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Every UK buy-to-let mortgage is stress-tested against an Interest Coverage Ratio (ICR) at a stressed interest rate — not the actual product rate. The framework comes from the Prudential Regulation Authority’s Supervisory Statement SS13/16, in force since 1 January 2017 and revised through 2024. The headline rule: rental income must cover the stressed mortgage interest by at least 125% for a basic-rate taxpayer or Limited Company borrower, and 145% for a higher- or additional-rate taxpayer borrowing in personal name. A typical stress rate is 5.5% or "product rate plus 2%" whichever is higher.

The ICR maths is straightforward: monthly rent × 12 × (1 − void allowance) ÷ (loan amount × stress rate ÷ 12 × 12) ≥ ICR threshold. Lenders typically assume zero voids for the stress calculation but apply an internal voids buffer when scoring the deal; some specialist BTL lenders explicitly require a 5% void deduction in the test. A £220,000 BTL with £1,200 monthly rent at a 75% LTV (£165k loan), 5.5% stress rate, 145% ICR for a higher-rate taxpayer requires £1,098 of monthly stressed interest and £1,591 of monthly rent — the rent passes by a thin margin.

The 5-year fixed-rate exemption is the single biggest lever for higher-LTV deals. PRA SS13/16 allows lenders to apply a lower stress rate (or even the product rate itself) for products fixed for 5 years or more, on the basis that interest-rate risk over the fixed period is removed. Many lenders apply 5.5% stress on 2-year fixed products but 5.0% (or the product rate) on 5-year fixes — enough to push a marginal deal across the line. The trade-off is a higher product rate and an early-repayment charge if you exit before the 5-year window.

Top-slicing is the second lever. A growing number of specialist BTL lenders (Paragon, BM Solutions, Foundation, Aldermore) accept "top-slicing" — using the borrower’s personal income to plug an ICR shortfall. The borrower must demonstrate sufficient surplus personal income (typically £30k–40k of net disposable income above household commitments) to cover the gap. Top-slicing turns a borderline deal into an approval and is essential for higher-LTV London / South-East purchases where rents struggle to hit the 145% ICR.

Portfolio landlord (4+ mortgaged BTL properties) rules add a further layer. PRA SS13/16 requires the lender to assess the entire portfolio — cash flow, leverage, geographic concentration — not just the property being purchased. Most lenders demand a portfolio business plan, a property schedule with rental coverage on every property, three years of SA302s and a stress test of the whole portfolio at a higher rate. Some withdraw entirely from portfolio lending once the borrower passes 10 properties; others (Paragon, Foundation, Together) specialise in 10–100 property portfolios.

Limited Company BTL has been the largest growth area since Section 24→ took full effect in April 2020. Ltd company borrowers benefit from full mortgage-interest deductibility and a lower 125% ICR threshold (treated as basic-rate for stress purposes), but pay a higher product rate (typically 0.3–0.6% premium over personal-name) and a higher arrangement fee. The decision is rarely about rate alone — the post-tax cash position usually favours Ltd above £50k of property profit, but personal name remains better below it. Use this calculator to test the maximum loan available to you in personal vs Ltd name before committing to a structure.