Replacing Shared Boundary Fence With Neighbour

You can't build a party wall without agreement from the other party, because the footings (at least) will be on their land.

Getting agreement and splitting costs may not be easy as cheap walls are ugly and nice looking walls are expensive. If I was your neighbour I would agree, let you pay for it, and spend my own money on making my side pretty (render or stone facing, or hide it behind a hedge).

If you can't reach agreement you can just build the wall entirely on your land, but even that may involve taking down the existing fence. So the worst case is that you end up paying for the new wall on your side AND a new party fence.

All IMO obvs.

Agree and let them pay for it? Really? What type of person does that?

This is pretty standard. The neighbour who wants the wall, pays for it.

I agree if the existing boundary fence is falling apart, then the cost of renewing it should be split, but if one side wants to "upgrade", then they should be prepared to foot the bill.

People can be funny about these things.

New neighbor called in to us a few years ago and asked about us going halves on a wall.

Now there is already an old wooden fence that we put in about 10 years ago and we have replaced panels and posts ourselves whenever needed, but we thought new neighbor moved in and in the name of good relations it might be nice to go halves on a wall with him, so we asked how much.

€10k he said without laughing. My Dad said, no way could we justify paying €5k to replace a perfectly serviceable fence. No 10k each he said.

My Dad just said, look we are happy with the wooden fence, do you want to go halves in the maintenance of it instead?

Neighbor hasnt spoken to us in years and has not done anything about their wall that they wanted.

I disagree that it is pretty standard, maybe it's your experience.