Lodgers and mortgages?

I am in england uk. so uk only answers plz ! if i was to get a lodger, do i need to tell my mortgage company or convert to a buy-to-let mortgage? whats the official answer plz people?
Answer
If you have a lodger then technically yes you do have to tell your mortgage company. since it will still be your home you do not have to have a buy to let mortgage.i seriously advise that you make sure that the lodger signs an agreement - these are available free google it.