How a Buy-to-Let Mortgage Works (and What ICR, LTV, and Yield Mean)

What is a buy-to-let mortgage?

A buy-to-let (BTL) mortgage is a specialist loan for landlords. The lender knows the property is being let to a tenant rather than lived in by the borrower, and that single fact changes the underwriting in three important ways.

First, the deposit is bigger. Most UK BTL lenders cap loan-to-value (LTV) at 75–80%, so you put down at least a fifth of the purchase price. Second, the rate is higher — spreads above the base rate are wider than residential rates, and product fees of 1–3% of the loan are common. Third, affordability is tested differently: instead of running a household income calculation, the lender stress-tests the rent against the interest. That last one is the interest coverage ratio (ICR) test, and it sits at the heart of every BTL application.

UK BTL lending sits under the Prudential Regulation Authority's supervisory statement SS13/16, published by the Bank of England in 2016 and still in force. It sets the underwriting standards every regulated BTL lender has to meet.

How a buy-to-let mortgage is calculated

There are four numbers the buy-to-let mortgage calculator returns. Each one answers a different question, so it helps to know what you're looking at before you read off the result.

The monthly payment

For an interest-only mortgage — the BTL default — the monthly payment is just the interest:

Monthly payment = Loan × (Annual rate ÷ 12)

For a capital repayment mortgage, the payment also chips away at the loan balance. The standard amortisation formula is:

P = L × r ÷ (1 − (1+r)⁻ⁿ)

Where L is the loan, r is the monthly rate (annual rate ÷ 12), and n is the total number of months. The repayment figure is always larger than the interest-only figure, because part of every payment now goes to the principal.

Most landlords pick interest-only because it keeps cash flow stronger. The trade-off is that you don't build equity from monthly payments — any equity gain has to come from price appreciation, or from refinancing into a repayment loan later in the life of the asset.

Loan-to-value (LTV)

LTV = Loan ÷ Property value

LTV decides which products you qualify for. Lenders tier their rates at the 60%, 65%, 70%, and 75% LTV bands; lower LTV unlocks better rates because the lender's downside is smaller. Above 80% LTV, BTL lending is rare and expensive, and most high-street lenders won't consider it.

Gross rental yield

Gross yield = (Monthly rent × 12) ÷ Property value

This tells you what the property earns relative to what it costs, before any costs come out. A 6% gross yield means £6,000 of annual rent for every £100,000 of property value. Gross yield is the headline number landlords quote because it's stable across regions; net yield depends on local letting agent fees, void rates, and your personal tax position, which makes it harder to benchmark.

Interest coverage ratio (ICR)

This is the metric that decides whether the lender will write you the loan at all.

ICR = (Monthly rent × 12) ÷ (Annual mortgage interest)

UK BTL lenders typically require:

• 125% for basic-rate taxpayers and limited companies
• 145% for higher-rate or additional-rate taxpayers

…stress-tested at a notional rate of 5.5%, or two percentage points above the product rate, whichever is higher. Even if the real product rate is 4.5%, the lender will check the ICR at 6.5%. If it doesn't clear the threshold, the loan is downsized until it does.

The 145% rule for higher-rate taxpayers comes from SS13/16. It was designed to absorb the income tax that's no longer offset by mortgage interest under the Section 24 reform — more on that further down.

Worked example

Let's walk through a typical case. Plug these into the BTL mortgage calculator and you'll see the same numbers come back.

• Property value: £250,000
• Loan amount: £187,500 (75% LTV)
• Interest rate: 5.5%
• Term: 25 years
• Monthly rent: £1,250
• Payment type: Interest-only

The calculator returns:

• Monthly payment: £859.38
• Annual interest: £10,312.50
• Annual rent: £15,000
• Gross yield: 6.0%
• ICR: 145.5%
• LTV: 75%

The ICR at 145.5% is the tightrope. If the rent fell to £1,200 a month, the ICR would drop to about 139.6% — below the 145% threshold for a higher-rate taxpayer, and the lender would either reduce the loan or decline. That's why landlords on this kind of structure tend to negotiate hard on rent and resist letting agents who'd accept the first tenant willing to pay £50 less than the asking rent.

Switch to a capital repayment mortgage at the same rate and term and the monthly payment jumps to roughly £1,151. That's nearly £300 more out of pocket each month, but you'd own about £4,200 more equity at the end of year one. Whether that trade is worth it depends on your other deployable capital and your view on house-price growth — neither of which a calculator can answer.

Factors that affect what you can borrow

Rental valuation, not asking rent

Lenders use the surveyor's rental valuation, not the rent you advertise. If you put the property on at £1,400 but the surveyor comes back at £1,250, the lender works the ICR off £1,250. The gap between "what I think it'll let for" and "what the lender will accept" catches a lot of new landlords on the first deal.

