Check your planning permission conditions. In a previous 2nd hand residence, we had only retaining wall to the side and nothing out front.
Planning permission indicated that I could do 6 foot wall all around the side and back of the house (i.e. for back and sides of house). For front of house I could do 3 or 4 foot (can't remember which).
It was in the planning permission so I could do what I wanted within that. I did get consent from neighbour and paid for it all myself (including plastering his side of the wall !)
But if he didn't want it, I don't believe he could have prevented it.
I know that in some planning conditions, walls are specifically not allowed, and specify hedging, fencing etc.
If there is no mention in planning permission, then there should be guidelines on Dept Environment or local planning office web site outlining what kind of boundary changes do not require planning.