The simple answer to this is the insulation in your loft/attic has not been continued far enough through the eaves and like the previous window answer, leaves a cold spot and causes mould to form.

This is normally down to two things:

1. Insufficient ventilation and you may need window trickle vents fitted.

2. Give the plaster around the windows a knock, are they hollow? If so, then the cavities around the window have not been closed and insulated. This causes a cold spot and then mould spaws form, which then spread. The plaster needs to be removed, filled with insulation and then replastered. Mould effected areas then need treatment to kill the spaws.

This can be a number of issues, but the most common we find as a company are:

1. Outside ground level is too high and either to close to the damp course or even sometime higher than the damp course.

2. The in place damp course has failed due to being old or poorly maintained (missing mortar in joints) and causing water ingress.

3. Internally the plaster on the walls is too low and therefore sitting below the damp course and the water in bridging across.