The most compact toilet of all is probably the stairs toilet. The toilet goes under the stairs, and one stair step has a hole in it that lifts up where you sit on it. A curtain is pulled round the user during use.
It is recommended that a walkway be maintained down the stairs while this curtain is closed, to avoid people barging in. With narrow stairs this will not be an option.
The toilet itself needs less than 2'x2', freeing up space for storage while providing the convenience of an extra convenience.
Handwashing facilities can be done one of a few ways. The most basic is to use the window. When it rains, put your hands out the window and wipe dry on the bogroll. Fancier facilities would include a second lift up hole where users can put their hand into the water of the cistern, enabling a wash during dry weather as well.
NT