BTL affordability and stress tests
Buy-to-let affordability works differently from residential mortgages. Lenders use stress tests and rental coverage ratios to decide how much you can borrow. This guide explains exactly how these calculations work and what you can do if the numbers don’t stack up.
In this guide
What is the interest coverage ratio (ICR)?
The interest coverage ratio is the key metric lenders use to assess buy-to-let affordability. It measures whether the expected rental income is high enough to cover the mortgage payments with a comfortable buffer.
An ICR of 125% means the rent must be at least 125% of the monthly mortgage payment calculated at the lender’s stressed rate. So if the stressed payment would be £800/month, the rent needs to be at least £1,000/month.
How do stress rates work?
Typical stress test parameters
5.5%
Common stress rate
The interest rate used to calculate affordability, regardless of your actual rate
125%
Basic-rate taxpayer ICR
Rent must cover 125% of the stressed mortgage payment
145%
Higher-rate taxpayer ICR
A stricter ratio due to the Section 24 tax impact
How do lenders calculate rental coverage?
Worked example: £200,000 BTL mortgage
Calculate the stressed payment
£200,000 at 5.5% interest-only = £916.67 per month. This is the hypothetical payment the lender uses, not your actual rate.
Apply the ICR
At 125% ICR: £916.67 × 1.25 = £1,145.83. The monthly rent must be at least £1,146 for a basic-rate taxpayer to pass the test.
Compare with market rent
If comparable properties in the area rent for £1,200/month, you’d pass comfortably. If rent is only £1,000/month, you’d fail and need to explore alternatives.
Factor in your tax band
At 145% ICR (higher-rate taxpayer): £916.67 × 1.45 = £1,329.17. The rent would need to be significantly higher to meet this threshold.
What if the rent doesn’t meet the stress test?
- A bigger deposit reduces the loan and therefore the stressed payment
- This is the most straightforward way to pass a tighter stress test
- Even a small increase can make the difference
Do you need personal income for a buy-to-let mortgage?
Most lenders require a minimum personal income of £25,000 per year, even though rental income is the primary affordability measure. This acts as a safety net, ensuring you have other resources to fall back on if the property is empty or needs unexpected repairs.
Some specialist lenders have no minimum income requirement and base their decision entirely on the rental figures. These are useful for landlords whose income comes mainly from property, or for those who are retired.
Tips to improve your BTL affordability
- Even 5% more deposit can significantly improve your ICR position
- Lower LTV also unlocks better interest rates
Related guides
More guides in our buy-to-let mortgage hub.
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