A house wash is the whole building envelope rather than one surface: walls, eaves, soffits, gutter faces, downpipes, window exteriors and the paths immediately around the house.
Why it is done as one job
Cleaning a wall without the path below it leaves a clean wall draining onto dirty concrete, and the tide mark shows. Doing the envelope in one visit also means the runoff from each surface lands on something that is about to be cleaned rather than something already finished.
Two sides of the same house, two different problems
On the coast the north and east elevations take airborne salt, which holds moisture against render and Colorbond and dulls window frames. The southern and shaded sides take mould and lichen instead, because they stay damp. The same house often needs two approaches, and we change as we work around it.
Almost all of this is soft washing. Render, paint, weatherboard and fibre cement are all damaged by pressure, so the walls are treated with a biocide at low pressure while only the hard paving sees a surface cleaner.