Amends the Property Tax Code. Provides that a county opting out of the special assessment programs to reduce the assessed value of certain residential real property shall not disqualify or shorten the maximum eligibility periods for any property approved to receive a reduced valuation prior to the county opting out. Requires that the special assessment programs be available to all qualifying residential real property regardless of whether or not the property has or is currently receiving any other public financing or subsidies or subject to any regulatory agreements with any public entity, or both. If an owner is approved for the reduced valuation prior to December 31, 2037 and the provisions are not subsequently extended, this shall not disqualify or shorten the maximum eligibility periods for any property approved to receive a reduced valuation. Provides that, if the chief county assessment officer has not created application forms, the chief county assessment officer shall make publicly available and accept applications forms that shall be available to local governments from the Illinois Department of Revenue. If a county Internet website exists, the application materials, as well as any other program requirements used by the county (such as application deadlines, fees, and other procedures required by the application) must be published on that website, otherwise it must be available to the public upon request at the office of the chief county assessment officer. On an annual basis, requires the Illinois Housing Development Authority to calculate and make available on its website the minimum per square foot expenditure requirements to be applicable statewide to be eligible for the reduced valuation, which shall include the historical annual expenditure requirements starting with calendar year 2021. Changes reference to improvements to existing residential real property to substantially rehabilitated residential real property. Makes other changes.