3. Understanding Buy-to-Let Mortgages
Buy-to-let mortgages are fundamentally different from residential mortgages. They are assessed primarily on risk, rental coverage, and asset performance — not personal affordability alone.
A residential lender asks: can this person afford these payments from their salary?
A buy-to-let lender asks: can this property pay for itself, even if rates rise?
Understanding how lenders think is the difference between building a resilient deal and being surprised at the application stage.
3.1 Interest-only vs repayment mortgages
The vast majority of buy-to-let mortgages are taken on an interest-only basis, but the choice is a genuine strategic decision, not a default.
| Interest-only | Repayment | |
|---|---|---|
| Monthly payment | Lower — interest only | Higher — interest + capital |
| Loan balance over time | Stays the same | Reduces to zero by end of term |
| Cashflow flexibility | Higher | Lower |
| Equity build | Only via capital growth | Via growth and capital repayment |
| Refinancing risk | Higher — full balance still owed | Lower — balance shrinks over time |
| Resilience to rate rises | Higher (lower payments) | Lower (higher payments) |
| Capital repaid on exit | Full loan due on sale/refinance | Little or none |
Interest-only is commonly chosen where cashflow resilience is the priority, capital is allocated elsewhere, and exit planning is conservative and realistic. Its central risk is simple: the loan balance never falls on its own, so the full capital must be repaid on sale or refinancing.
Repayment suits lower-risk strategies and steadily reduces leverage — but the higher monthly payment cuts into operating margin and leaves less room to absorb shocks.
The strategic point: this is not "right vs wrong." It's a trade-off between monthly cashflow and long-term leverage reduction. Highly-geared portfolios usually favour interest-only for the buffer; investors deleveraging toward retirement often shift to repayment.
3.2 The stress test — how lenders size your loan
This is the single most important mechanic in buy-to-let lending, and the one that catches most people out.
Lenders do not size your loan against the rate you'll actually pay. They size it against two things:
- A stressed interest rate — a higher "what if rates rise" rate.
- An Interest Coverage Ratio (ICR) — the margin by which rent must exceed that stressed interest.
The framework comes from the Prudential Regulation Authority's Supervisory Statement SS13/16, in force since January 2017.
Interest Coverage Ratio (ICR)
The ICR is the buffer lenders require between your rent and the stressed mortgage interest. It depends on your tax status — and this is where higher-rate taxpayers get penalised:
| Borrower type | Typical ICR |
|---|---|
| Basic-rate taxpayer (personal name) | 125% |
| Limited company / SPV | 125% |
| Higher-rate taxpayer (personal name) | 145% |
| Additional-rate taxpayer (personal name) | up to ~165% |
| HMOs / multi-unit blocks | 125%–175% (lender-dependent) |
The reason higher-rate taxpayers face 145% rather than 125% traces directly back to Section 24 (covered in Module 6): they can no longer deduct mortgage interest in full, so their after-tax margin is thinner and lenders demand a bigger cushion. This is a major reason many higher-rate landlords now buy through limited companies, which are typically stress-tested at 125%.
Stress rate
| Product type | Typical stress rate applied |
|---|---|
| 2-year fix or shorter | 5.5% floor, or pay rate + 2% (whichever is higher) |
| 5-year+ fix | Often lower — sometimes pay rate + 1%, occasionally the pay rate itself |
| HMO / specialist | 6.0%–7.0% |
The 5-year-fix carve-out matters: a borderline deal that fails on a two-year product can often pass on a five-year fix, because the lender is allowed to stress it more gently. This alone can be the difference between a deal working and not working.
3.3 Worked example — minimum rent to pass
Figures below use a 5.5% stress rate. Correct as of July 2026 — always confirm current lender criteria, as stress rates move with the market.
Property: £250,000, bought at 80% LTV → £200,000 loan.
Stressed annual interest = £200,000 × 5.5% = £11,000
| Borrower | Calculation | Minimum rent needed |
|---|---|---|
| Basic-rate taxpayer (125%) | £11,000 × 1.25 = £13,750/yr | £1,146 / month |
| Higher-rate taxpayer (145%) | £11,000 × 1.45 = £15,950/yr | £1,329 / month |
Same property, same loan — but the higher-rate taxpayer needs £183 more rent every month just to pass the test.
3.4 Worked example — the higher-rate borrowing penalty
Now flip it around. Same rent, how much can each borrower actually raise?
Rent achievable: £1,200/month → £14,400/year. Stress rate 5.5%.
| Borrower | Max stressed interest (rent ÷ ICR) | Max loan (÷ 5.5%) |
|---|---|---|
| Basic-rate (125%) | £14,400 ÷ 1.25 = £11,520 | ≈ £209,450 |
| Higher-rate (145%) | £14,400 ÷ 1.45 = £9,931 | ≈ £180,560 |
On identical rent, the higher-rate taxpayer can borrow roughly £29,000 less — which usually means finding a larger deposit or buying a cheaper property. This is one of the clearest illustrations of why ownership structure (Module 6) should be decided before you start viewing.
3.5 Loan-to-value (LTV) explained
LTV is the proportion of the property value funded by debt.
LTV = Loan Amount ÷ Property Value
Buy-to-let LTV typically caps at 75%, though the best rates and easiest stress tests sit at 60–65%.
| Higher LTV (e.g. 80%) | Lower LTV (e.g. 60%) |
|---|---|
| Less upfront capital needed | More deposit required |
| Higher rate, harder stress test | Better rates, easier to pass |
| More sensitive to rate rises | More resilient |
| Less refinancing flexibility | More options at renewal |
Experienced investors generally prioritise LTV discipline over squeezing out maximum leverage — the resilience is worth more than the extra borrowing.
3.6 Costs, fees and the portfolio threshold
Two practical points routinely missed by beginners:
Arrangement fees. Buy-to-let product fees are often high — commonly £1,000–£3,000, sometimes charged as a percentage of the loan. Adding the fee to the loan preserves cash but increases your balance and interest. Always factor fees into your yield, not just the headline rate.
The four-property threshold. Once you have four or more mortgaged buy-to-let properties, you're classed as a portfolio landlord. Fewer lenders will consider you, underwriting looks at your whole portfolio's stress position, and paperwork increases significantly. Plan for this before you hit it, not after.
Top-slicing. Some specialist lenders (Paragon, Precise, Aldermore among others) allow surplus personal income to bridge a shortfall where rent alone doesn't quite meet ICR. Useful, but lender-specific — a good broker earns their fee here.
3.7 Remortgaging and refinancing
Remortgaging replaces an existing loan with a new one — usually to secure a better rate, release capital, or adjust terms. Key risks:
- Valuation shortfalls — a down-valuation shrinks how much you can raise.
- Tighter criteria — the deal that passed last time may not pass now if stress rates have climbed.
- Rate environment — you refinance into today's market, not the one you bought in.
A large cohort of landlords who fixed cheaply in 2021 have been rolling onto materially higher rates, and some are finding their rent no longer clears the current stress test even after rent rises. Treat refinancing as optional upside, not a guaranteed step in your plan — and never build a strategy that depends on being able to refinance at a specific date and rate.
Module summary: In buy-to-let, the lender's stress test — not the headline rate — usually decides how much you can borrow. Know your ICR (125% or 145%), assume a stress rate around 5.5%, and model your deal against it before you offer. Structure and LTV discipline decided early save expensive corrections later.
Educational information only. This does not constitute financial, tax, mortgage or investment advice. Lender criteria and stress rates vary and change frequently — verify current figures with a qualified broker before acting.