Engineers and collateral warranties: what they are and their PII implications

~5 min read

Reviewed by Matthew Bartlett, Director · Last reviewed 2026-07-06

Collateral warranties are contractual instruments issued by engineers (and other consultants) to third parties who are not the primary appointing client — typically funders financing the project, purchasers acquiring the completed asset, tenants taking a long lease, or subsequent owners in a chain of ownership. The warranty extends the pool of parties who can bring a direct claim against the engineer for breach of the appointing contract, and it does so on contractual terms negotiated separately from the primary appointment. From a professional indemnity insurance perspective, collateral warranties materially affect engineering firms' claim exposure and are a live consideration at proposal stage and at every material project engagement. This entry sets out what warranties are, why they matter, and what engineers and their brokers should look for.

Why collateral warranties exist

The legal problem collateral warranties solve is the doctrine of privity of contract. A third party who is not a signatory to the engineer's appointment with the developer client generally cannot enforce the contract directly against the engineer. A tenant taking a lease of a completed office building, if the building turns out to have a defect the engineer negligently missed, would have no direct contractual route to the engineer without either a collateral warranty from the engineer or a benefit under the Contracts (Rights of Third Parties) Act 1999. The warranty provides the direct contractual bridge.

Warranties are typically required in three scenarios. First, funders providing project finance require warranties from the design team to protect their security interest — if the project fails, the funder wants a direct route to the professional advisers. Second, purchasers of the completed asset (either off-plan or on completion) require warranties to protect their acquisition. Third, first tenants taking substantial leases may require warranties as a condition of taking the lease.

What the warranty contains

A collateral warranty is a separate contract in its own right. It typically contains a covenant by the engineer that the engineer's work was carried out with reasonable skill and care, a mirror of the primary appointment on liability limits, defined step-in and assignment rights for the warranted party, and specific provisions on insurance maintenance and notification. The commercial terms are usually more limited than the primary appointment — the warranty is not intended to duplicate the primary liability, but to provide a direct route where the primary appointment cannot be enforced.

Two provisions matter most from a PII perspective. First, the net contribution clause — see the parallel entry on the net contribution clause. A warranty that omits net contribution reintroduces joint-and-several liability for the warranted party even where the primary appointment carried the clause. Second, the assignment provisions — a warranty that permits unrestricted assignment allows the warranted party to sell the benefit of the warranty to subsequent purchasers, materially widening the potential claim pool.

The insurer's perspective

PII insurers underwriting engineering firms ask about collateral warranty practice at proposal stage. Three questions recur. First, does the firm issue warranties, and if so on what terms — the standard RIBA/CIC wording, a bespoke wording, or a client-supplied wording? Second, how many warranties has the firm issued in the last five years, and on what value of projects? Third, does the firm's warranty wording carry a net contribution clause?

Firms with disciplined warranty practice — a standard wording, clear internal sign-off on any deviation, a register of live warranties — receive better underwriting than firms whose warranty practice is inconsistent. Insurers occasionally require, as a condition of cover, that warranties be issued only on approved wordings.

Third-party rights and the alternative

The Contracts (Rights of Third Parties) Act 1999 provides an alternative to collateral warranties. Where a primary appointment expressly permits a third party to enforce a specific term of the appointment, that party can do so without needing a separate warranty. In practice, warranties remain the market-preferred mechanism because they are drafted specifically for the warranted party and can include bespoke commercial provisions (assignment restrictions, notification obligations, step-in rights) that a general third-party rights clause cannot easily replicate.

Warranty limits and the BSA overlay

The Building Safety Act 2022 has changed the residential warranty position materially. Section 135's extension of DPA limitation to 30 years retrospectively and 15 years prospectively means that residential warranties now carry claim windows aligned with the DPA rather than with the primary contract's own limitation period. Engineers issuing residential warranties are exposed to claims for longer than they were before the BSA. Some engineers have narrowed their willingness to issue residential warranties in response, either declining to issue them or issuing them only on terms that expressly limit the DPA exposure.

Assignment restrictions

A warranty that permits unrestricted assignment is materially worse for the engineer than one with restricted assignment. Insurers prefer warranties limited to one or two assignments in total, or restricted to certain classes of assignee (e.g. only to the first purchaser or first tenant). Firms should not accept unrestricted assignment provisions without explicit consideration of the PI exposure they generate.

The volume question

Individual warranties are not usually problematic. A high volume of warranties on the same or related projects can be. A firm that has issued 50 collateral warranties on a single 500-unit residential development is exposed to claims from up to 50 parties on a defect that manifests across the development. Aggregation on the primary policy — whether related claims count as one under the each-and-every limit — becomes the load-bearing question.

Worked example

Illustrative only. A four-engineer structural consultancy has been asked to issue collateral warranties on a 120-unit residential development completing in 2026. The developer's proposed warranty wording omits a net contribution clause and permits unrestricted assignment. The engineer's own primary appointment contains a net contribution clause. Broker action: warranty wording review with the client's solicitor and the developer, resulting in a warranty that mirrors the primary appointment's net contribution position and limits assignment to two assignees per warranty. Insurer notified of the volume of warranties being issued; the primary PII policy's aggregation position confirmed as robust for a systemic defect scenario. Firm's warranty log updated.