If your subcontract package includes any element of Contractor’s Designed Portion (CDP)-whether it is an M&E installation, structural steel connections, roofing details, or glazed partitioning-you are taking on professional design risk. You carry Professional Indemnity (PI) insurance to cover that exposure.
What commercial managers rarely tell you is that a single small-print tweak buried inside a bespoke Schedule of Amendments can render your PI insurance policy completely worthless the moment a design issue emerges on site.
That trap lives inside JCT Clause 2.19.
1. The Technical Clause: Reasonable Skill and Care vs Fitness for Purpose
Under unamended standard forms (such as the JCT Design and Build Subcontract DBSub):
Clause 2.19 provides that insofar as the Subcontractor designs any part of the Subcontract Works, they owe the Contractor an obligation equivalent to the standard duty of an architect or professional designer: reasonable skill and care.
However, main contractors regularly strike this baseline out in their bespoke Schedule of Amendments, replacing it with an absolute guarantee:
“The Subcontractor warrants that the CDP works shall, when completed, be fit for their intended purpose as set out or reasonably inferred from the Subcontract Documents.”
2. Plain English Translation: What This Means on Site
The difference between these two legal standards determines who pays when something fails:
- Reasonable Skill and Care: You are assessed against whether a competent specialist designer in your trade would have done the same thing. If the system fails despite you following British Standards, manufacturer guidance, and standard engineering practice, you have not breached your duty.
- Fitness for Purpose: You guarantee an absolute outcome, full stop. It does not matter if you were not negligent, followed approved engineering standards, or took every reasonable precaution. If the finished installation fails to achieve the client’s operational purpose (such as an HVAC system struggling to hit temperature during an unseasonal heatwave), you are in strict breach of contract.
3. The Financial Risk: The Uninsurable Liability Trap
This is where trade subcontractors suffer catastrophic losses:
Almost every standard UK Professional Indemnity insurance policy explicitly excludes liability assumed under contract that goes beyond reasonable skill and care.
When you agree to a bespoke “fitness for purpose” warranty in JCT Clause 2.19:
- Your PI insurer can legitimately refuse to indemnify the claim because you agreed to a higher contractual standard than common law requires.
- You are left personally liable to cover the cost of redesign, strip-out, reinstatement, and employer losses directly out of your business cash reserves.
- A £40,000 package design defect can rapidly escalate into a £150,000+ uninsured corporate loss.
Unamended vs Amended Clause 2.19: Comparison Table
| Contract Element | Standard JCT DBSub Form | Aggressive Bespoke Amendment | Action Required |
|---|---|---|---|
| Design Standard | Reasonable skill, care, and diligence | Absolute “fitness for purpose” warranty | Strike out. Reinstate standard duty of care. |
| PI Insurance Alignment | 100% compliant with standard UK PI cover | Voids PI policy coverage for design failure | Insist. Match subcontract terms to your PI schedule. |
| Upstream Performance Guarantees | Excluded unless expressly specified | Flows down client operational performance specs | Delete. Limit duty strictly to specialist design scope. |
| Disclaimers for Other Designs | Subcontractor not liable for main contract designs | Subcontractor warrants all interface drawings | Clarify. Exclude liability for prior designer errors. |
The Ready-to-Negotiate Redline
Before signing an amended JCT agreement with CDP elements, audit Clause 2.19 and any related warranty schedules. If you see “fitness for purpose” or performance warranties, push back with this redline:
“Strike out all references to ‘fitness for purpose’ and substitute: ‘The Subcontractor’s duty under Clause 2.19 in respect of design shall be limited strictly to the exercise of reasonable skill and care to be expected of an appropriately qualified and competent specialist subcontractor carrying out similar work. Nothing in this Subcontract shall impose an absolute obligation of fitness for purpose.'”
Internal Linking & Commercial Balance
Design disputes rarely happen in isolation on site. Main contractors frequently combine alleged design shortfalls with delayed valuations and aggressive deductions. Ensure you understand how bespoke terms affect your entire commercial position:
- Read how commercial teams slash interim applications using bespoke JCT Clause 4 Pay Less Notice traps.
- Learn how to protect your extension of time claims against JCT Clause 2.26 3-day condition precedent time-bars.
Audit Your Subcontract Before You Sign Away Your Margin
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