How much can I borrow on a buy to let?

It is worked out from the rent rather than from your salary, which is the main difference from a residential mortgage.

The lender applies an interest coverage ratio: the rent has to cover the mortgage interest by a set margin, tested at a rate higher than the one you are paying. The margin and the test rate vary by lender and by whether you are a basic or higher rate taxpayer.

That is why two lenders can look at the same property and the same rent and offer very different amounts. It is also why a small change in the test rate can move the maximum loan a long way.