I phoned Allianz some years ago when I got my attic converted and asked this question, they advised I should just increase my building cover to reflect this.
Although not officially a room, they consider it an additional bedroom and would note this on my policy.
They said, having an attic conversion in no way effects my home insurance policy, all benefits and cover remain the same.
I asked them to put all that in an email which they did, I keep this email with my policy documents.
My bank knew about it, attic conversion and home improvements was the reason given on the application form for the top up mortgage that paid for it, they had no problem with it.
I would definitely mention it to your insurers.