Build new WC downstairs under the staircase.
Tiling as required
Fit piping, tape, W/C and sink as essential.
Avoid FINEST FINISHES at all cost !!! Unprofessional, untrustworthy, poor skills, poor workmanship, shy aways from committment, always need to chase up, no clear logical way of working. Always running out of money and need money in advance without doing work. Promises material of high spec but delivers low spec components. Terrible service. Uncompleted scope of work....& damaged other parts.... In a nutshell "avoid" at all cost.