Usually you can modify everything inside the condo (walls, floors, ceilings) as long as it doesn't compromise the structure of the building. In smaller condos (4-6 units) sometimes you can change the siding of the outside or the roof of your part too, depends on the building.

A house isn't so different in that there are restrictions, since you have zoning codes and stuff. It's ownership in that you own the right to live inside it and sell it and modify it within reason.