S.C. Code Ann. § 6-1-980

(A) The impact fee for each service unit may not exceed the amount determined by dividing the costs of the capital improvements by the total number of projected service units that potentially could use the capital improvement. If the number of new service units projected over a reasonable period of time is less than the total number of new service units shown by the approved land use assumptions at full development of the service area, the maximum impact fee for each service unit must be calculated by dividing the costs of the part of the capital improvements necessitated by and attributable to the projected new service units by the total projected new service units. (B) An impact fee must be calculated in accordance with generally accepted accounting principles.