I think the crown owns all the land beneath you, or something.
For Scottish law at least you nominally own the land
a coelo usque ad centrum which roughly translated means from the centre of the Earth to the sky above. This was taken from the Roman civil law.
However it's not without limits because your feu superior previously may have reserved the mineral rights, planning law may restrict you, you will find that there are limits regarding aircraft etc. In Roman times it wasn't necessarily important to put on such restrictions because that would be dealing with problems that didn't exist then but now you'll rapidly find your rights curtailed if you're really pushing the boundaries of what you can do with your land.
I should also point out that with flats outside the top and bottom of the block you own that particular section of "air" space, your rights above and below don't exist generally speaking. It can all get quite complicated and you may have responsibilities for upkeep and maintenance but that's a very quick overview.