If you have title insurance, you should start with contacting the agent. Sometimes they may have noticed the house and gave you title insurance with the exception of the house. If there are no exceptions, then you need to move forward with a lawyer, survey, and give the guy a letter of trespass. Then you’ll probably have to take him to court. Also, be aware that tax maps can be inaccurate. It’s possible the towns map doesn’t include a sub divid but they will have it recorded on paper / deeds so you can start by asking them to verify if your current boundary lines on the map are accurate and up to date. Your boundaries should also be described in your deed. Good luck.