Theoretically you will need the approval of the local council. This will involve getting plans drawn up and paying fees. Your neighbours will be consulted and will probably object. You would then use the plans to ask for approval from your OC. You will find that the OC will probably not approve it because you are modifying common property by adding a shower and basin. If this requires a new floor waste this may need an engineers approval anyway etc etc…….
So while it is theoretically possible it is usually too hard to do. Because it can be so hard there are specialized building companies that will take it on for $$$$$.