When I did an extension couple of years ago yes I informed my insurance company as I knew I had to and knew as well they would greatly reduce the cover I had while the works were in place. If you are using just the one contractor then their insurance should cover the works, only if it was direct labour and you were the boss as such and hiring all the trades would you need self build insurance. That said I'm not never heard of it being used for extensions, it's usually new builds and you have to apply for it before the work starts. My house insurer actually required a copy of my builders policy to be sent to them beforehand.