You should get in touch with your insurer before the work is carried out, so they can assess the risk to the property and adjust your premium if necessary. Allow at least a couple of weeks before the work is due to start.
Depending on what you're planning to have done, your insurer may change the terms of your policy or suggest completely new cover.
Make sure you have all the necessary information and documents to hand – your insurer will likely want to know things like:
The type of work you're having done
The estimated cost of the work
How long it will take
If the building will be empty while the work is being carried out
If the builders have adequate public liability insurance.