For residential mortgage loans in which the interest rate may vary, the reasonable ability to pay the principal and interest on the loan shall be determined based on a fully indexed rate, which rate shall be calculated by using the index rate prevailing at the time of origination of the loan plus the margin that will apply when calculating the adjustable rate under the terms of the loan, assuming a fully amortizing repayment schedule based on the term of the loan.
For loans that allow for negative amortization, the principal amount of the loan shall be calculated by including the maximum amount the principal balance may increase due to negative amortization under the terms of the loan.
205 ILCS 635/5-6