Retiling ground floor with tile over tile (primed with Mapei Eco-Prim Grip). In one room (at least 300sf) there is currently carpet over concrete slab that will get the same tile as rest of floor. Being that this room doesn't have the existing tile, it is lower height. What is the best method to raise this floor by the height of the existing tile & mortar so that new tile floor will continue level? 1. Cement board seems a bit too thick, and concerned about securing large sheets to the concrete slab. 2. Levelling compound seems like it would require far too much and not sure I can ensure proper height. 3. Laying a layer of cheap base tiles first seems most effective, but just a lot of added work, time, and expense. Not worried about the time part. 4. Find a line to let new tiles slant down, potentially causing a negative visual effect. Suggestions?