Answers · UK 2025/26

How does a buy-to-let mortgage stress test work?

A buy-to-let mortgage stress test checks whether the expected rental income would still comfortably cover the mortgage payments even if interest rates rose, typically requiring rental income to reach around 125-145% of the mortgage payment calculated at a notional "stressed" interest rate (often higher than the actual rate offered), rather than the actual pay rate alone. This protects both the lender and the landlord from the property becoming unaffordable if rates increase.

Last updated: 2 July 2026Last reviewed: 2 July 2026How we verify

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Buy-to-let mortgage affordability is assessed quite differently from a standard residential mortgage, focusing primarily on rental income rather than the borrower's personal salary. **Why lenders stress test buy-to-let mortgages** Since the property's rental income is generally the main source of funds used to repay a buy-to-let mortgage, lenders need to check that this income would remain sufficient even under less favourable conditions -- particularly if interest rates rise during the mortgage term, increasing the required monthly payment. **How the interest cover ratio (ICR) test works** Lenders typically require the expected monthly rental income to reach a set percentage -- commonly around 125% to 145% -- of the mortgage interest payment, calculated using a "stressed" notional interest rate that is often higher than the actual rate being offered on the mortgage (for example, testing affordability at 5.5% or higher, even if the actual product rate is considerably lower). This is known as the Interest Cover Ratio (ICR) test. **Why the stressed rate is often higher than the actual rate** Using a higher notional rate for the affordability calculation, rather than the actual (often lower) rate on the mortgage product, builds in a safety margin -- ensuring the rental income would still cover payments even if rates rose significantly during the mortgage term, or if the borrower needed to remortgage onto a higher rate in future. **Higher and lower stress test percentages** Higher-rate taxpayers are often subject to a higher required rental cover percentage (commonly around 145%) than basic rate taxpayers (commonly around 125%), reflecting the fact that mortgage interest tax relief for buy-to-let is now restricted to a basic rate tax credit under Section 24, which changes the effective after-tax cost of the mortgage for higher-rate taxpayers. **Special products for lower rental yields** Some lenders offer specific products for landlords in lower-yield areas (such as London and the South East, where property prices are high relative to achievable rents), sometimes using a lower stress rate or allowing top-slicing (using the landlord's personal income to support the affordability calculation if rental income alone does not meet the standard ICR requirement). **Worked example** A property is expected to achieve £1,200 per month in rent. At a stressed notional rate of 5.5% and a 145% ICR requirement, the lender calculates the maximum mortgage payment the rental income can support, then works back to the maximum loan amount available -- this maximum loan may be considerably lower than what the same borrower could raise on a residential mortgage based on personal income. **Practical tip** Use the Buy-to-Let calculator alongside realistic local rental estimates to check whether a prospective purchase is likely to pass a lender's stress test before making an offer, since failing the affordability check is a common reason buy-to-let purchases fall through.