How do I repaint exterior house walls?

I have old water based paint on the outside walls of my home and I want to repaint them. Do I need to do any preparation first?

The paint would need to be cleaned by pressure washing and any mould would need to be removed with bleach. Lightly sand all surfaces to a dull finish, then wipe with a damp cloth. If the paint is peeling or powdery, sand to remove loose paint or dust, then apply one coat of British Paints PREP 4 in 1 (oil based) then two coats of British Paints 4 Seasons. If the paint is in good condition then apply 2-3 coats of British Paints 4 Seasons after cleaning and sanding any gloss surfaces to a matt finish.