Tiling to old cottage bathroom walls

[Ttt1601]

im hoping someone has a idea i haven't thought of for this one, tomorrow i night i have to go and price up a bathroom in an old cottage, i did the floor earlier this year, ended up having to put down an epoxy based dpm, with all relevant parties contacted, tec depts, a damp specialist etc etc. now the customer wants the ground floor bathroom fully tiled, the walls are flint lime and horse hair, im pretty sure it had that corrugated bitumen wall dpm installed in the 50s/60s then hardwall and skim over that. but not all areas have this as there is still a couple of little damp spots showing through on 1 external wall. my big question is, it is small room already i really cant afford the space to build a stud framework inside of the existing walls, if i batton it off the walls and fix to that i will penetrate any existing damp measures that are in place and i really wouldnt trust directly fixing to the existing walls.

so you bunch of genisus's any ideas???

 

[Ttt1601]

Right, went in to see this one and have a chat, he has decided he only wants half hieght round the room, so was thinking, d n d backer board to height then rile, no real issue? Only thing I couldn't think of was best way to finish the top? Would be too deep for a trim with the addition of the board, but not deep enough for a capping tile, he doesn't want a chamfered tile cut on top mitred at 45 to finish flat with top of tile and wall