A. Mike Ducato. I hope you are willing to spend a pile of money for this. When you have very limited space such as city lots with narrow side lots and small backyards this might be a consideration since adding on is not an option due to a lot of restrictions. An average basement can run anywhere from $20,000 to $50,000. Now add to this the cost of relocating existing mechanics and the other possible structural considerations. The liability issue is also a potential consideration since cave-ins are certainly possible. Consider this alternative very carefully and get all the expert advice you can before embarking on this.