Where and how to run the many pipes needed in the bathroom.
Where you put your sanitaryware will depend on the waste pipes. The water supply pipes to most items is of secondary importance as they are much smaller and can be positioned more easily.
Showers and baths need a 40mm waste pipe, and wash hand basins a 32mm waste pipe, both of which are then routed to a main 100mm soil pipe. Both NI and ROI use metric measurements for waste pipes.
As with all waste pipes, you want to minimise any bends or junctions as they are potential locations for blockages. Preferably, bends should be accessible and fitted with a means to open them.