Accommodation question

My partner is currently on a contract for one in his rental apartment. The landlord has already stated that it is fine for us to live there together and that we can draft up a new contract for two people once their term runs out. If we would like to start the fiancé application prior to this what documents would you recommend acquiring? We're thinking of getting a written confirmation from the landlord that we have already agreed on these new terms and the house is eligible for two people living there.