The lender, which is part of Co-operative Bank, has also joined several lenders in increasing rates on residential mortgages.
It has increased selected two-, three- and five-year fixed rates at 60-90% loan-to-value (LTV) by up to 0.15%.
For buy-to-let remortgage applications without additional borrowing, Platform will assess cases using a minimum of 128% rental cover at a notional interest rate of 5.50%.
This is down from its usual assessment criteria of 145% rental cover at a notional interest rate of 5.50%, which will remain for all other BTL mortgage applications.
The lender said the arrangement “supports landlords who already had a mortgage before the new Prudential Regulation Authory (PRA) affordability guidelines came into effect in January 2017, and who are looking for a pound-for-pound remortgage without any additional capital raising”.
Paul Norcott, head of mortgages and insurance at the Co-operative Bank and Platform, said: “We have been closely following developments in the BTL market since the new PRA guidelines were published, and we know that BTL remortgaging has been more difficult for some landlords since the market adopted those guidelines.
“These changes to our BTL criteria demonstrate that, while we remain extremely committed to responsible lending, we also want to take a common-sense approach that supports landlords looking for a straightforward like-for-like remortgage.”