The plans for your current home should be on file at the Southampton Town Building Department. They are open till 4 PM weekdays. It is best to call ahead and ask them if they have the plans on file in their office because sometimes they need to get them from storage. The main number is 631 283-6000 and ask for the Building Department Representative. A Building Permit will be needed so pick -up a form or download one ahead of time from their website and there is a fee to apply. Also, check if your septic system will allow an additional bath. If you need an more assistance feel free to contact me. Good Luck!