Answering for the UK, as it looks like you come from there...
Sheathed cable (such as twin+earth) can be directly buried in walls
according to following rules:
At a depth less than 50mm behind the surface of the wall, you can
do a horizontal or vertical run from any visible wiring accessory,
or within 150mm of a corner or below the ceiling (but not the floor).
Oval plastic conduit or capping is often used, but is not a regulatory
A wire can be buried anywhere in a wall if protected by an earthed
metal sheath, eathed armour, eathed metal conduit or equivalent, or
buried deeper than 50mm behind the wall surface.
Singles without a sheath (e.g. separate live, neutral and earth)
must be in conduit.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.