Washing a house is not rocket science but it does take some knowledge of equipment, methods, and house cleaners used in order to give you the best results possible. Many people wash their own homes but do not get the results that a professional house washing company would deliver. We have seen homeowners only wash the bottom half of their two story house year after year. The upper parts of the house get neglected to the point that when they do call a professional, it may cost them more to have it cleaned and it may not clean up as well as the bottom portion.
The biggest problem with DIY house washing, is that most homeowners are not going to use contractor grade house cleaners. Most will but some cheap big box store house wash or just use nothing at all. The house wash sold to consumers has to be safe to handle, unfortunately this makes it pretty worthless as far as actually cleaning your house siding.