## Understanding Buy-to-Let Refinance Headroom and Interest Coverage
Related check: [Gross Net Yield Calculator](/tools/gross-net-yield-calculator).
When seeking to remortgage a buy-to-let property, lenders assess your ability to service the mortgage debt, particularly under stressed interest rate conditions. This assessment is primarily focused on the property's rental income relative to the proposed loan amount. The key metrics are the Interest Coverage Ratio (ICR) and the lender's specific stress rate.
Lenders must adhere to underwriting standards to manage risk. For firms regulated by the Prudential Regulation Authority (PRA), supervisory statements outline minimum standards, including expectations for ICR tests and interest-rate affordability stress tests. These standards are designed to ensure that borrowers can manage their mortgage payments even if rates rise significantly. You can find details on these underwriting expectations in statements relevant to PRA-regulated firms. [Underwriting standards for buy-to-let mortgage contracts](https://www.bankofengland.co.uk/prudential-regulation/publication/2016/underwriting-standards-for-buy-to-let-mortgage-contracts-ss) provide a framework for these assessments.
Separately, the Financial Conduct Authority (FCA) sets out conduct rules and guidance for regulated mortgage and home-finance activities. It is important to confirm the regulatory classification of any proposed mortgage case rather than relying solely on product labels. The [FCA Mortgages and Home Finance Conduct of Business sourcebook](https://www.handbook.fca.org.uk/handbook/MCOB/) details these requirements.
### How Lenders Calculate Affordability
Lenders typically require that the gross rental income generated by a property must be a certain multiple of the monthly mortgage interest payment. This multiple is the ICR.
In addition to the ICR, lenders apply a 'stress rate'. This is a hypothetical interest rate used to assess affordability under adverse conditions. For instance, InterBay uses a stressed rate of 5.5%, or the initial pay rate plus 1.55%, whichever is higher, for standard BTL mortgages.
### Determining Loan Limits
Lenders use these metrics to set a maximum borrowing amount. The simplified formula to determine the loan ceiling based on rental income is:
**Loan Ceiling = Qualifying Annual Rent / (ICR x Annual Stress Rate)**
For a property with an annual rent of £18,000, a required ICR of 145% (or 1.45), and a stress rate of 5.5% (or 0.055), the calculation would be:
Loan Ceiling = £18,000 / (1.45 x 0.055) = £18,000 / 0.07975 = approximately £225,705.
This figure represents the maximum loan the rental income can support at the specified ICR and stress rate, before any Loan-to-Value (LTV) restrictions are applied.
### Worked Example - LTV Cap vs. Rent Support
Consider a property valued at £180,000. A lender offers a maximum LTV of 75%, meaning the maximum loan based on property value is £135,000 (£180,000 x 0.75).
Using the same rental income and stress parameters as above (£18,000 annual rent, 145% ICR, 5.5% stress rate), the maximum loan supported by rent is £225,705.
In this scenario, the LTV cap of £135,000 is the binding constraint, as it is lower than the rent-supported loan amount of £225,705. The actual maximum loan you could secure would be £135,000.