It is important that your Contractor is insured as follows;
PUBLIC LIABILITY INSURANCE
If there is an accident on site involving a member of the public it will be covered by the Insurance.
If one of the workers has an accident on site, again it is covered by Insurance.
That the building insurance covers the newly built structure until it is handed over to the Client on completion.
It would be normal practice to get your builder to email a copy of his insurance to your insurance broker or architect if you have one and let them check it is adequate.