When special assessments are levied by the percentage of tax value of the property assessed or by the foot frontage of the property bounding and abutting upon the improvement, and there are lands subject to such assessment which are not assessed for taxation, the Director of Finance and Public Record shall fix, for the purpose of such assessment, the value of such lots as they stand and of such lands at such depths as the Director of Finance and Public Record considers a fair average of the assessed value of other lots in the neighborhood, and where lands are not subdivided into lots but are assessed for taxation, the Director of Finance and Public Record shall fix the value and the depth in the same manner; but the above rule shall not apply in making a special assessment according to benefits.