One of the most expensive coordination mistakes in a bathroom renovation is tiling before waterproofing properly, because correcting it afterwards means chipping off the tiling already laid.
The correct order
Waterproofing is applied to the substrate (levelling screed or cement board) before the tiling is laid, covering especially the shower tray, the first centimetres of wall around it and any pipe penetration. The tiling is laid afterwards, directly on the already-cured waterproofing.
Why you can't waterproof "over" the tiling
Tile is not waterproof in itself: water can seep through the silicone or tile adhesive joints over the years, so if there is no waterproof layer underneath, that water reaches the substrate directly and can leak into the flat below. Waterproofing over tiling that is already laid doesn't solve that underlying risk.
Coordination between trades
This is one of the points where it pays most to coordinate the bricklayer, waterproofer and tiler well from the quote stage, making clear which trade carries out each phase and in what order, so nobody covers another's item with material before it has been completed.
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