Buy-to-let mortgages work very differently from the residential mortgages most people are used to. Rather than basing how much you can borrow on your personal income, BTL lenders primarily look at the rental income the property itself can generate - measured against a stressed interest rate using something called the Interest Coverage Ratio, or ICR.
In practice, this means a basic-rate taxpayer typically needs the rental income to cover around 125% of the mortgage interest at a "stressed" rate (often the higher of the pay rate plus 2%, or a floor of around 5.5%), while higher-rate taxpayers usually need closer to 145%. Understanding how this calculation works - and how it differs from lender to lender - is essential before you start house-hunting, because it directly determines how much you can actually borrow.
This guide walks through how BTL mortgages are assessed, what deposits and rates typically look like in 2026, the difference between interest-only and repayment options for landlords, and the questions worth asking a broker before you commit.
The full guide will cover: