Overhead Power cable from my house to my barn

not illegal, in the sense of 'point to the law it breaks please'

We cannot see the route, and there may be a good reason why overhead is tricky, but that is not a legal issue.  A span of 25m unsupported would need a fairly bosky stay wire, think 5 or 6mm stainless steel, depending on the cable weight, and it will impose significant lateral forces on the mounts - it is important to realise that you cannot have a perfectly straight line, as that requires an infinite tension, (think of the drop of a heavily loaded washing line.),. Ideally  the cable forms a graceful catenery curve, over 25m aim for maybe 0.5m droop from the straight line at the mid point. If the building is not up to the side forces involved then the usual solution is poles and guy lines at the ends, which on private property are usually scaff poles planted in cement footings rather than the electricity company style telegraph poles that require most of a tree each.

As above, you want to be high up, say 4,5m or so  if full sized vehicles may go beneath less maybe OK if it is cars only or you have some other feature like an arch that limits heights,  though  if possible a route over a hedge or a ditch is better as that reduces that risk to almost nil (or coming underground for any section that crosses a driveway)

Where cables come indoors they need to loop down first, so that water running on the cable drips off, rather than is channelled indoors.

Most things are possible, you do not need to be limited by your sparks' imagination.

Mike.

not illegal, in the sense of 'point to the law it breaks please'

We cannot see the route, and there may be a good reason why overhead is tricky, but that is not a legal issue.  A span of 25m unsupported would need a fairly bosky stay wire, think 5 or 6mm stainless steel, depending on the cable weight, and it will impose significant lateral forces on the mounts - it is important to realise that you cannot have a perfectly straight line, as that requires an infinite tension, (think of the drop of a heavily loaded washing line.),. Ideally  the cable forms a graceful catenery curve, over 25m aim for maybe 0.5m droop from the straight line at the mid point. If the building is not up to the side forces involved then the usual solution is poles and guy lines at the ends, which on private property are usually scaff poles planted in cement footings rather than the electricity company style telegraph poles that require most of a tree each.

As above, you want to be high up, say 4,5m or so  if full sized vehicles may go beneath less maybe OK if it is cars only or you have some other feature like an arch that limits heights,  though  if possible a route over a hedge or a ditch is better as that reduces that risk to almost nil (or coming underground for any section that crosses a driveway)

Where cables come indoors they need to loop down first, so that water running on the cable drips off, rather than is channelled indoors.

Most things are possible, you do not need to be limited by your sparks' imagination.

Mike.