Adverse possession, even in Mo is different than an agreement reached between 2 property owners. One is viewed as being hostile toward the true owner of the property and the other is an amicable agreement between 2 or more parties.
Boundary lines and adverse possession
In Missouri as in other states, a boundary fence that has been in place for 10 or more consecutive years can, in effect, become the boundary by the legal doctrine of adverse possession (RSMO chapters 516 and 527). Adverse possession means that the fence, for all practical purposes, becomes the property line, and so you cannot just remove or move the fence.
A survey does not take precedence over adverse possession. If a fence has been in place for more than 10 years, you can't just move it. A fence that has been in place less than 10 years, however, can be moved based on a property survey. The fence has to be in place and meet the minimum definition of a legal fence for this to qualify for adverse possession. So, rotted posts in the ground or a fence lying on the ground likely will not meet that definition.
What is required for adverse possession:
In addition to the 10-year requirement, for adverse possession to take effect, the neighbor's occupation of the land must be
Actual, which means the neighbor acted in the manner of an owner;
Open and notorious, so in plain sight and capable of being seen by the true owner and the public;
Exclusive, in that the true owner doesn't possess it at any time during the 10-year period;
Hostile to the true owner's rights; and
under claim or right, in that an error in title or actual use caused it.
For adverse possession to be complete, such that the property line is legally determined, it has to be proved in court. The judge will hear evidence from both sides. What evidence is allowed is decided by the judge and might include the survey, pictures and witnesses as to how long the fence has been there. This process generally puts a new landowner in a tough legal position unless all or some of the evidence supports his or her claim.
Can my neighbor and I agree to put a fence in place on the current boundary line without risking adverse possession later?
Yes, you and your neighbor can place the fence in an agreed-upon location and, to avoid adverse possession, state that it is there "for convenience purposes only" (have an attorney draft the specific wording) on a quitclaim deed, which must be signed by you both and recorded in the county recorder's office. Such a deed can also be recorded concerning a fence that is already in place if you and your neighbor agree.
https://extension2.missouri.edu/g811
