Amends the Property Tax Code. Provides that to receive a reduction in assessed value, an owner, for the purpose of the initial application and only until the building is put in service, may provide proof of either a deed restriction or participation in a government program that includes legally enforceable affordability requirements comparable to the requirements of this Code and the chief county assessment officer shall furnish a letter of intent to the applicant indicating that a preliminary assessment of the new construction or qualifying rehabilitation indicates that it will meet all eligibility requirements. Modifies "assessed value for the residential real property in the base year" to mean the assessed value used to calculate the tax bill, as certified by the Board of Review, for the tax year immediately prior to the tax year in which the building permit is issued; for property assessed as other than residential property, the "assessed value for the residential real property in the base year" means the assessed value that would have been obtained had the property been classified as residential as derived from the Board of Review's certified market value (currently, the value in effect at the end of the taxable year prior to the latter of: (1) the date of initial application; or (2) the date on which 20% of the total number of units in the property are occupied by eligible tenants paying eligible rent). Modifies "maximum income limits" to include when a property may be deemed to have satisfied the maximum income limits with a weighted average if municipal, state, or federal laws, ordinances, rules or regulations requires the use of a weighted average of no more than 60% of area median income for that property. Modifies "maximum rent" to include that a property may be deemed to have satisfied the maximum rent with a weighted average if municipal, state, or federal laws, ordinances, rules or regulations requires the use of a weighted average of no more than 60% of area median income for that property.