NEC4 X15 limits the Contractor's liability for its own design to a professional skill and care standard, rather than the stricter duty to meet the Scope regardless of fault. The NEC4 title is "The Contractor's design", and the page heading above is our own short description of it.
The NEC3 ECC's heading is longer and describes a limit on design liability to reasonable skill and care. The NEC4 version does much more: it has clauses on use of material, keeping records and professional indemnity insurance. It's one of the most heavily negotiated NEC secondary Options on design and build work.
Why it matters
Under the core clauses, a Contractor that designs something is expected to make it comply with the Scope. If the finished work doesn't comply, there's a Defect, whether or not the design work was carefully done. Professional indemnity policies generally respond to negligence, not to a fitness or strict compliance obligation, which is the gap X15 helps to close. For the core position, see the Contractor's design.
NEC3 and NEC4 compared
| Point | NEC3 ECC | NEC4 (as in the subcontract text) |
|---|---|---|
| Standard | Not liable if it proves it used reasonable skill and care | Not liable unless it fell below the standard of a competent professional designing comparable works |
| Burden of proof | On the Contractor to prove care was used | The wording reads as though the party alleging a failure has to show it. Take advice |
| Correcting a Defect it's not liable for | Compensation event, X15.2 | Compensation event, X15.2 |
| Reuse of material | No provision | The Contractor may use its material elsewhere, subject to stated limits |
| Keeping design records | No provision | Yes, for the period for retention |
| Insurance | No provision | Professional indemnity cover, with a certificate on each renewal |
This page cites only X15.2 for NEC4. Look up the other sub-clauses in your own contract before you cite them.
A worked example
A fictional case with illustrative figures. A Contractor designs a retaining wall. The Scope requires a stated drainage performance, and X15 applies. Ten months after Completion, before the defects date, seepage appears, because ground conditions differed subtly from those the designer could have predicted.
- The Supervisor notifies a Defect before the defects date, and the Contractor corrects it for £90,000.
- An independent review shows that the designer did what a competent professional would have done.
- The Contractor isn't liable for the Defect, and so its correction is a compensation event under X15.2. It recovers the assessed change to the Prices.
- If the review had found that the designer failed to take normal care, the Contractor would have borne the cost and could look to its insurance.
The same facts without X15 would mean no compensation event, because the wall simply doesn't comply with the Scope.
What the Client should think about
- Risk returns to the Client. Under X15, a design failure with no negligence sits with the Client. It could be a concern where the Scope sets a performance outcome, such as an energy target.
- Who proves it? Expect the question to take time, and expect experts to be involved.
- Keep the insurance alive. Diarise renewal certificates and the end of the stated period. X15 cover is only useful while it runs.
- Read Option X18 together with this one. Option X18 can cap design liability for Defects not listed on the Defects Certificate.
What the Contractor should think about
- Back it up in the supply chain. A designer's appointment needs the same standard. If the appointment is weaker than the main contract, there's a gap.
- Keep your design records. They'll be the main evidence if liability is ever in question.
- Don't assume it covers everything. It's limited to Defects arising from the Contractor's own design. A workmanship Defect remains the Contractor's responsibility.
- Check the Z clauses. Fitness for purpose wording in the Scope or a Z clause can undercut X15. Reading the Z clauses guide first will help you spot this.
Related guides
- NEC Secondary Options: X, Y, W and Z Clauses Explained
- X16: Retention
- X17: Low Performance Damages
- X18: Limitation of Liability
- X13: Performance Bond
- X14: Advanced Payment to the Contractor
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This guide is general information in our own words, not legal advice. Your contract and its Contract Data always govern. NEC, NEC3 and NEC4 are trade marks of Thomas Telford Ltd; Contract Status is not affiliated with or endorsed by NEC.