The sun does not reach the north side of a house or building, and there can often be an excess of mildew and moss (especially here in CT).  To apply paint to this kind of unwashed surface is to invite trouble.   Normally, power washing with chemicals will eliminate this kind of problem, while increasing the life span of your paint job.  

Out of concern for our clients, we use power washing products that will not harm plants, shrubs, or grass.
Besides having a clean house power washing also helps the paint on your exterior last longer.