "any one or more Insured" claims.

Q8. How does limit adequacy work for architects? Test the worst-case project value against the limit. For a £10 million residential project, a £2 million limit is plainly inadequate if the project has remediation issues. For larger projects, limits of £10 million to £25 million or higher are standard.

Q9. What is the typical run-off requirement? Most appointments require 6 or 12 years of run-off. Some local authority and housing association appointments require 12 years. Read each appointment.

Q10. What is the most important practical step? Read your policy's project definition and your appointment's PI requirements. Make sure the two align. Discrepancies create coverage gaps that the architect carries personally.

Aggregation in ARB and RIBA architect PI wordings