Shower niche depth

Yes.

Niche's are built by building it into the wall. Effectively there is no wall beyond the plaster board and tiles. Hence it will be the width of the wall - 10mm (plaster) -x for tile width 2-3mm ? So if its a 90mm wall it will be 77mm deep etc.

So if its on a wall that joins the bedroom or another room there is no wall.

If its on the outer wall (or if you request) they can't do that so they will build a secondary wall and place it within that. For which it will be any width you really want (but generally the normal wall width). but you are losing shower space unless that moves to compensate.

Just ask, and make sure its listed as a variation. They normally accomodate.