If a main contractor hands you a JCT Design and Build Subcontract (DBSub/A), you are legally responsible for any part of the works you design. Under standard JCT terms, your obligation is balanced. However, main contractors frequently insert custom amendments into Clause 2.17 that completely rewrite the legal rules of the game. If you miss this change, you could find yourself personally liable for catastrophic design failures that your corporate insurance will not cover.
The Standard Rule: Reasonable Skill and Care
By default, unamended JCT frameworks state that a subcontractor must exercise reasonable skill and care in their design. This means your work is judged against the professional standards of an experienced consultant or engineer. If something goes wrong, the main contractor must prove that you were negligent or careless in your calculations. Crucially, standard Professional Indemnity (PI) insurance policies are custom-built to cover this exact standard of risk.
The Trap: “Fitness for Purpose” Overrides
The commercial exposure happens when the main contractor amends Clause 2.17 to include the phrase “Fit for Purpose” or hooks your liability directly to the main contract standard.
This single wording change shifts your design obligation from a performance duty to an absolute guarantee. Under a fitness for purpose clause, it does not matter if your engineering calculations were flawless or if you followed every British Standard to the letter. If the final installation fails to achieve the main contractor’s intended end-use or operational purpose, you are in breach of contract.
The Insurance Black Hole
This is where trade profit margins are destroyed. Almost all standard UK Professional Indemnity insurance policies contain an absolute exclusion for Fitness for Purpose guarantees. PI policies cover negligence; they do not cover absolute commercial guarantees of functionality. If you sign an amended Clause 2.17 that elevates your design exposure to fitness for purpose, you are operating completely uninsured. If a design dispute arises, the legal costs and damages will come directly out of your company cash flow, risking instant corporate insolvency.
How to Protect Your Trade Margin
Before you sign any JCT Design and Build Subcontract, you must inspect Clause 2.17 and ensure it explicitly keeps to a standard of reasonable skill and care. If you spot amendments attempting to insert absolute suitability or purpose guarantees, strike them out immediately or demand a formal amendment protecting your policy limits.
Main contractors routinely rely on busy subcontractors skim-reading their design packages without spotting custom text variations. Do not take an uninsurable gamble with your design risk. Before you sign your next draft agreement, use the JCT contract checker on Trade Contracts Simplified to run an instant, automated health check on your paperwork. Our system scans your document in seconds, identifying hidden liability upgrades, unfair design transfers, and uninsurable terms before they damage your company cash flow.
Behind the Tech: Built for the Trades
I custom-built this automated contract scanner through my parent technology consultancy, Your Tech Neighbour. Operating out of Lyde Green, I specialise in building straight-talking, practical AI tools and custom workflow automation systems that eliminate manual admin bottle-necks for UK construction firms, commercial specialists, and independent tradespeople.
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