Thanks :) Yeah decided to go with tile. So sand cement is ok over existing tiles if I prep them first? Only problem is, the new pad can only be about 30mm at it's thickest (leaving up to 15mm for tiles) otherwise it'll end up being higher than the hallway floor. So I either need to make the fall shallower or not bother with the waste and make it all level. If I made it level, could I use levelling compound at that thickness or would deck mud/sand cement be the best option? Also, should I be putting in mesh for strength? (I know I should (and will) ask the local hardware what they suggest, but it's great to have suggestions from the Houzz community also)

Some inspiration, the last pic is clever with the bookcase, you would have a lip on the shelves to stop books falling off every time you open the door, but it sure serves two purposes

Kind of funny really how many people would feel exposed and would not want to use the bathroom. The more I think about it, it's a bit like a public restroom with stalls (as stall walls rarely go to the ceiling or floor and have the added issues of gaps between the wall panels and doors -- which thankfully we won't have!). Regarding the humidity control issue, I should have clarified this is only for a toilet and small hand basin (not a full bathroom with shower). The main room on the other side of the proposed toilet is our dog whelping room where we will have young litters of puppies (when we have them). Further down the hall, removed from this room, is our training room where there are likely to be other visitors but the proposed bathroom would be quite removed from that area. It would be wonderful to find more windows like the ones that are up near the ceiling! That would certainly solve the issue. I'll have to scour the demolition/restoration places.

I think your layout is generally good. With a one bedroom it's nice for guests to have access to the ensuite without having to walk through the bedroom. One thing I do notice though is that there is a lot of hallway/storage/circulation space in the center of the building (in the two bedroom portion) that might be better used as living space. You could pretty easily make the ensuite in the one bedroom larger for example. The wash house space seems unnecessarily large to me as well. You could push the wall dividing kitchen from wash house almost all the way back to where the toilet door is in this plan (you'd then have the toilet door open into the wash room to the right from this viewpoint rather than straight ahead). This would leave much more kitchen to porch access, which would be nice.