1: the survey will be non-invasive, will be a series of heavily caveated opinions, nothing more.
2: I take a different view to my clients than mf1 re unauthorised developments (UDs): don't buy without them being fixed.
3 In addition, no lender will mortgage a house with Uds.
4: Don't know the relevance of the 10 years in the OP's head, while there is a 7 year limit to LA enforcement of UDs, the problem doesn't go away after 7, retention or demolition is required.
5: Lastly, even if you are a cash buyer with a serious discount achieved, the UDs need to be resolved before you can apply for planning for anything else.
6: Retention fees are normally 3 times the normal planning fees so a substantial UD could cost a few shillings