There are a couple of ways you can go about covering your house. You can approach your existing insurer and ask them to cover your existing property (you will need to tell them about this work either way) and then see if your builder has a contractors all risk policy (CAR) which he should have, to cover the structure and extension as he is working on it. If you are going direct build, you can take out a specialist policy that will cover your liability and property exposure in one policy.
There is also specialist policies that will cover your property whilst the extension is being done and also your contents (your existing insurer will only give basic cover whilst the extension is being done and will generally not cover contents)
A building under course of construction policy is a different type of policy and usually refers to a build on a green field site.