If North Carolina is similar to Virginia, then this information might help lead you further down the path of gathering more information. Tax records through your municipality's governmental tax assessors office (or your equivalent) would need to correct this information. Usually home values are reassessed according to a pre-determined municipality schedule based upon local ordinances; these time frames vary widely.
A word of caution: if your Dad still lives here (or the current resident/owner if he has moved), this reassessment of value will more than likely raise property taxes and the assessed value of this property. The current owner might also inquire as to what constitutes a legal bedroom according to the assessor's standards and municipal codes. Legal bedroom has various answers for different people. So find out this definition first to better aid your Dad/current owner in determining the accuracy of the tax records.
It's also possible that the information imported into the tax records was imported incorrectly or even typed incorrectly.
Please let me know if you're looking for a professional agent in your area.
Sherrie Hartzell 540-903-0212
Licensed real estate salesperson in Virginia with Keller Williams Preferred-Stafford, VA