Is vertical crack the height of garden wall a sign of subsidence?

You obviously have movement somewhere. Without looking at the job, it could be a number of things but my guess would be insufficient footings. Just because the garden wall has failed, it doesn't mean your house is going to start subsiding! Best thing to do is post your job on this site so somebody can come around and put your mind at rest. Hope this helps Graham

From what you describe it does seem the crack is due to the wall moving by either the footing has cracked underneath thus cracking the wall, but it could also be the fact that there is no expansion joint in the wall. As there is no cracking in the house wall, subsidence seems out of the question for the property and just a problem with the wall. Without looking at the wall I doubt if it is anything other than the above. If you are still worried I would contact a local builder to put your mind at rest.