35 ILCS 200/10-166
Except in counties with more than 200,000 inhabitants that classify property for the purpose of taxation, to the extent any portion of any lot, parcel, or tract of land is (i) registered in perpetuity under Section 16 of the Illinois Natural Areas Preservation Act, or (ii) encumbered in perpetuity by a conservation right, as defined in the Real Property Conservation Rights Act, if the conservation right has been conveyed and accepted in accordance with Section 2 of the Real Property Conservation Rights Act, recorded under Section 5 of that Act, and yields a public benefit as defined in Section 10-167 of this Act, upon application under Section 10-168, the portion of the lot, parcel, or tract of land registered or encumbered shall be valued at 8-1/3% of its fair market value estimated as if it were not registered or encumbered; and any improvement, dwelling, or other appurtenant structure present on any registered or encumbered portion of land shall be valued at 33-1/3% of its fair market value. Beginning with the 1995 tax year in counties with more than 200,000 inhabitants that classify property for the purpose of taxation, to the extent any portion of a lot, parcel, or tract of land is (i) registered in perpetuity under Section 16 of the Illinois Natural Areas Preservation Act or (ii) encumbered in perpetuity by a conservation right, as defined in the Real Property Conservation Rights Act, if the conservation right has been conveyed and accepted in accordance with Section 2 of the Real Property Conservation Rights Act, recorded under Section 5 of that Act, and yields a public benefit as defined in Section 10-167 of this Code, upon application under Section 10-168, the portion of the lot, parcel, or tract of land registered or encumbered shall be valued at 25% of that percentage of its fair market value established under this Code, by an ordinance adopted under Section 4 of Article IX of the Illinois Constitution, or both, as the case may be; and any improvement, dwelling, or other appurtenant structure present on any registered or encumbered portion of the land shall be valued at that percentage of fair market value established under this Code, by an ordinance adopted under Section 4 of Article IX of the Illinois Constitution, or both, as the case may be. To qualify for valuation under this Section, the registration agreement or conservation right establishing an encumbrance shall prohibit the construction of any other structure on the registered or encumbered land except replacement structures, no larger than the previous structures which are replaced, that do not interfere with or destroy the registration or conservation right.
The valuation provided for in this Section shall not apply to any land that has been valued as open space land under Section 10-155.
35 ILCS 200/10-166