S.C. Code § 5-3-110
Whenever the whole or any part of any street, roadway, or highway has been accepted for and is under permanent public maintenance by a city, a county, or the Department of Transportation, that portion of any right-of-way area not exceeding the width thereof lying beyond but abutting on the corporate limits of the city may be annexed to and incorporated within the city by adoption of an ordinance so declaring, without necessity for election of any sort, upon prior consent in writing of any public agency other than the city engaged in maintenance of the right-of-way area to be annexed. Consent on behalf of the Department of Transportation may be given by the director. Consent on behalf of any county may be given by its county commissioners, county board of directors, or other local county agency or governing body having jurisdiction over county roads.
S.C. Code § 5-3-110