Every general reassessment of real estate in a city or county shall be made by (i) a professional assessor appointed by the governing body, who is either an employee qualified by the Department or an independent contractor holding valid certification issued by the Department; or (ii) a board of assessors of not fewer than three members, with not more than one member from each district for the election of a member of the governing body within such city or county appointed by the governing body. The assessors shall be designated on or after July 1 in the year immediately preceding the year in which the general reassessment of real estate is required to be made.