wooden floor,

[Etherglide]

Hi guys. Ive been called out to a 1st floor bathroom, part of a three year old extension on to an original building. Floor is 60 x 30 marble tiles and there are several cracked. The extension has visibly "dropped". There are gaps around the edge of the floor, gaps at the sink and a gap at the bath. I went there initially to replace 4 floor tiles but on inspection even the tile installation appeared crap. Customer has decided to replace the whole floor including underfloor heating. I can Hardiebacker it and then insulate it. I can then install the heat mat or cable and then self level prior to tiling. Problem is the centre of this floor is fully one inch lower than the edges due to the "settlement". Id love to help the customers out but in your opinion is this a step too far for a tiler and maybe they should be seeing an engineer of sorts. Can you lose an inch of level over a 1st floor wooden floor?