It's a downstairs (ground floor) loo with a concrete floor, tiled
walls. Currently there is a loo in the corner, which may or may not
be moved depending on the design of the wet room and whether it is
necessary or not. The loo has an outflow which goes vertically down
into the concrete.

Actually I just remembered I have bought a new bathroom suite and the
new loo has a horizontal outflow, I had decided to move the loo and
have the outflow going out through the wall.

There is also a sink in the other corner.

But anyhoo, the point is, how do you design and construct a wetroom
from this starting position because the drain for the wetroom will
have to be embedded into the concrete floor.

Same goes if I have a posh stone shower tray installed, I don't want a
shower tray on legs, I want a shower tray that sits directly onto the
concrete floor. So I have to dig out a channel into the concrete?

How do you do this without damaging the DPM?
And what do you do if you do damage the DPM?

cheers