Hello, welcome to the website. My name is Tony, I can assist you with this.
You cannot usually build any fence wall like this in excess of 6 ft without planning permission. Do you know whether he got planning permission?
Yes, you would usually get correspondence, but I suppose correspondence can go missing. It's worth checking, but if he's building a 12ft wall, then yes, he is likely to need planning consent.
He sounds like a bit of a troublesome neighbour.
Is he on your land at all>
Okay. Generally, this is something that the council should be taking up for you, it being a potential breach of the planning regulations. Have you informed the council?
This is your starting point - it costs nothing and they have a duty to look into these things and, if appropriate, take action on your behalf.
However, you could potentially have a nuisance action against him yourself, which you could run with without the council, but much will depend on how bad the situation is when judged by a court.
You're more than welcome. Is there anything else you would like me to answer for you?
You too - I hope you manage to sort it quickly!
Good luck to you. Hope you have a good weekend.