The bathroom is the smallest room and the one that causes the most damage when it is done badly. Two decisions carry most of the outcome: the shower tray and the extraction.
A wet room is not automatically the answer
Tanking the whole room sounds efficient and adds a waterproofing layer that has to survive road transport and a building that flexes. A one-piece moulded tray with an upstand does not care about either, and it is the detail that leaks least in practice.
If you do want a wet room, insist on seeing how the membrane turns up the walls and how the linear drain is fixed to the floor cassette. Those two details are the whole job.
Size the shower for the person, not the plan
A 700 by 700 tray fits the drawing and not an adult. 800 by 800 is the point where a shower stops being a compromise, and 900 by 800 is noticeably better again. In a compact plan those extra centimetres come out of the corridor, which you use for four seconds a day.
Head height matters as much as footprint. Under a loft, check the clear height at the shower head before you accept the layout — a 1.9m ceiling over a tray is unusable for a tall person.
Extraction is what keeps the room alive
A compact bathroom with no dedicated extract puts every shower's worth of vapour into the rest of the house. A balanced ventilation system with an extract valve in the bathroom handles it; a humidity-triggered fan is the minimum if there is no MVHR.
Run the fan on an overrun timer and leave a gap under the door for make-up air. A sealed bathroom with a strong fan and no inlet simply does not move air.
Toilets: flush, macerating or composting
A standard flush toilet needs a gravity fall to the outlet, which the floor build-up may not allow. A macerator solves that and adds a pump that will eventually need service. A composting toilet removes the black water question entirely and adds a routine the user has to keep.
None of the three removes the grey water from the shower and basin. That is the discharge the authority actually asks about, so settle it before choosing the toilet.
Hot water is a capacity question
An instantaneous electric heater is compact and draws a lot of current at the moment of use, which matters on a limited supply or off-grid. A small cylinder spreads the load and gives you a genuine second shower.
Whichever you pick, insulate the run and keep it short. In a small house the wait for hot water is the difference between a good shower and a resented one.