Why Your Buy-to-Let Mortgage Offer Is Smaller Than Expected: ICR, Rent Stress Tests and Section 24 Explained

Hi Angela. We are unpacking "Why Your Buy-to-Let Mortgage Offer Is Smaller Than Expected: ICR, Rent Stress Tests and Section 24 Explained" today and tying it back to the wider Cost Saver ecosystem, including tools like UK Buy-to-Let Mortgage Engine · ICR + Section 24, so you can turn insights into action quickly.

Exactly. Treat this chat as a smart estimate only, not as professional financial guidance. Always check important details with official sources or a qualified expert before making any big decisions.

Thanks Angela, always good to be back. And yeah — you've basically described it perfectly. That moment when the offer comes through and it's just... lower. Sometimes significantly lower. And people immediately assume something's gone wrong.

Usually not, no. That's the — well, that's the thing that surprises people. It's rarely your credit score. It's rarely the property being down-valued. It's almost always the rent. Buy-to-let lenders size loans on what the rent can cover, not just what the property is worth. Which is, um, a fundamentally different way of thinking about it compared to a normal residential mortgage.

Exactly. A residential mortgage leans heavily on your salary. A buy-to-let mortgage leans mainly on the rent. Lenders still look at you as a borrower — they're not ignoring you completely — but the rent is doing most of the heavy lifting.

That's exactly the assumption people make. And it's not wrong exactly, it's just... incomplete. Because lenders are running several tests at the same time, and the one that produces the lowest number is the one that wins. So you might sail through the loan-to-value test, and then a completely separate test based on the rent just quietly trims it back.

Three main ones. You've got the loan-to-value — which is what most people are familiar with, often capped at 75% for standard buy-to-let products. Then you've got something called the interest coverage ratio, which everyone shortens to ICR. And then there's a stressed interest rate. And those last two, um, work together.

Yeah, of course. So the ICR is basically asking: does the rent cover the mortgage interest by a comfortable enough margin? And it's expressed as a percentage. So if your annual rent divided by the annual mortgage interest gives you, say, 150%, that means the rent is one and a half times the interest payment. Does that make sense?

Because things go wrong. Void periods, repairs, rate changes — they want to know there's headroom. So they set a minimum threshold. And this is where it gets really important — different lenders set different thresholds, and different thresholds apply to different types of borrower.

Mainly your tax band. So the common ranges you'll see — and these are typical ranges, not fixed rules, lenders change them — are 125% for limited company applications and some basic-rate individual landlords. Then 140% to 145% for higher-rate or additional-rate taxpayers borrowing personally. And 150% for some products or more complex cases. Have I explained that clearly?

It's a really good question. And it — it comes down to Section 24. Which is, honestly, one of those things that hit a lot of landlords harder than they expected.

So Section 24 is the tax rule that changed how individual landlords can claim mortgage interest. Since April 2020, if you're an individual landlord of residential property, you can no longer deduct your finance costs — so your mortgage interest — from your rental income when you're working out your taxable profit. Instead you get a basic-rate tax credit of 20% on those costs.

Exactly. For a basic-rate taxpayer the effect is often modest. But for a higher-rate taxpayer it can raise the tax bill substantially. Let me give you a simplified illustration — I'm going to ignore the personal allowance to keep it clear. Say rent is £20,400 a year, running costs are £2,000, and mortgage interest is £12,000.

Under the old rules, taxable profit would be £6,400 — because you'd deduct the interest. Under Section 24, taxable profit is £18,400, because the interest isn't deducted anymore. A higher-rate taxpayer then pays 40% on that £18,400, which is £7,360, then subtracts a 20% credit on the £12,000 of interest — that's £2,400. Net tax comes out at about £4,960.

About £2,560. So that's roughly £2,400 more tax every year on a single property. Over a decade, that's £24,000.

It really isn't. And lenders know this. They know Section 24 reduces the after-tax cash left over for higher-rate landlords. So they apply a higher ICR — that 145% — to higher-rate individual borrowers. They want a bigger rent cushion because those landlords have less cash left after tax. It's kind of... logical, once you see it. But it's not obvious from the outside.

It really does. And on the same property, same rent — a basic-rate taxpayer might face a 125% ICR where a higher-rate taxpayer faces 145%. And that difference is around £41,000 in borrowing capacity.

Yeah, so — let's take a property worth £400,000 that rents for £1,700 a month, so £20,400 a year. A 75% LTV gives you a £300,000 limit. We'll assume a stressed rate of 5.5% for illustration. At a 125% ICR, the maximum loan based on rent comes out at about £296,700 — just under the LTV limit. Close enough. That works.

That's fine. But at 145% ICR — so now you're a higher-rate taxpayer — the maximum loan falls to about £255,800. And at 150% with a 6% stressed rate, it drops to about £226,700.

More than £70,000, yeah. Nothing changed about the property. Nothing changed about the rent. Only the lender's assumptions. That's — I mean, that's exactly why borrowers feel blindsided.

Often not, no. They use a higher notional rate — a stressed rate — to check that the rent would still cover interest if rates rose. So even if your actual rate is lower, they're testing against a tougher number. Which is why a higher stressed rate shrinks your maximum loan.