The construction phase (assuming it is 12 months or less) is exempt from the ATR requirements so you would base your calculation on the permanent phase.
Comment 43(e)(2)(iv)-1: For a qualified mortgage, the creditor must underwrite the loan using a periodic payment of principal and interest based on the maximum interest rate that may apply during the first five years after the date on which the first regular periodic payment will be due. Creditors must use the maximum rate that could apply at any time during the first five years after the date on which the first regular periodic payment will be due, regardless of whether the maximum rate is reached at the first or subsequent adjustment during the five year period.