Under the stairs if they are enclosed is the least intrusive space. You could also steal a bit of space from the kitchen if necessary. This may not be possible as I do not know where the fridge/freezer etc is located. Looking at the bathroom above you would want to keep it in roughly the same location downstairs as moving items like the soil pipe is very expensive. It depends on your future plans for the kitchen though.

Agree with Isla, under stairs is the best/only option. Hopefully the pitch of the stairs will give you the head height. Then get a small
Sink and loo with a large mirror. Plumbing will be tricky as your soil pipe is likely in the corner.

Thanks Isla and Siobhan,

The electricity box is situated under the stairs and as it's small, the kitchen wall may need to be moved a couple of feet, as you suggest. Would the box cause issues?

I'm not sure about issues but it might be costly. Is it on the inside or outside wall? I mean is it facing you as you open the door or to your left?

Yes, on the left, on the wall connected to the kitchen so not outside wall.

It's most likely possible to move it into the kitchen then. You will need to contact a good electrician to do this work for you and advise you further.

Is the kitchen layout staying as is? If not, I'd take half of it for a proper size shower room with laundry space, adding thd other half to the dining room for a good sized eat-in kitchen.

The image isn't completely accurate. The agents failed to depict the garden door on the right as you enter the kitchen so if we can we may get 2 feet more before the door frame.

Maybe you could carve it out from thenright hand side of the kitchen, so the connection to existing soil pipe is easiest, and still have narrow(er) u- shape layout (if needed make window bot narrower which is not expensive as you dont need structural work) Or if you want open up to have kitchen diner then have peninsula with breakgast bar there?