When buying a property in a co-ownership building ('copropriété'), such as a block of apartments, it is important to distinguish between the parts that are designated as 'held in common' and those that you own outright ('parties privatives').
Generally you own outright your own apartment, and also acquire a share in the building's common parts, the share being calculated in relation to the size of your apartment and included in your property deeds. The building's common parts include the ground it is built on, foundations, gardens, driveways, entrance, hallway, corridors, stairways, service areas (for dustbins etc), lift, roof structure, technical services (heating, ventilation, TV aerial etc), and communal facilities such as a swimming pool or tennis court.
What you own outright is the space within your own four walls, within which you can do more or less what you like (such as re-arranging the layout or installing a new kitchen), provided you do not endanger or damage the building's infrastructure, for example by removing a supporting wall.
It is important to note the position in relation to balconies and terraces. In some buildings, you may find that they are designated as a 'partie commune' of which you have exclusive use. Under a 1965 law it was decided that you own the floor of a balcony or terrace (so you could re-tile it, for example) but not its structure (so cannot remove it). You also do not have the right to sell your balcony or terrace separately from your apartment!
As a result, it is invariably necessary to ask the syndic (building management) for permission to alter a balcony, loggia or terrace, for example if you wish to enclose it. There are also rules about changing the floor, in relation to drainage from the building's common parts and the risk of flooding an apartment below yours.
The syndic may also have strict rules about what you can and cannot do on your balcony, for example relating to shrubs and plants, sunblinds - and the hanging of washing. My own syndic has recently voted to ban estate agency For Sale and To Let signs in the interest of aesthetics, using their right to apply rules in respect of the balconies which are defined in our syndic's constitution as common parts.