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If the owner who owned your property also owned the lot next door at one point up to the foreclosure, this is going to be a problem, because you cannot use a process called "tacking" for the purposes of claiming "adverse possession" which is your only way to lay a legal claim to that land next to you.
Under GA law, in order for you to have a claim pursuant to adverse possession, you had to openly/notoriously, continuously, and against the rights of the true owner (hostile) possess the land for a period of 20 years (since you do not have any claim under some type of alleged title to that land which would reduce the time to 7 years). Openly, means it had to be visible and continuous use of the property that is seen by all. Continuously, means you had to use the land as your land on a regular continuing basis. Hostile means that the owner of the land could not have given consent or permission to use the land (this is why when the lots were owned by the same owner their possession was not hostile so it could not be used to add to your time possessing or using the land).
Since you do not have color of title (a document claiming to transfer both lots to you) the statutory time in GA for adverse possession is 20 years, whereas if the documents you had stated anything about the other lot then you could claim 7 years under color of title. So having those documents closely examined by a local real estate
attorney to make sure you cannot fit this into a color of title adverse possession would be in your best interests.
Other than adverse possession claim, you are going to have to negotiate this with the new owner and you start by contacting the assessor to see what he paid for it so you know how to negotiate with him to try to get him to sell to you.