I’m no expert, but what about that corner of your dining area on the left that abuts the hall? You could create a doorway into it from the hall and just be taking a small bit of the dining room.

It's quite small, already. This is the room... If I ate into the room, the wall would be next to the fireplace.

If you don't mind walking through the living room to get to the kitchen diner you could block up the kitchen door from the hall and have a loo in the area under the stairs and across the width of the hall, with a door from the hallway.

ArchGee, I haven't moved in yet to measure it but it is shorter than me (I'm 172cm). There is about 140cm of space under the stairs but it's literally triangular. No flat ceiling above it.

Pmasmith3 that could work but wouldn't be too aesthetically pleasing.

One thing I did consider is knocking down the porch, having the staircase brought forward by 50-100cm and then having the room built under the stairs? Does anyone know how much this would probably cost?

The key element in adding loos is generally guided by the position of existing soil pipes.... unless you add a Maserati ( ah love auto correct - that can stay in !) a macerater ( saniflow or similar ) but most people don’t like the fallout when they get jammed ! So do you have an image showing next floor(s) up too? And are you thinking of extending out at any point ?

One Plan, this is the upstairs. I would consider extending outwards (in line with the conservatory, as long as it wasn't too expensive).

Did you ever find a solution to this? We have a similar layout and would like to have a downstairs toilet