Thank you for your question.
There are a number of things most likely going on. I am unabel togive you a thourgh determination, since I am not the prpoperty tax assesor, and do not know what he or she is thinking.
But lets look at how the SC property tax law treats these kinds of property. What first comes to mind is the so called 100,000 dollar exemption.
100,000 exemption: SC allows, automatically, a 100,000 dollar exemption from property tax for your primary home. When you convert a primary home to rental use, you lose that exemption. The exemption does not apply to vacation homes, second homes, or rental property.
How the property is assesed will be based on it statuts at anytime during the year; so we could expect that the state would reassess or adjuste your assessment without the 100,000 exemption. This could be expected to raise your tax by the millage rate at least by the 100,000.
In addition, the assesmetn on rental property includes what the property is expected to bring in on revenues for the year.
Try this out and see if it fits. If not, lets look deeper.
I would need to know the actual appraised value of the property (assesed value) at least from your last property tax and realestate property tax bills.
I would also need to know what items were included in your property tax bills in question.