I presume the apartment is in an old building. In this case, you will form a company, and the apartment will become an asset of your company.Your company will then own the apartment and however many square metres it says in the deeds, forever. No problems, no catches.
Bo is not quite right. First of all only a Bulgarian person, or a company set up and registered in Bulgaria can own land/property.
Either you buy the apartment with a percentage of the land through a Bulgarian company. Or as with most off plan developments, you buy the apartment in your own name, without a company. This way you are buying the "right to build" from the original land owner. They will, in effect still own the land that the apartment block is on, but will have sold their rights to build to the new owners/developers. The original owners will not be able to do anything with the land, even if the whole block collapses, because they have sold their rights to build on it.