My own house has a self contained apartment at the side of it.It has its own front door and rear door.My planning permission states that that it has to be used as part of the main house and cannot be rented or used for commercial purposes.The banks will not accept it as a rental property due to the planning permission not allowing it.I also work in the building trade and all extension / granny flats I have done in the past are subject to similar planning terms.
There is a loophole in planning regs where if the building is over 7 years ( i think) you can apply for a change of usage for the building.To the best of my knowledge , and please correct me if i'm wrong , a folio number refers only to planning documentation .It is land registry that the banks are mainly concerned with.
You can show the bank that you intend to rent one room of your house out, and they will accept this as part of income proposal.They will not be concerned as to which room in particular you're lodger stays in.