Location of new wiring

Hello, Difficult to nail a proper response without seeing your job. If it was in my house? As a rule of thumb you would want to install the cables in earthed containment depending on depth. If you've just had the C/U upgraded it should be fed via an RCD? But for good practice it should be fed through a source of containment would be my advice.

Running cables below ground floor level is quite unconventional and ill-advised for a number of reasons. What if you have a flood? Certainly Part P of building regs is clear about the need to either bury unprotected cables more than 50mm or provide mechanical protection to prevent penetration with nails or screws if less than 50mm deep. Normally much easier to go up the wall into the ceiling, along and down again.