Blog 042 - Don't become the Designer

Beware the Contractor’s Designed Portion (CDP). I’m seeing this mis-used by clients in two key ways:

1. Signing Specialist Contractors up to Design & Build (D&B) contracts without them realising or needing to be.

2. Pushing subcontractors into design responsibility via technical submittals and “proposals.”

   
   

The key is understanding your obligations. Don’t drift into D&B if you’re only meant to be detailing. The liability gap is huge. Even where contracts say “reasonable skill and care,” performance specs or warranties can drag you into “fitness-for-purpose” territory.

I see many Specialists — especially in complex or safety-critical trades — being appointed D&B when detailing would do. Example: asking an MEP subcontractor to specify boilers or pumps. Unless your scope clearly requires you to design, and you’ve got the PI cover to back it up, plant selection belongs with the Client’s professional team.

If you are taking on design, check your PI limit, exclusions, retroactive date, and run-off. Otherwise, you risk being uninsured.

Don’t let detailing become design

Clients will often push subcontractors to “submit a proposal” to plug gaps in their design. That’s how liability creeps. Shop drawings and tech subs should show compliance — but propose a deviation, and you risk becoming the designer.

Remember  - selecting plant can push a subcontractor into design liability.

If you must submit a Tech Sub, add a qualifier like: “Submission is for coordination only and does not extend our design responsibility beyond the scope of the subcontract.”

Detailing vs design

Clients will claim service routes, locations, ironmongery or finishes mean you’re D&B. Often that’s just detailing, you’re using your experience to improve buildability. But once your choices affect performance, compliance, or safety, it can become design.

Trades are problem-solvers — but don’t let that turn you into a designer unless you’re paid, insured, and resourced for it.

The trap of fitness-for-purpose

Even following standards may not save you. In MT Højgaard v E.ON (2017), a contractor complied with international codes, but because they were equally tied to a performance outcome, they were held liable on a fitness-for-purpose basis, despite following the approved code of practice.

Final point: Under the Building Safety Act, anyone that participates in design functions also take on statutory duty holder obligations, especially on higher-risk buildings. That makes “helping out with design” even riskier. Stay in your lane, specialist installation unless you are appropriately competent, compensated and insured to undertake design.

If you are unsure about your current design liabilities within your contract, drop me an email and lets have a chat.