A: Yes, you should definitely tell your insurance provider before the work begins. Why before? Basically, it is to ensure you have the correct protection in place, not only for your current buildings and contents, but also for the additional building work that is being carried out.
Your insurance provider should also make sure you are covered in case, for example, building materials waiting for installation are stolen.
It is important you carefully read all the finer points of the contract you enter into with the builder. Quite often the contract will make you, the customer, responsible for any hired-in plant, site huts, caravans or temporary buildings; so you will need to discuss these with your insurance provider to ensure you are aware of your liability.