Do you think, maybe the drawing is a delaying tactic?
Just kidding. You will need to check the roof construction, whether it is trusses or joist and rafters. If it has joists, are they big enough and where are the bearing walls under the ceiling? A 2 X 6 should not be expected to span more than 8' and a 2 X 8 no more than 12'. Bearing walls and knee walls in the attic can be considered support for the joists.
This is a major project and, if you could send me a copy of your floor plan showing any existing plumbing, I would be happy to draw the plumbing layout for you.