Bathroom costs tiler 15 sqm (without material)

Or exactly right, because he took the time to work carefully and precisely. With a flat rate, he wants to finish quickly. The worst is flat rate and providing the materials yourself. Then consumption suddenly increases incredibly...

Or exactly right, because he took the time...

Can backfire very easily :cool: We've already had a few examples of that in this forum. A good qm calculation takes good quality and the time needed to execute it into account. The advantage is that there are no "bad" surprises later on.

Well, maybe yes, maybe not, then the tiler just says yes, if the screed had been even, but we had to level so much. Or so many cuts were necessary, or the tiles weren't completely straight/were so easily broken blah blah blah. If the craftsman is good and not too greedy, it probably doesn't matter how he charges, then you can always be satisfied. If he's a screw-up, he's a screw-up.