A buy-to-let only works if the rent comfortably covers the mortgage. Not just today, at the rate you happen to be offered, but if rates move against you. That margin of safety has a name. Lenders call it interest cover, and they test it before they lend a penny. The investors who sleep well are the ones who run the same test on themselves first.
What interest cover actually measures
Interest cover is simply the rent set against the mortgage interest. It is usually written as a ratio. If the monthly rent is 145% of the monthly interest payment, your interest cover ratio is 145%. The higher the number, the more room you have before the rent stops covering the loan.
Lenders use this ratio to decide how much they will lend. A common approach is to want rent worth at least 125% of the interest, and often more for higher-rate taxpayers or limited company purchases. The exact figures move around and vary by lender, so treat any percentage here as illustrative and check the current criteria with a broker. The principle does not change: the rent has to clear the interest with a margin to spare.
Why lenders stress-test at a higher rate
Here is the part that catches new investors out. A lender does not test your cover at the rate you are paying. It tests it at a higher, assumed rate, to see whether the property would still hold up if borrowing got more expensive. So a mortgage advertised at one rate might be assessed as though it were two or three percentage points higher.
That is not the lender being awkward. It is the lender asking the only question that matters over a long hold: if money gets dearer, does this property still pay for itself, or does it start eating into your pocket every month? A deal that only works at the lowest rate on the market was never really safe.
Run the test yourself before you offer
You do not need a lender to do this. Take the purchase you are weighing up and work it through in three steps.
- Start with the realistic rent. Not the optimistic figure, the one the area actually pays. The void periods post on this blog explains why that matters.
- Work out the interest at a stressed rate. Take your likely loan amount and calculate the monthly interest at a rate two or three points above what you expect to pay. This is your worst-case payment.
- Compare the two. Divide the monthly rent by that stressed interest figure. If the rent still clears it with margin to spare, the deal has resilience built in. If it only just scrapes through at today's rate, you are buying fragility.
A worked example
Say a property would let for around 900 a month and you expect to borrow at a rate that costs roughly 500 a month in interest. At that rate the rent covers the interest comfortably. Now stress it. Assume rates climb and the interest payment rises to 700 a month. The rent of 900 still covers it, but the margin has thinned. Push the assumption further and you can see exactly where the deal would stop working. These numbers are illustrative, not a quote, but the exercise is the point. You want to know the breaking point before you own the property, not after.
What good cover buys you
Strong interest cover is not just a lending hurdle to clear. It is breathing room. It is the difference between a rate rise being an inconvenience and a rate rise being a crisis. It lets you ride out a void or an unexpected repair without the property going underwater. Thin cover does the opposite. It turns every small shock into a monthly loss.
This is also why the cheapest-looking, highest-yield deal is not always the best one. A property in a weak area might show a big headline yield and still fail a sensible stress test, because the rent is unreliable and the margin is thin. The numbers that survive a stress test are the numbers worth buying.
Stress test: rent needed on a £100,000 interest-only loan (illustrative)
| Stressed rate | Monthly interest | Rent needed at 125% ICR | Rent needed at 145% ICR |
|---|---|---|---|
| 5% | £417 | £521 | £604 |
| 6% | £500 | £625 | £725 |
| 7% | £583 | £729 | £846 |
Figures are illustrative, to show the method. Lenders' stress rates and ICR requirements vary.
Common questions
What is a buy-to-let mortgage stress test?
It is how lenders check that the rent will comfortably cover the mortgage even if interest rates rise. They apply an assumed (stressed) interest rate and require the rent to exceed the payment by a set margin, usually expressed as an interest cover ratio (ICR).
How can I pass a buy-to-let stress test?
Buy in a higher-yielding area where rent covers the loan with room to spare, put down a larger deposit, or use products designed for lower stress rates. The stronger the rent is relative to the loan, the easier the test.
Stress-testing the numbers is part of how we assess every property before we recommend it. We would rather walk away from a deal that only works at the best rate than hand you a problem in two years. If you want a second pair of eyes on a purchase, let us run the numbers with you.
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