Not sure what you mean by the building lines. If the home was not built on the property being sold, then it is on someone else's property and a real estate lawyer is needed.
If it was not built within the setback, then a variance from the jurisdiction is needed by the current owner prior to the sale.
QUESTION: How old is the survey? If it is not current, then a new survey is required. Also, if new, have you discussed this with the person who performed the survey?
NOTE: This reply is not to be construed as legal advice.