I never quite understood why when wiring an out building it is not advised to use the main property earthing . In most cases the only option is an earth spike that is a dubious alternative?

It will only become common in UK general building if the building control approved docs get updated to say 'leave two ends of Rebar poking up in diagonally opposite corners for the electricians.  Dress the damp course round this to the satisfaction of the LA inspector'

About 6 months after that everyone will do it if it  is needed or not.

Then we'll likely end up with a plain concrete foundation with just two 6" lengths of useless rebar sticking up in the least accessible corners of the building.

I think the UK has a couple of problems with the foundation earthing approach:

• We don't have a tradition of using rebar for underground/foundation concrete in ordinary domestic work - there's normally little need for it in UK conditions (no seismic requirements) unless on very weak soil (in which case it normally gets done by specialist contractors rather than ordinary builders). Most ordinary builders seem to think that putting steel underground is asking for trouble from corrosion - and may 70s concrete buildings designed with say 50mm of concrete cover to prevent the rebar corroding seem to have illustrated that point.
• UK builders are far less 'regulated' than say their German counterparts. There's much less of a formal education process where new ideas can be introduced and properly explained (just look how thermal insulation is often installed, it's there but with so many gaps that the draughts just go around it). In the UK there's far more just relying on old tried and tested approaches and distrust of anything new, so getting them to do anything new and do it reliably, is going to be quite a challenge.

   - Andy.

It will only become common in UK general building if the building control approved docs get updated to say 'leave two ends of Rebar poking up in diagonally opposite corners for the electricians.  Dress the damp course round this to the satisfaction of the LA inspector'

About 6 months after that everyone will do it if it  is needed or not.

Then we'll likely end up with a plain concrete foundation with just two 6" lengths of useless rebar sticking up in the least accessible corners of the building.

I think the UK has a couple of problems with the foundation earthing approach:

• We don't have a tradition of using rebar for underground/foundation concrete in ordinary domestic work - there's normally little need for it in UK conditions (no seismic requirements) unless on very weak soil (in which case it normally gets done by specialist contractors rather than ordinary builders). Most ordinary builders seem to think that putting steel underground is asking for trouble from corrosion - and may 70s concrete buildings designed with say 50mm of concrete cover to prevent the rebar corroding seem to have illustrated that point.
• UK builders are far less 'regulated' than say their German counterparts. There's much less of a formal education process where new ideas can be introduced and properly explained (just look how thermal insulation is often installed, it's there but with so many gaps that the draughts just go around it). In the UK there's far more just relying on old tried and tested approaches and distrust of anything new, so getting them to do anything new and do it reliably, is going to be quite a challenge.

   - Andy.

Agreed there Andy, rafts on rough or reused groundwork seem to be used only occasionally in our area. 
In my experience Builders just want to cover up as fast as possible. Not so much ‘engineering a structure’ as ‘getting it done ASAP’. I’m sure that doesn’t apply to all of them especially on architectural sites, but perhaps specialists ground work contractors are an answer. 
What are your thoughts on this, (earthed cage hard stand) in reference to EV’s?  Thanks 

SL1 said:

What are your thoughts on this, (earthed cage hard stand) in reference to EV’s?  Thanks 

It depends on what earth electrode resistance you can actually achieve. In single-phase installations, using the formulas in A722, we're talking really low (e.g. below a couple of ohms, with larger installation demand, perhaps even well below an ohm).