The Short Answer
Tiling over existing tile is possible when the existing tile is firmly bonded with no hollow spots or loose tiles, the substrate is structurally sound, and adding the extra thickness will not cause problems at thresholds, door clearances, or fixtures. The downside is added height — tile over tile raises the floor or wall surface by 3/8-5/8 inch, which can create transition problems at doorways and must be accounted for around vanities and toilets. In wet areas like showers, tiling over tile is generally not recommended because it traps moisture between layers.