With interest rates still high, what's actually the best way to calculate if a buy-to-let mortgage is even affordable now, especially with the 125%/145% stress tests? Am I missing something with how lenders work it out?
Quick Answer
Buy-to-let mortgage affordability is assessed against Income Cover Ratios (ICR) and stress tests, not personal income. Lenders typically require rental income to cover 125% or 145% of mortgage interest at a higher notional rate, usually 5.5%.
## Demystifying Buy-to-Let Mortgage Affordability Calculations
Buy-to-let mortgage affordability for lenders is primarily determined by Income Cover Ratios (ICRs) and stress tests, not directly by a borrower's personal income. As of December 2025, the Bank of England base rate is 4.75%, with typical BTL fixed rates ranging from 5.0-6.5%, impacting these calculations significantly. The standard BTL stress test requires 125% rental coverage at a notional 5.5% rate for basic rate taxpayers, which for higher or additional rate taxpayers can rise to 145% at 5.5% or even 6-7% depending on the lender. This means the rental income must be sufficient to exceed a calculated mortgage payment by the specified percentage.
Lenders calculate an 'affordable' maximum loan amount by taking the expected rental income, dividing it by the stress test percentage (e.g., 1.25 for 125%), and then dividing that figure by their notional interest rate (e.g., 0.055 for 5.5%). This provides the maximum loan amount they would consider. For example, a property generating £1,000 per month in rent, under a 125% stress test at 5.5%, would be assessed as £1,000 / 1.25 / 0.055 * 12 = £174,545 max loan, rather than a simpler sum that would be £218,181 based on market rates. This ensures the property can still cover its costs if rates rise or income dips slightly. The Section 24 restriction, removing mortgage interest deductibility for individual landlords since April 2020, also influences these calculations for many, as it reduces taxable income but not the direct rental income used for ICRs.
### Can you still get a good BTL mortgage deal now?
Yes, it is possible to still secure viable buy-to-let mortgage deals, although the criteria are stricter due to higher Bank of England base rates at 4.75%. Lenders are adjusting their notional stress test rates, with some still using 5.5% for basic rate taxpayers, but many applying higher rates for higher/additional rate taxpayers. For instance, a property with £1,600 monthly rental income targeted by a higher rate taxpayer might need to cover 145% of the mortgage interest at a 6% notional rate. This significantly reduces the maximum loan amount available for a given rent compared to previous years.
Rental yield calculations remain paramount; a property needs to generate sufficient rent to pass these tests. On a £250,000 property, earning £1,500 rent, this may not be enough for a 75% LTV mortgage if the stress rate is high. This requires investors to meticulously calculate rental income against potential mortgage payments at the stress-tested rate, not just the pay rate. Some lenders offer products specifically for limited companies, where Corporation Tax rates (19% for profits under £50k, 25% over £250k) still allow for interest relief, potentially offering better ICRs.
## Potential Pitfalls in BTL Mortgage Calculations
* **Overlooking the Lenders' Notional Rate:** Many investors only factor in the actual pay rate (e.g., 5.5% fixed) but ignore the lender's higher notional stress test rate (e.g., 5.5% for basic rate, 6-7% for higher/additional rate taxpayers) used for affordability. This is a common error in 'landlord profit margins' calculations.
* **Ignoring the ICR Variation:** The assumption of a universal 125% ICR is incorrect. Higher-rate and additional-rate taxpayers, and properties in HMOs or with shorter fixed terms, often face 145% stress tests, or even higher notional rates up to 7% with some lenders. This needs careful attention for 'rental yield calculations'.
* **Underestimating Additional Costs:** Beyond the mortgage, SDLT (5% additional dwelling surcharge for most BTLs), legal fees, and unexpected repair costs are not factored into the lender's affordability calculation, but significantly affect an investor's true 'BTL investment returns'.
* **Reliance on 'Guidance' Stress Rates:** Lenders' specific stress rates and ICRs are often higher than advertised, especially for specific property types or borrower profiles, always confirm with a broker what your specific stress calculation will be.
## Investor Rule of Thumb
Always calculate buy-to-let affordability using the specific lender's stress test criteria (ICR and notional interest rate) applicable to your tax bracket and property type, not just the promotional pay rate, to assess true viability before making an offer.
## What This Means For You
Understanding these nuanced calculations is fundamental for successful property investment in the current climate. Most investors don't lose money because they rush into deals, they lose money because they don't grasp the underlying financial mechanisms that lenders use. If you want to accurately assess your BTL deals and ensure they meet strict affordability criteria, this is exactly what we teach inside Property Legacy Education.
Steven's Take
The shift in BTL mortgage affordability isn't just about higher interest rates; it's about how lenders interpret those rates through their stress tests. I've consistently seen investors trip up by only looking at the 'pay rate' rather than the 'stress rate'. From April 2020, Section 24 removed mortgage interest deductibility for individual landlords, which means for higher rate taxpayers, the higher 145% ICR at a notional 5.5%+ rate is commonplace. This makes many deals previously viable under the old rules unviable today. You need to use a specialist BTL mortgage broker to understand the current landscape.
What You Can Do Next
Step 1: Contact a specialist buy-to-let mortgage broker (find one via NACFB.org.uk or PTFS.co.uk) to obtain up-to-date ICRs and notional stress rates for your specific tax position and property type.
Step 2: Calculate the maximum loan amount your target property's rental income can support using the broker's confirmed stress test figures.
Step 3: Review your tax position with a property tax accountant (search for one at ICAEW.com or ACCA.org.uk) to determine if personal ownership or a limited company structure is more tax-efficient under Section 24 and Corporation Tax rules (19% under £50k profit, 25% over £250k).
Step 4: Create a comprehensive deal analysis spreadsheet that incorporates the stress-tested mortgage payment alongside all other costs, including the 5% additional dwelling SDLT surcharge and potential void periods, to assess true profitability.
Get Expert Coaching
Ready to take action on financing & mortgages? Join Steven Potter's Property Freedom Framework for comprehensive, hands-on property investment coaching.