Exterior mould usually starts small and in a spot that doesn't get much direct sun, a shaded corner, under an eave, or behind a downpipe where moisture lingers longer than it should. Left alone, it spreads across the surface and can work into porous render or timber over time.
Early signs include dark speckled patches that look like dirt but don't brush off easily, a slightly musty smell near the affected wall, and discolouration that's noticeably worse in damp or shaded sections compared to the rest of the exterior.
Catching it early makes removal far simpler, a light surface clean is usually enough. Left for a season or two, it often needs a more thorough pressure and chemical treatment to fully clear, and by then it may have already started affecting the paint or render underneath.
