55 ILCS 5/5-1109
The county board of any county having a population of less than 1,000,000 inhabitants may whenever in the opinion of the board it becomes necessary, retain the services of a surveyor who shall be registered under the provisions of the Illinois Professional Land Surveyor Act of 1989, as amended, or a person experienced in the preparation of assessment maps or plats, to prepare assessment maps or plats of all or any part of the real property in any or all of the townships in such county. Such maps shall show each separately assessed parcel of real estate together with the area thereof. Subdivided property in recorded plats shall be given the same designation as is contained in the plat recorded, except that the surveyor may designate by letter or number any assessed parcel within such recorded plat which cannot be identified without describing it by metes and bounds. Assessed parcels not within recorded plats shall be designated by lot numbers or letters. The county board in each county may make such further regulations concerning this work as are deemed necessary. A copy of the books containing such maps or plats shall be filed with the county assessor or supervisor of assessments, with the recorder and with the county clerk, and a copy of the maps or plats for each township shall be filed with the assessor of such township, all of whom shall maintain and preserve these copies subject to the provisions of the Local Records Act, as amended. Upon the filing of the books as aforesaid, the county clerk, the township or county assessor, the supervisor of assessments, the board of review, and all other persons whose duty it is to assess property within the area covered by the maps, shall, beginning with the next quadrennial assessment year as set forth in Section 9-95 of the Property Tax Code, assess the parcels of land by identifying them in accordance with the description and designation set forth in such assessment map or maps. All maps filed in accordance herewith shall be designated as "Supervisors' Assessment Maps .... Township".
In any county adopting the provisions of this Section, a surveyor, who shall be registered under the provisions of the Illinois Professional Land Surveyor Act of 1989, as amended, or a person experienced in the preparation of assessment maps or plats, shall be retained by the county board and shall prepare supplemental or correction maps showing all changes in assessment descriptions made subsequent to the preceding maps and prior to November 15 of the year preceding each quadrennial assessment year. Supplemental or correction maps shall be prepared only of those pages upon which corrections or changes are to be made and shall conform to the original maps filed except as to such changes. Copies of such supplemental or correction maps or pages, properly indexed and identified, shall be bound in one volume, if practical; shall be filed in the same manner as is herein provided for copies of the original maps; and shall be known as "Supplemental Supervisors' Assessment Maps for the year (insert year)".
The expense of making such maps or plats and copies thereof shall be borne by the county.
55 ILCS 5/5-1109