A garden room with bathroom is where a very airtight, very well insulated building meets the wettest activity in a house. In a masonry house a bathroom forgives a lot. In a timber or panel building with a continuous vapour control layer, moisture that gets into the wall build up has nowhere to go, so a shower room out here is a construction detail before it is a plumbing job.
Keep the moisture out of the structure
A shower enclosure in a lightweight building needs a proper tanked substrate behind the tiles, a vapour control layer that is continuous and correctly sealed at every penetration, and no gaps where warm wet air can be pushed into the insulation. Penetrations for the extract duct, the shower valve and the lighting are the places it goes wrong.
Size the extract for a shower, not for a cloakroom
A shower produces far more moisture than a WC, and it produces it fast. That means a fan with the right extract rate, ducted to outside in insulated ducting so it does not condense inside the duct, on an overrun timer. Background ventilation has to be there too, because a fan with nowhere to draw air from moves very little.
Heat the room, not just the water
A bathroom that goes cold between uses condenses moisture on every surface. Some background heat, a heated towel rail or underfloor heating in the wet area, keeps the surfaces above the dew point and does more for the room's longevity than any amount of sealant.
Hot water: local or from the house
An electric shower or a small unvented cylinder in the building is simple, avoids a long hot run and adds electrical load that the supply cable has to carry. Bringing hot water from the house means a long insulated buried run, a wait, and heat lost every time. Neither is wrong, and the choice has to be made before the supply cable is sized.
Questions people ask about garden room with bathroom
Does a garden room with a bathroom still count as an outbuilding?
For planning, a bathroom on its own does not make it a dwelling, and the incidental use test still applies. What changes the answer is a kitchen alongside it and use as a separate household, which starts to look like an annexe.
Will it get damp?
Only if the construction and the ventilation do not match the use. A shower room built with a continuous vapour control layer, a properly sized extract and background heat is no damper than one in the house. One added into a sealed building as an afterthought usually is.
How much bigger does the room need to be?
A workable shower room takes up meaningful floor area once the enclosure, the basin, the WC and a door swing are drawn. On a small garden room that can be a third of the building, which is a good reason to draw it before choosing a size.