Crack in wall leading to and around window sill

Hi David Cracks can appear due to movement in the structure or simply if the plaster is very old. It depends what type of wall you have. I would mark two straight pencil lines either side of the crack with a straight edge and cut out the cracked section to examine what is underneath. Hopefully this is a small repair and you don’t have any major issues. Cheers Dean.

First off all you need to find the cause of this, often is due to a leaking or broken pipe work below ground which causes movement bcus it will have turned the ground to sludge

It sounds a little bit like sub sidence, if you get cracks as much as u are saying. If it is a bay that has dropped it always seems to go on the 4 x corners of a window, if it's going across the middle of the wall u might want to get a structural engineer around or building control. Not trying to scare you but it sounds a bit wrong, its hard to judge without seeing pictures or seeing it in real life. Hope it's not sub sidence. Good luck

Your main question is why is it happening where do you live is there any old mine shafts near you! Have you a problem with drainage and it’s causing subsidence Some picture would help,above that a site visit to for recommend remedial work.