Vicki is absolutely correct. This is not a pressing issue even if you are planning on selling. If you do add on, you should get an appraisal or have the assessor verify the current square footage of your home so that the difference between the current assessment and current square footage doesn't get rolled into the new assessment based on the addition. You can use the appraisal as evidence for the assessor in such an occurrance.
If we haven't answered your question to your satisfaction, please give me a call or drop me an email and I'll be happy to come discuss your concerns with you.
It's very common for the size to be incorrect in the tax rolls. That's why the listing agents have the option of stating various sources for square footage in the MLS ("per appraisal" ir "per owner" or "estimated") Don't worry about it being accurate - it doesn't really matter (except maybe to Zillow, which isn't very accurate anyway!)
You can call the county to reassess your property based on the corrected square footage of your house and your lot but expect an increase in property tax. Good Luck