In most of the western world it's illegal to excavate any significant volume without a bunch of permitting and oversight. Additional regulations surrounding liv

In most of the western world it's illegal to excavate any significant volume without a bunch of permitting and oversight. Additional regulations surrounding living spaces. Plus the fact that you don't generally own the volume underneath your lot in the first place. If you make it past all that (regardless of whether you selected the legal or illegal route) there's the expense of doing so safely. Then add on the expense of maintenance (by which I mostly mean water management).