In addition to what ufmace said, the vast majority of buyers in the UK do what has just been changed to a RICS Home Survey. These are reports written by charter

In addition to what ufmace said, the vast majority of buyers in the UK do what has just been changed to a RICS Home Survey. These are reports written by chartered surveyors, meaning that these are not just best effort opinions, they have legal implications. Some mortgage providers will not agree to lend without having a fresh one to consider. They list all kinds of small details: a single double glazed window containing condensation, or a gutter leaking onto an exterior wall. Experienced buyers would know which of the details are actually important, but it's still a sobering document. They encourage buyers and mortgage providers to act cautiously and are typically used as a tool for negotiating the price down. If you are a cash buyer and just don't care, then fair enough. But whether or not it's a sound piece of engineering, a DIY tunnel (without planning permission?) dug underneath the property would be a massive red flag for your solicitor and your surveyor who would likely provide professional advice against buying the property. And mainstream mortgage advisors and insurers and may even refuse to do business with you due to perceived risk.