Your tax band

Higher-rate and additional-rate taxpayers face the 145% ICR threshold; basic-rate taxpayers and limited companies face 125%. The same property can be financeable inside a limited company but not in a personal name, simply because the threshold is lower. This is the main reason new BTL purchases since the Section 24 phase-in have skewed heavily toward limited-company structures.

Stress rate

The stress rate of 5.5% (or product rate plus 2%) means the affordability calculation always runs at a higher rate than your actual rate. In a low-rate environment that gap is wide and shrinks the loan; when rates climb, the gap narrows but the headline ICR also tightens because real interest costs eat more of the rent.

Property type

Standard houses and flats are the easy cases. HMOs (houses in multiple occupation), short-let or holiday operations, ex-local- authority flats, flats above commercial premises, studio flats below 30 m², and properties caught up in cladding remediation all face tighter LTV caps and a smaller pool of lenders. Specialist BTL lenders will go where the high street won't, but at higher rates.

Personal income

Most BTL lenders want to see at least £25,000 of personal income outside the rental income. The logic is that if the property goes through a void, you can still service the mortgage. A handful of no-minimum-income specialist lenders exist for portfolio landlords, but the rates reflect the wider lender risk.

How to improve your BTL position

If the calculator is throwing back numbers that don't pass the lender's tests, the levers worth pulling are:

• Bigger deposit. Drop the LTV from 75% to 65% and you cut the absolute interest cost by about 13%. Most lenders also drop the rate at lower LTV bands, so the saving compounds.
• Longer term. A 30-year term doesn't change the interest-only payment — interest-only is term-independent — but it does cut the capital-repayment payment by easing the amortisation schedule. Worth running the BTL mortgage calculator at different terms to see the effect.
• Buy in higher-yielding areas. If your current target area returns 4.5% gross, an area returning 7% lets you borrow more against the same monthly rent. The trade-off is usually capital growth — the highest-yielding areas often see slower price appreciation, and tenant turnover can be higher.
• Limited-company structure. A Ltd Co. faces the 125% ICR threshold, not 145%. Mortgage interest also stays a deductible expense, since Section 24 doesn't apply to companies. The downside is corporate accountancy costs, lender fees that tend to run slightly higher, and the eventual question of how to extract profits.
• Stress-test before you offer. Run the calculator at 1% above the current product rate and at 90% of the surveyor's likely rent. If the deal still works, you've built a margin of safety into the numbers.

Common mistakes

Forgetting the 5% stamp duty surcharge

In England and Northern Ireland, buying an additional residential property triggers an extra 5% on top of the standard SDLT bands. On a £250,000 property, that's £12,500 of stamp duty before the standard bands even start to bite. It's the single biggest line item many first-time landlords miss when they sketch a deal on the back of an envelope. HMRC publishes the current bands at gov.uk/stamp-duty-land-tax.

Confusing gross yield with cash flow

A 6% gross yield doesn't mean 6% cash in your pocket. After agent fees (8–12% of rent), insurance, repairs, voids (budget 4–6 weeks a year), and mortgage interest, net yield is often half the gross figure. After income tax, you might keep a third. The calculator shows the gross — you have to do the haircut from there.

Ignoring the Section 24 effect

Since the 2020 tax year, individual landlords no longer deduct mortgage interest as an expense. They receive a 20% basic-rate tax credit instead. For a higher-rate taxpayer, this can push paper income into the higher band even though the cash position hasn't changed. Some landlords end up worse off net than they were pre-reform, and only realise it when the next year's tax return lands.

Chasing the highest yield

Yields above 9% are usually a signal, not a feature. Either the area has structurally weak demand, the property has hidden problems (cladding, leasehold issues, EPC under E), or the tenant pool is high-risk. Solid 5–7% gross yields with reliable tenants beat nominally higher yields with three months of voids and a court appearance.

Skipping the ICR check before offering

Putting in an offer before checking the ICR is the most common own-goal. A property that "stacks up" at the asking price might fail at the lender's stress rate, leading to a downsized loan, a rushed deposit top-up, or — worst case — a fall-through that costs the survey and legal fees you'd already paid for.

When to seek professional advice

The BTL mortgage calculator handles the maths. It can't tell you:

• Whether to buy in your personal name or through a limited company — that's a tax adviser's call, and the right answer depends on your other income, your existing portfolio, and your succession plans.
• Which lender will actually approve your case — a whole-of-market BTL broker has real-time data on lender criteria, including the soft factors (lender appetite for HMOs, recent applicant rejections, current turnaround times) that public-facing rate tables don't reveal.
• Whether the underlying investment is sensible — a calculator can't price local rental demand, capital-growth prospects, or the operational drag of managing a property at distance.

The calculator is a screen. If a property doesn't pass the screen, don't buy it. If it does, the next step is a broker conversation, not an offer.