I have had ceramic tiles installed in my kitchen, but I feel they are not level. The company is telling me that the level is acceptable within their terms of business. What would be a typical acceptable level of unevenness i.e how much higher should one tile be than the one next to it?
There is not much a requirement for tiles to be on a "level" (i.e level with a spirit level) but the "flatness" should be not out more than 1mm for a 5 mm joint (i.e. lip edge of tile higher than adjacent tile - often called "lippage").
However, some larger format tiles (e.g. 600x300mm) may be slightly convex - i.e. the corners are 1mm higher than the centre. This is a feature of manufacture of some porcelain tiles, and is usually dealt with by the tiler by avoiding half tile brickbond patterns (offsets usually kept to 1/4 of tile length).
Good tilers pride themselves in achieving the flattest tiling possible. On some tiled surfaces like polished porcelain it is particularly important to avoid lippage - it looks terrible to the trained eye. On other tiled surfaces lippage is a latent feature (e.g. riven surfaces like slate tiles will always have lippage because it is part of the beauty of the tile that each one is a different thickness and has sheared surfaces).
What now with wet screeds and so on it is much easier to tile, if you were to walk into a period property chances are that the floors would be all over the shop, you can only do so much with self levelling compounds.
The larger the tile over floor area I would say is harder to get level, I don't think I have ever come across a perfectly level floor, so in my opinion that if the finished floor looks level and feels good to walk on then the customer should have no gripes, however if your catching toes and heals get the company back and ask for it to be rectified.
What type of mosaic have they install ? All the tiles should be perfectly level to the substructure of the original floor . Is your floor install on concrete or on ply wood? Have they use dittamatting? L