The space you actually need
A usable cloakroom needs roughly 800mm width and 1400mm depth for a WC and small basin, with the door swinging out or sliding. Under-stairs spaces are often deep but taper in height, so the WC should sit where headroom is greatest and the basin under the lower run.
Drainage decides the cost
A gravity connection into an existing soil stack within a few metres is the cheap outcome. Where that is not available, a macerator allows a WC almost anywhere but adds a powered component, a maintenance obligation and a restriction on what can be flushed. Gravity first, macerator only where gravity is genuinely impossible.
Ventilation and building regulations
A WC without an openable window requires mechanical extraction ducted to outside, typically with overrun. Any new WC installation is notifiable work in building regulations terms, and the electrics must be certified. These are not optional extras to be value-engineered out.
Making a small room feel deliberate
Wall-hung or back-to-wall pans free floor space and simplify cleaning. A compact projection basin, a mirror on the longest wall, and a single continuous floor finish carried through from the hall make a tiny room read as designed rather than squeezed.