Corner,
Firstly I am not an insurance expert or intermediary.
You need to be advised by an insurance expert or intermediary.
Part of your conversation should cover the below issues as a minimum.
You may need to take legal advice regarding employment law for Direct Labour.
You are effectively the Main Contractor so you will probably need all his insurances: Employers, Public Liability and All Risks.
It may also be appropriate for you to take out non-negligence cover where, even where everything is done correctly something occurs.
If you are designing the plans or elements of the building in the absence of a qualified Designer, you may also need to get Professional Indemnity cover.
Take a look at the Self-Build FAQ on this forum for an overview of some of the issues involved [this is a work in progress].
You are obliged by law to familiarise yourself with and understand your responsibilities under the Health and Safety legislation.
You are deemed to be acting as the Contractor under H&S legislation and may have responsibilities as Designer in the situation noted above.
Some responsibility may pass to specialist suppliers in relation to the design of components like stairs, but the law is not 100% clear on this point.
However if there's no-one else to take the responsibility, the buck stops with you.
FWIW
ONQ.
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