Neighbor's fence
Firstly, you need a survey. Has there ever been a survey done on the land alone, or the land and current existing housing structures, etc.? It's less costly to have whoever did the original survey (make sure they are reputable) do an updated survey, should be almost 1/2 the cost. If you are not comfortable with that, shop around for a new survey. You need that survey for your own safety, also, if you end up in court over this matter.
In my area a new one cost $300, updated $150. Try approaching the neighbor, in person, and placing them on notice (verbal at first) that (if indeed it reflects from the survey) there is an encroachment and they do not have your permission for it to remain. Give them, say, 30 days to remove it.
If you desire to place them on notice in writing they have failed to remove it from your property and on such-in-such a date you will take it down.
Now. Here is where you tempt fate. You can take it down your self or hire someone to do it. At risk --> they can, who's to say if they would prevail at all, come back on you and say you "damaged" their fence. Stranger things have been known to happen. He might win in court.
Or--> you can take it down, replace their materials, put a new one (yours) up in the proper place and put the ball back in their court. Let him worry about getting an attorney, coming up with a retainer and having the worry of what will happen, when he was encroaching to begin with.
Or--> if the verbal request renders no action, you can file through the proper court, by let me tell you. You never know what's on a judge's mind. You don't know how they will rule. That entire process can drag out for months, even a year or more.
There is such a thing as "reverse possession." You want to be careful not to let an encroachment situation exist too long. Ck the laws in your area. If a structure has existed long enough, property might actually belong TO that neighbor that you thought was yours.
We have a beautiful new home. We paid $2,000 for a beautifully constructed wooden fence...placed at our instruction WELL inside OUR property boundary. A nasty young fella next door immediately ran a front wooden fence across his property, up to ours. Which is fine, it never touched ours, only by a
fraction of a hair, and I mean a
fraction .
The problem was, when he did so, he closed off all possible means for us to excess our side property, which was now part of his backyard. He called it "overhang." Never heard of such. I advised him that he did not have my permission to fence off my access to my property and I would remove anything he placed up.
I had a survey performed, immediately had the new wooden fence SAWED into sections and thrown in a pile I later sold at my leisure, and had a 6' chain link fence put up, THIS TIME placed on the current property line which enclosed all of MY property. His greed now affords him a view that looks like Little Sing Sing.
He was livid. I told him to chill, I could have easily taken in a little more -- you know, "overhang." From his side of course.
The day they sank the post for the chain link he came home from work in a tiff, made a scene and instructed my fence contractors "Don't put that up." I'm standing there with a smirk instructing them <<<Sink the posts, sink them NOW.>>> Which they did. He called the law, they laughed --> <<<the woman has a survey>>>
He removed, immediately that evening, the section on my property. The next day they ran the rest of the fence...80' of it.
Come to find out, my insurance company does not pay to replace wooden fencing damaged in storms, etc., it decomposes naturally they say. The chain link they will replace if ever damaged. In addition, it'll stand forever. I came out better.
Granted, you should be able to go through the justice system, shamelessly, swiftly, and emerge vindicated and given full restitution for any costs...
I've been married to a law enforcement officer for 30 years and I can tell you my dear..."The most odious of all oppressions are those which mask as justice." Sometimes you have to use your wits, just don't break the law yourself in the process.

Hope it's all helped and good luck.