I have the same question also, ended up not doing. Because the terrace house is old, and plumber recommend to install new septic tank and re-route everything new again.

But I asked them to quote me the cost to install a new WC, it's about RM7k.
Works involve digging into ground, locate the septic tank inlet, and install new pipe to old septic tank.

Personally, I'd rather have the sink closest to the door, then the WC, then the shower at the end. Cause I prefer the wet shower area to be as far away from the door as possible. Also don't prefer to walk across a wet area to get to my WC.

But that would mean more relocation of the existing things and would cost more.

Yes, ground floor only.

They need to dig new and route a new pipe, for new WC.
And since it's ground floor, and old house, they need locate the septic tank, because not very sure where is it.

Yes, there's one bathroom behind, no wc. Want to install 1 WC there is no easy feat.
They also recommended to just bury a new septic tank, total cost is roughly RM10-15k. Because no excavator can go in (middle terrace), will need labor excavating.