According to IL Publication 108 Illinois Property Tax Credit, on page 2:

You must own your residence in order to take this credit. If you and your spouse each have a principal residence or if you had two principal residences during the tax year due to the sale of your home, you may claim the tax paid on both residences when figuring this credit. See "What if I sell my house?" for more information. 

Therefore, if you have owned and paid property tax on two primary residences in IL, you can enter the other property number in Line 4d of Schedule ICR Illinois Credits since your adjoining lot on your residence is already entered in Line 4c.

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