Do I have to pay if a neighbour decides to put up a fence between our properties?

This is definitely a new one, if my neighbour put up a fence then told me I had to pay for it, she would very quickly be told to go take a hike. It's her that's putting up the fence so it's her that needs to cough up. She knew when she got the property there was no fence separating the garden so live with it. Now if she had come to me and asked if I wanted to go halfers on a fence then maybe I would have been more sociable.

Let her waste her time and money with small claims, you have no obligation to pay. It would have been reasonable and neighbourly to discuss the issue first and as ADR state she is bang out of order. She will get nowhere with small claims, it was her decision and therefore her own financial loss.

Hi you have no obligation to pay for any fence, yours or not, if you do not wish to, even if the fence lies on a shared boundary and your neighbor wants it replacing for a new one. You still don't have to pay a penny, the only issue regarding new fencing with neighbors is that it does not stand higher then 6ft and if new fence will be on a shared boundary, both neighbors have to agree on the new fence can be put up. APM Fencing

On the other hand, if she is asking you to pay the total bill for the fence, then you could of course take ownership of it and in my opinion if that was the case and you didn't like 'your' fence once paid for, you could get rid of it :-)