Any taxpayer or his duly appointed representative may apply to the board of equalization for the adjustment to fair market value and equalization of his assessment, including errors in acreage, and any county or city through its appointed representative or attorney may apply to the board of equalization to adjust an assessment of real property to its fair market value and to equalize the assessment of any taxpayer. An executed and properly notarized letter from the property owner designating an appointed representative for the taxpayer shall be presumed to be a valid designation from the taxpayer, and the person whose signature is notarized shall be presumed to have the authority to designate such representative on behalf of the taxpayer.