It is becoming increasingly popular to incorporate a toilet or full shower room into a garden room building. An expensive option, the addition of these facilities does increase the usability of the building now and in the future.
There are two options open to you, an off-grid system or a system connected to the mains supply. The latter option is more expensive as there are considerable groundworks involved. If you work with your supplier on a turnkey basis, they will handle these connections for you.