765 ILCS 605/5
Simultaneously with the recording of the declaration there shall be recorded a plat as defined in Section 2, which plat shall be made by a Registered Illinois Land Surveyor and shall set forth (1) all angular and linear data along the exterior boundaries of the parcel; (2) the linear measurements and location, with reference to said exterior boundaries, of any buildings improvements and structures located on the parcel; and (3) the elevations at, above, or below official datum of the finished or unfinished interior surfaces of the floors and ceilings and the linear measurements of the finished or unfinished interior surfaces of the perimeter walls, and lateral extensions thereof or other monumental perimeter boundaries, where there are no wall surfaces, that part of every unit which is in any building on the parcel, and the locations of such wall surfaces or unit boundaries with respect to the exterior boundaries of the parcel projected vertically upward; (4) the elevations at, above, or below official datum and the linear measurements of the perimeter boundaries, of that part of the property which constitute a unit or a part thereof outside any building on the parcel and the location of the boundaries with respect to the exterior vertical boundaries of the parcel, projected vertically upward. Every such unit shall be identified on the plat by a distinguishing number or other symbol; (5) if the Registered Illinois Land Surveyor does not certify that such plat accurately depicts the matters set forth in subsection (3) and (4) above, such a certification for any particular unit or units as built shall be recorded prior to the first conveyance of such particular unit or units as part of an amended plat, thereby complying with the requirements of subsections (3) and (4) of this Section; (6) when adding additional property to an add-on condominium, the developer, or in the event of any other alteration in the boundaries or location of a unit, any building on the parcel or the parcel authorized in this Act, the president of the board of managers or other officer authorized and designated by the condominium instruments shall record an amended plat of survey conforming to the requirements of this Section, or shall provide a certificate of a plat previously recorded that is in accordance with the certification requirements of this subsection. Such amended plat or certificate shall be certified by a Registered Illinois Land Surveyor as to accuracy in depicting changes in boundary or location in the portions of the property set forth in subsections (1), (2), (3) and (4) above, and that such changes have been completed.
765 ILCS 605/5