In simple terms, everything that’s within the space of an apartment is the responsibility (and ownership) of whoever owns that space, and that will not therefore be covered by the body corporate as common property.
This means that the owners of the apartment are responsible for the maintenance and repairs of their own property. It also means that they’re able to renovate the space, however there are a couple of considerations here:
1) The balcony area may have restrictions, depending on whether it affects the aesthetic quality of the apartment building, or might potentially be dangerous to other apartments.
2) Living in an apartment should not affect the amenity of those in other apartments. For example, placing down hardwood floors without adequate sound insulation could be the cause of an amenity complaint, which could result in the body corporate becoming involved to demand better soundproofing within the offending apartment.
Home contents insurance will cover everything inside one of these apartments, with the owner being responsible for purchasing and maintaining an insurance policy.