would a better way to put it in this case be that the interest on the old mortgages (pre 2017) will be fully deductible. if he buys a new house with a $500K mortgage before he sells and pays off the mortgage on his old primary mortgage, none of the interest would be deductible since the balances on the old mortgage exceed $750K. when he sells and pays off the mortgage on the old primary interest on $350k of the $500K new mortgage would be deductible.
in all probability, the calculations will be similar as to those under the old law just with new limits. so say you pay off the old primary on October 31,2018. let's assume for November the average balance on the mortgage on your second home is now down to $395K interest on $355K of new mortgage would be deductible.
