Buy-to-let mortgages explained: ICR, stress tests and why deals get declined

This guide is for anyone applying for their first buy-to-let mortgage, or anyone who got declined and wants to know why. We cover how lenders actually decide what to lend you, with the real maths behind it. If you have not bought your first rental yet, start with our first buy-to-let guide and come back.

BTL lending is a different game

A residential mortgage is assessed on your salary. A buy-to-let mortgage is assessed mainly on the rent the property can earn. That one difference explains almost everything below.

Two other things surprise first-timers:

• You need a bigger deposit. The market standard in 2026 is 25% down, so the lender covers 75% of the value (75% loan-to-value, or LTV). Some lenders go to 80% or beyond, but rates climb fast above 75%, and the widest choice of deals sits at 75% LTV or below.
• Most BTL lending is not FCA-regulated in the way residential mortgages are. It is treated as business lending. A small slice called consumer buy-to-let (for example, letting out a home you once lived in) is regulated under the Mortgage Credit Directive Order 2015. Either way, mortgage brokers themselves are FCA-authorised, and you should use one.

Why interest-only is standard

Most BTL mortgages are interest-only. You pay only the interest each month and the loan balance never shrinks. The capital gets repaid at the end, usually by selling the property or remortgaging.

That sounds risky, but there are rational reasons landlords choose it:

• Cash flow. Interest-only payments are far lower, so the rent covers them with room to spare for repairs, voids and insurance.
• Tax. For individual landlords, only the finance cost (the interest) attracts any tax relief. Capital repayments get you nothing. So there is no tax incentive to repay early.
• Flexibility. You can overpay when you choose to, rather than being locked into a repayment schedule.

Nothing stops you taking a repayment BTL mortgage. It just makes the affordability test below harder to pass, because lenders test the interest either way but your monthly outgoing is bigger.

The ICR test: the number that decides everything

The Interest Coverage Ratio (ICR) is the ratio of monthly rent to the monthly mortgage interest, calculated at a stressed rate rather than your actual rate. Typical thresholds in 2026:

• 125% for basic-rate taxpayers and limited company borrowers
• 145% for higher-rate taxpayers borrowing in their own name (some lenders go higher still for additional-rate taxpayers)

Why the difference? Since April 2020, individual landlords cannot deduct mortgage interest from rental income before tax. Instead they get a flat 20% tax credit (the Section 24 rules, from the Finance (No. 2) Act 2015). A higher-rate taxpayer therefore keeps much less of the rent after tax, so lenders demand a bigger rental cushion. Companies still deduct interest as a normal business expense, which is why they get the 125% test, and a big part of why so many landlords now buy through limited companies.

Stress rates: the rate they test, not the rate you pay

Since 2017, the Bank of England's Prudential Regulation Authority has required lenders to stress-test BTL affordability under supervisory statement SS13/16, which remains in force in 2026 (it was updated in January 2026 with the core expectations unchanged). The PRA expects lenders to assume rates rise by at least 2 percentage points, and to test at a minimum rate of 5.5%, unless your rate is fixed for 5 years or longer.

In practice, in mid-2026:

• 2-year fixes and trackers are typically stressed at pay rate plus 2%, or a floor of around 5.5%, whichever is higher. In mid-2026 that typically lands somewhere between 6.5% and 8%, depending on the lender and the pay rate.
• 5-year fixes escape the mandatory uplift, so many lenders stress them at or near the pay rate. This is why 5-year fixes dominate BTL: they let you borrow more, not because landlords love long fixes.

A worked example

Say you are a higher-rate taxpayer buying in your own name. Property price £200,000, so a 75% mortgage is £150,000. The lender stresses at 5.5% with a 145% ICR.

If the letting agent says the realistic rent is £950, you fail. The lender will not decline you outright; they will cap the loan at what the rent supports:

Maximum loan = (£950 x 12) / (5.5% x 1.45) = £11,400 / 0.07975 = roughly £142,900

So you would need to find about £7,000 more deposit. Run your own numbers in our free BTL stress-test calculator. Note that a limited company borrower at 125% would only need rent of £859.38 on the same loan, which is exactly why the company route passes tests that the personal route fails.

Top-slicing: when your salary rescues the deal

Some lenders allow top-slicing: if the rent falls just short of the ICR, they count your surplus personal income to plug the gap. SS13/16 explicitly allows personal income to be used, provided the lender properly verifies it and tests your own affordability. Not every lender offers it, the paperwork is heavier, and it works best for applicants with strong salaries and few commitments. A broker will know who does it.

Product fees: how headline rates lie

BTL deals often carry chunky arrangement fees, commonly a flat fee of around £1,000 to £4,000, or a percentage fee of 3% to 7% of the loan on the lowest-rate products. Lenders use big fees to buy down the headline rate, partly because a lower pay rate can help the deal pass the stress test.

Quick comparison on a £150,000 loan over a 2-year fix:

• 4.5% rate with a 3% (£4,500) fee: interest £13,500 + fee £4,500 = £18,000
• 5.5% rate with no fee: interest £16,500 = £16,500

The "cheaper" headline rate costs £1,500 more. Always compare total cost over the fixed period, not the rate.

Portfolio landlords: the rules change at four

Under SS13/16, once you have four or more mortgaged buy-to-let properties (counted across all lenders, including any held in a company), you are a portfolio landlord. Every new application then triggers a full portfolio review: a property schedule, business plan, cash flow figures, and an affordability check across the whole portfolio, not just the new purchase. It is manageable, but turn up organised. Properties owned outright do not count towards the four, though lenders will still want them disclosed.

Why first BTL applications get declined

The most common reasons, roughly in order:

1. Rent fails the ICR at the stress rate. By far the biggest one. Check before you offer.
2. Property type. Flats above takeaways, non-standard construction, some new-build flats, tiny studios and ex-local-authority high-rises all narrow the lender pool.
3. Minimum income rules. Many lenders want around £25,000 of personal income, separate from the rent.
4. First-time buyer and first-time landlord combined. Many lenders want you to own your own home first, to stop people using BTL to dodge residential affordability checks.
5. Credit blips and undisclosed debts. Even small ones matter more than people expect.
6. Deposit source. Lenders want a clean, documented trail (savings, sale proceeds, documented gifts).

Mistakes people make

• Offering on a property before running the stress test, then scrambling for extra deposit when the loan gets capped.
• Comparing headline rates and ignoring percentage fees.
• Assuming the 145% test applies to them when the 125% test does, or vice versa. Your tax band drives it, so check.
• Forgetting that yield and mortgage affordability are different questions. A property can pass the ICR and still be a poor investment. See the yield guide.
• Going direct to one lender instead of using a whole-of-market, FCA-authorised broker who knows which lenders fit your exact situation.

Sources: Bank of England, SS13/16 Underwriting standards for buy-to-let mortgage contracts, gov.uk, Restricting finance cost relief for individual landlords, legislation.gov.uk, The Mortgage Credit Directive Order 2015, legislation.gov.uk, Finance (No. 2) Act 2015, Section 24

Education, not financial advice. For mortgage advice, speak to an FCA-authorised broker.