Before approving a buy-to-let mortgage, lenders check that the rent covers the payment by a comfortable margin even if rates rise, using an Interest Coverage Ratio (ICR) and a notional stress rate. This guide explains how the test works, why your tax position changes the numbers, and how to improve how much you can borrow.
What the Stress Test Is
A buy-to-let mortgage stress test checks that the expected rental income on the property would still cover the mortgage payment even if interest rates increased, rather than relying on today's pay rate. Lenders calculate a notional monthly payment using a stress rate that is usually higher than the actual rate on offer, then compare that notional payment against the rent to decide the maximum they will lend.
The Interest Coverage Ratio
The Interest Coverage Ratio (ICR) expresses monthly rent as a percentage of the notional interest-only payment. A common requirement is rent at 125%-145% of that notional payment: for example, if the stressed monthly interest payment would be £1,000, a 125% ICR requires at least £1,250 in monthly rent. Lenders publish their own ICR and stress rate policies, and both vary by lender, property type, and borrower circumstances, so it is worth comparing more than one lender if a first application falls short.
Why Your Tax Rate Matters
Since mortgage interest relief for individual landlords was restricted under Section 24 to a basic rate tax credit rather than a full deduction, higher and additional rate taxpayers keep proportionally less rental profit after tax than basic rate taxpayers earning the same rent. To reflect this, many lenders apply a tougher ICR (often 145% rather than 125%) to applicants who declare themselves higher or additional rate taxpayers, while limited company borrowers -- taxed under Corporation Tax rather than the Section 24 rules -- are often assessed at the lower ICR regardless of the director's personal tax band.
How to Improve How Much You Can Borrow
A larger deposit reduces the loan amount and therefore the notional payment being tested. Choosing a product fixed for 5 years or more can unlock a lower stress rate at some lenders, since the payment cannot change within the fixed period. Some lenders also offer "top-slicing," using surplus personal income to support a shortfall in rental cover, though this is capped and not offered everywhere. Comparing several lenders' ICR and stress rate policies before applying can materially change the maximum loan available for the same property and rent.
Frequently Asked Questions
What is a buy-to-let mortgage stress test?
It is the calculation lenders use to check that the rental income on a property comfortably covers the mortgage payment, even if interest rates rise. Rather than testing the payment at your actual pay rate, the lender applies a higher "notional" rate and a minimum rental cover ratio (the Interest Coverage Ratio, or ICR) to decide the maximum loan.
What is the Interest Coverage Ratio (ICR)?
The ICR is the rent as a percentage of the monthly mortgage interest payment that a lender requires. A typical requirement is that rent must be at least 125%-145% of the interest-only payment calculated at the stress rate, though the exact figure varies by lender and by the borrower's tax position.
Why do higher-rate taxpayers need a higher rental cover ratio?
Since the Section 24 restriction on mortgage interest relief for individual landlords, higher and additional rate taxpayers keep less of their rental profit after tax than basic rate taxpayers on the same rent. Most lenders therefore apply a higher ICR (often 145% instead of 125%) to individual landlord applicants who pay tax above the basic rate, or ask about tax status on the application.
Show 5 more questionsShow fewer questions
What stress rate do lenders currently use?
The notional stress rate is usually the higher of the lender's own reversion (standard variable) rate plus a margin, or a fixed minimum such as 5.5%-8%, even when the actual mortgage rate on offer is lower. Lenders review their stress rate periodically in response to Bank of England base rate changes, so always check the specific rate quoted on an individual application rather than assuming a single UK-wide figure.
Does a 5-year fixed rate mortgage get a lower stress test?
Often yes. Some lenders stress test a buy-to-let mortgage at the pay rate (rather than a higher notional rate) if it is fixed for 5 years or more, since the payment cannot rise within that fixed period. This can materially increase the amount you can borrow compared with a 2-year fixed product.
Does the stress test apply to limited company buy-to-let mortgages?
Yes, though limited companies are not affected by the Section 24 individual landlord tax restriction, so lenders sometimes apply a lower ICR (commonly 125%) to limited company borrowers regardless of the underlying director's personal tax rate, since the property profit is taxed at Corporation Tax rates within the company rather than the director's Income Tax rate.
How can I improve my ICR if the rent does not cover the loan I want?
Options include putting down a larger deposit to reduce the loan amount, choosing a longer fixed-rate deal that may unlock a lower stress rate, using a lender with a lower ICR requirement for your tax bracket, or in some cases offsetting the shortfall against other personal income if the lender allows a "top-slicing" assessment.
What is "top-slicing" in buy-to-let lending?
Top-slicing lets a lender use your surplus personal income (after your own living costs and other mortgage commitments) to support a buy-to-let application where the rental income alone falls just short of the ICR requirement. Not all lenders offer top-slicing, and those that do usually cap how much personal income can be used.
Disclaimer: Stress rates and ICR requirements are set individually by each lender and change over time. This guide is for general information only and is not mortgage advice. Speak to a mortgage broker or lender and check gov.uk and the FCA for current rules before applying.
