What legal action can I file against this guy?

In today's world, it doesn't surprise me that a person's neighbor can be so rude- I'm giving you the link to Soft Shoulder Advice: http://www.softshoulderadvice.com/ Perhaps there will be a peaceful way to solve this concern, they also have legal advice for free.

The only recourse you might have would be: 1) He has violated planning permission (if it was necessary for the building) 2) He has significantly taken your light 3) He has made it impossible to maintain your wall satisfactorily 4) The building is used in such a way as to be an annoyance or health hazard Otherwise he has a perfect right to build on his property without consulting you. It is rude of him not to say anything but not illegal, so long as he doesn't need planning permission.

You will have to consult a good lawyer. The neighbors do not seem to care for you at all. Otherwise they would have taken your consent. Noe only a court judjement can force then demolish illegal part.

While it would have been courteous if he had asked you first, I really don't think there is a law against rudeness. If it is on his own property and he got the proper permits needed to build, I am thinking there is little that you could do. What is it about this building that makes you feel unsafe?

Go to the zoning department or planning department and report your grievance to them. Make sure to take pictures of what is going on and the building in question. If he built that violating zoning laws then you should have an easier time with this. If he somehow got permission via a variance, then that must have been a sloppy planning commission (or city council) depending on how your jurisdiction does it. Usually adjacent property owners are notified in case of a request for property variance, especially if it can cause foreseeable difficulty for the neighbors. I'm just perplexed at how this played out if the latter scenario is the case. It seems like he did it illegally and if that was the case all you have to do is take photos and talk to the zoning department/planning department. They should help you out, especially the zoning inspector. Usually zoning violators are warned for anywhere from weeks to months. If they continue to violate the department automatically takes legal action depending on how grave the violation is. Minor things are usually a cycle of fine and warn and some people actually live with zoning violations like this. Good luck with your issue Mermaid.