because the water could seep through the joints between the new tiles and sit between the two layers and affect the seal and also raise the height of the floor and affect the slope to the waste outlets..but, why do you want to retile if you're not using the bathroom??...for immediate resale consider bathroom wall to wall bathroom carpet, new wall paint, new vanity, taps and light fittings which should also be cheaper than tiling over the top and, if it's not allowed by the body corporate, it will be a risk to future sales as it should not satisfy a proper building inspection

The size of floor tile: 300 x 300 mm and the size of wall tile: 300 x 600mm The floor tile and wall tile do not align at all. It looks unprofessional. Should the tiler redo the job? Thanks for your advice.

Yes I have by Precious Tiling. For my home, we first did the concrete flooring then laid the vinyl tiles. For a quality job, the substrate must be clean, flat, and very smooth. To fit tile around pipes or obstacles, cut a pattern from cardboard and then use a contour gauge or compass to transfer the pattern onto a tile.

You may be lucky & the tiles may come off easily if just laid (we just did this in our shower). Worst case when they pull them off they may damage the membrane, in which case they will need to repair it, so it is just the time of the repair & drying, before they can relay the tile. If it's important to you, get them to fix it, as long as you can accept the worst case scenario of the extra time (that is what we did, and then the membrane was fine anyway). Good luck.