How lenders stress test a buy-to-let mortgage
Most first-time landlords work out their deposit, find a rent figure that gives a tidy cashflow, and assume the mortgage follows. Then the offer comes back smaller than they asked for — or doesn't come at all. The reason is almost always the same: the deal failed the stress test, and the stress test caps your loan before cashflow is ever considered.
Here's exactly how it works in 2026, so you can run it yourself before you pay for a survey.
The test is on the rent, not on you
A residential mortgage is underwritten on your income. A buy-to-let mortgage is mostly underwritten on the property's income. The core check is the Interest Coverage Ratio (ICR) — sometimes called rental cover — which asks a single question: does the rent comfortably clear the mortgage interest, with a safety margin, even if rates rise?
Two numbers decide it: the stressed rate and the ratio
1. The stressed rate (not your pay rate)
Lenders don't test affordability at the rate you'll actually pay. They add a buffer — typically your pay rate plus 2%, with a floor of around 5.5%. So a 5.5% product is usually stress-tested at about 7.5%. The important consequence: the rate you're quoted is not the rate the loan is sized on.
There's one common exception. On many five-year fixed products, lenders stress at a lower rate — sometimes the pay rate itself, or a flat ~5.5% — because you're locked in. That's why a longer fix often unlocks a bigger loan on the same rent.
2. The ratio: 125% or 145%
Which ratio applies depends on how the tax works for you:
| Borrower | Typical ICR |
|---|---|
| Basic-rate taxpayer | 125% |
| Limited company (SPV) | 125% |
| Higher-rate taxpayer (personal name) | 145% |
The gap is because of Section 24: a higher-rate taxpayer holding property in their own name loses more of the rent to tax, so lenders demand a bigger cushion. It's often the single biggest reason investors buy through a limited company.
A worked example
Say you want a £150,000 loan. Your product rate is 5.5%, so the lender stresses at 7.5%. The stressed monthly interest is:
Now apply the ratio to find the rent you need:
| Required ICR | Rent needed |
|---|---|
| 125% | £1,172 / mo |
| 145% | £1,359 / mo |
If the market rent is £1,100, a 145% lender won't give you £150,000 — full stop. It isn't a matter of a better credit score or a bigger deposit story; the rent simply doesn't stretch that far under the test.
What actually happens when you "fail"
Lenders rarely decline outright. Instead they cap the loan at the amount the rent supports. You can work that cap out yourself by rearranging the test:
With £1,100 rent, a 145% ratio and a 7.5% stress rate, the biggest loan the test allows is:
So on a £200,000 purchase, the rent caps you at roughly 61% LTV — not the 75% you planned. That's a £28,621 hole you'd have to fill with more deposit, a lower price, or higher rent. Finding that out at the mortgage application stage, after you've paid for a valuation, is an expensive way to learn it.
Five ways to pass the test
- Put in a bigger deposit. A smaller loan is easier for the rent to cover — the bluntest fix.
- Use a five-year fix. The lower stressed rate often lifts the maximum loan meaningfully.
- Buy through a limited company. The 125% ratio (vs 145%) can be the difference on the same rent — weigh it against running costs and get advice.
- Push the rent, honestly. A small, evidenced rent increase moves the maths; an invented one just fails at valuation.
- Choose a lender that stresses lower. Criteria vary widely — a broker earns their fee here.
Enter your rent, rate and deposit and see the biggest loan the ICR test will actually allow — before the lender tells you.
Run the stress test free →This guide explains general lending practice and is not financial advice. Lender criteria vary and change often — confirm the stress rate and ICR with the specific lender or a mortgage broker before you rely on any figure.