The department of assessments, if and when established, shall be headed by a commissioner of the revenue or a supervisor of assessments, who shall exercise the powers conferred and perform the duties imposed by § 15.2-521 upon the director of finance. In addition to the powers and duties hereinabove conferred, the governing body of any county which has provided for a department of assessments headed by a supervisor of assessments may, in lieu of the method now prescribed by law, provide for the annual assessment and equalization of assessments of real estate by such department. All real estate shall thereafter be assessed as of January 1 of each year. The provisions of this section shall not, however, apply to any real estate assessable under the law by the State Corporation Commission.