Do we have two flats or one house?

We have bought a house that has been split into two flats. Each flat has seperate utilities and we pay council rate for each flat. However the title deeds just have the house as a whole. We hope to upgrade one flat to a Furnished Holiday Let, with the potential to be able to sell it later on. The current situation has been the case for at least 10 years.
I am assuming that we do not need to go through planning as the council must already recognise the two flats.

1) Do I need planning to create a Furnished Holiday Let?
2) How do I get the two flats recognised on the title deeds?

Any help appreciated.