What is a Buy to Let Top Slicing Mortgage? The Mortgage Market Review (MMR) introduced not just an affordability test for mortgages but also a stress affordability test to check that you can still afford a mortgage if rates increase. Many BTL mortgages are assessed based upon the rental income to cover mortgage payments. What if mortgage interest rates increase but rents do not? Top slicing of your income is becoming popular for BTL mortgages.