Landlords coming off a fixed rate tend to shop on the headline number, then discover the application fails on something they were not looking at. The buy to let stress test, not the advertised rate, is what decides whether a remortgage completes. It is set out in the Prudential Regulation Authority’s underwriting standards for PRA regulated lenders, with the FCA running a parallel regime for non business consumer buy to let that mirrors the same approach, it has been in force since 2017, and it is the reason a property that is comfortably profitable in your bank account can still be declined.
How the stress test works
Lenders assess a buy to let against an interest coverage ratio. The PRA defines it as the ratio of the expected monthly rental income to the monthly interest payments, with those payments calculated using an assumed future interest rate rather than the one you are being offered.
So there are two variables and neither is the headline rate. The first is the rent, which the lender will verify independently through a valuer, an automated model or an existing tenancy agreement rather than taking your figure. The second is the stressed rate, which is deliberately higher than what you will actually pay. The ratio between them has to clear the lender’s minimum threshold, which the PRA notes has an industry standard of 125 per cent and which it has explicitly said it does not expect to see fall.
In practice thresholds sit above that floor for many borrowers, because the same guidance tells lenders to build in the costs of running the property when setting them. The PRA lists management and letting fees, council tax, service charges, insurance, repairs, voids, utilities, gas and electrical certificates, licence fees and ground rent.
Where the 5.5% floor comes from
The stressed rate is not invented by each lender. The PRA’s supervisory statement tells firms to look at likely future rates over at least five years from the start of the mortgage, and to assume a minimum increase of 2 percentage points on buy to let rates. It then sets a hard floor: even if that calculation produces a rate below 5.5 per cent, the firm should assume a minimum borrower interest rate of 5.5 per cent.
That floor is why falling rates do not translate straight into easier applications. A lender can cut its product rate and the affordability test barely moves, because the test was never run at the product rate.
There is one exemption that matters and it is the most useful lever most landlords have. The five year look forward does not apply where the interest rate is fixed or capped for five years or more. That is the mechanism behind the widely observed fact that five year fixes stress more generously than two year ones. It is not a lender being kind, it is the rule.
Why a remortgage fails on rent
Run the arithmetic and the failure mode becomes obvious. Take a £150,000 loan stressed at 5.5 per cent. The notional annual interest is £8,250, which is £687.50 a month. At a 125 per cent threshold the property needs £859 a month of rent. At 145 per cent it needs £997.
A landlord paying an actual rate well below the stress figure may be clearing that loan comfortably in cash terms every month and still fall short on the test, because the test is not measuring what they pay. It is measuring what they would pay in a worse world.
Which points at the lever people neglect. Rent is the numerator, and a tenancy that has not been reviewed for three years is quietly shrinking your borrowing capacity. Reviewing the rent to market before applying does more for a marginal case than shaving a few basis points off the product. The other levers are the fix length, as above, and reducing the loan, since a smaller numerator on the interest side needs less rent to cover it.
The 2027 tax change feeds in
One thing worth watching, because it connects two changes landlords tend to think about separately.
The PRA’s standards already require lenders to take account of the tax liability associated with the property when setting the minimum ratio, and note that firms may simply assume every borrower is a higher rate taxpayer. Tax is therefore not outside the stress test, it is an input to it. From 6 April 2027 property income is taxed at 22, 42 and 47 per cent under the new property rates set by section 7 of the Finance Act 2026, two points above the current bands.
A higher tax input pushing against an unchanged ratio can only tighten the test. How far, and how quickly each lender reflects it in its thresholds, is not something anyone can tell you yet, and no lender has published a rule for it. But a landlord assuming that the 2027 change touches only their tax return is looking at half of it.
What this means for property investors
Test the deal at 5.5 per cent before you buy, not when you refinance. A purchase that only works at today’s product rate is a purchase that may not refinance in five years, and refinancing risk is something the PRA explicitly tells lenders to consider at the end of a fixed period. If the rent does not cover the stressed interest at your likely threshold, the deal is thinner than the yield suggests.
That is a straightforward argument for buying on rent rather than on price. Higher yielding stock clears the ratio with room to spare and gives you options at every refinance. A low yielding property in a strong area can leave you unable to move lender at all, which is the worst position to be in when your fix ends. It is the same reason we concluded that buy to let still works on income rather than on hoped for growth.
If you hold four or more mortgaged buy to let properties the PRA treats you as a portfolio landlord, and lenders apply a specialist underwriting approach that looks at the whole portfolio rather than the single property. Expect to evidence the lot.
Every property we take on is assessed on rent that a lender would actually recognise. You can see what is currently available on our investment property listings, or join the insider list for deals before they are published.