Neighbour being a bit unreasonable?

[LeeUK]

Who said it is the OP's wall?

And it is no absurd. Normally when you build a wall you have to take care of the water flow which may mean adding draining holes to the bottom.

It is half OPs wall then. Boundary structures are no matter what people seem to think who owns what just because "they" erected it unless it is fully on neighbours land.

Building a wall with drainage is one thing, but drilling a hole so you garden no longer floods but now your neighbour's garage does is another thing.