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

Summary

A buy-to-let mortgage smaller than expected usually comes down to the rent-based limit. Lenders apply several limits at once: loan-to-value, an interest coverage ratio (ICR) and a stressed interest rate. The lowest result sets your maximum loan, and for many landlords that is the rent-based test rather than the deposit. Section 24 tax rules and your choice between personal and company ownership affect which ICR the lender applies, which can move your loan by tens of thousands of pounds.

Why Is My Buy-to-Let Mortgage Smaller Than Expected?

You find a property worth £400,000, you have a 25% deposit ready, and you assume a £300,000 mortgage is a formality. Then the lender comes back with an offer of £255,000, and you are left wondering where £45,000 went. You now have to find that money from savings, renegotiate the price or walk away, often after paying a valuation fee you will not get back. It is a frustrating moment, and it catches out first-time and experienced landlords alike.

The answer is rarely your credit score or the property's value. It is usually the rent. Buy-to-let lenders size loans on what the rent can cover, not just on what the property is worth. If you want to test your own numbers as you read, try the UK Buy-to-Let Mortgage Engine · ICR + Section 24. It takes about five minutes to enter a property's price, rent and your tax band. If you are also concerned about how lenders assess your wider finances, our guide on preparing for a mortgage stress test is a useful companion.

How Lenders Actually Size a Buy-to-Let Loan

The three limits working together

A residential mortgage leans heavily on your salary. A buy-to-let mortgage leans mainly on the property's rent, although lenders still look at you as a borrower. That is why two properties with the same price can support very different loans.

Lenders usually apply three separate constraints:

• Loan-to-value (LTV). This is the share of the property value the lender will finance, often capped at 75% for standard products.
• Interest coverage ratio (ICR). This checks whether the rent is high enough to cover the interest on the loan by a set margin.
• Stressed interest rate. This is a higher notional rate that lenders use in the ICR calculation instead of your real rate.

Each test produces its own maximum loan. They do not add together. Instead, they compete with each other.

Why the lowest number always wins

The binding limit is the smallest figure the three tests produce. A property might support £300,000 on LTV but only £255,000 on rent. In that case the lender offers around £255,000, and your deposit has to rise to fill the gap.

A simple way to picture the process is as a series of gates:

1. The lender works out the maximum loan from the LTV limit.
2. The lender takes the rent it will accept for the property.
3. The lender applies its stressed interest rate and ICR to that rent.
4. The lender compares the results and uses the lowest one.
5. The lender then checks your personal circumstances, such as experience and existing portfolio.

This catches people out because the LTV figure feels like the headline number. In reality, it is often just the ceiling, and the rent test is where many applications are quietly trimmed.

Understanding the Interest Coverage Ratio

What ICR measures

The ICR shows how many times the rent covers the lender's assumed mortgage interest. It is calculated by dividing the annual qualifying rent by the annual stressed interest, and the result is shown as a percentage.

Take a property earning £18,000 a year in rent. If the lender calculates stressed annual interest of £12,000, the ICR is 150%. That passes a 125% requirement comfortably, but it may fail with a lender asking for more on a different product.

There is no single ICR for every UK buy-to-let mortgage. Common thresholds you will see include:

• 125%. This is often used for limited company applications and some basic-rate individual landlords.
• 140% to 145%. This is frequently applied to higher-rate or additional-rate taxpayers borrowing personally.
• 150%. Some lenders use this for certain products or for more complex cases.

Treat these as typical market ranges rather than fixed rules. Each lender sets its own criteria, and they change with market conditions.

The stressed interest rate

The ICR is not usually calculated on the rate you will actually pay. Lenders often use a higher notional rate to check that the rent would still cover interest if rates rose. A higher stressed rate or a higher ICR both shrink your maximum loan.

On some longer fixed-rate products, however, lenders may use the actual pay rate instead, which can allow a larger loan. A five-year fix can be assessed more generously than a two-year fix or a tracker, because the rate is locked in for longer. The loan you can borrow is therefore tied to the product you choose, not just the property.

The simple formula

A rough way to estimate the maximum loan under the ICR test is to divide the annual qualifying rent by the ICR multiplied by the stressed interest rate. Written out, it looks like this: maximum loan = annual rent ÷ (ICR × stressed rate).

This is an approximation. Real lenders may use different rent assumptions, interest rate floors, product rules and tax treatments. Even so, it gives you a good feel for how sensitive your borrowing is to each input.

A Worked Example With Real Numbers

Comparing three scenarios

Let us take a property worth £400,000 that rents for £1,700 a month, or £20,400 a year. A 75% LTV gives a £300,000 limit. We will assume a stressed rate of 5.5% for illustration.

Here is how the rent test changes the answer at different ICR levels:

1. At a 125% ICR, the maximum loan is about £296,700, which is just under the LTV limit.
2. At a 145% ICR, the maximum loan falls to about £255,800.
3. At a 150% ICR with a 6% stressed rate, the maximum loan drops to about £226,700.

The same property and the same rent produce three very different loans, with a gap of more than £70,000 between the best and worst case. Only the lender's assumptions changed. This is exactly why borrowers feel blindsided.

How one landlord closed the gap

Consider an illustrative case based on the figures above. Priya, a higher-rate taxpayer in Leeds, applied for a two-year fix on the £400,000 property and was offered about £255,800 at a 145% ICR. Rather than find another £44,000 in cash, she asked her broker for products assessed at the pay rate.

A five-year fix at 4.5%, still tested at 145%, changed the outcome considerably:

• Original offer. About £255,800 on a two-year fix assessed at a 5.5% stressed rate.
• Revised calculation. The rent now supported about £312,600, so the loan was capped by the £300,000 LTV limit instead.
• Result. Priya borrowed £300,000, closing a £44,200 shortfall without adding to her deposit.

The trade-off was a longer commitment and early repayment charges if she sold within five years. For a long-term hold, that was a price she was comfortable paying.

When rent is the real problem

Now consider a property with lower rent. Say it is worth £300,000 and rents for £1,200 a month, or £14,400 a year. A 75% LTV allows £225,000.

At a 125% ICR and a 5.5% stressed rate, the rent supports roughly £209,500. At 145%, it supports only about £180,600. In both cases, the rent limits the loan before the LTV does, and the second scenario leaves a shortfall of more than £44,000.

This reveals a pattern worth remembering. Lower-yielding properties hit the ICR ceiling first, while higher-yielding properties are more likely to be limited by LTV.

What lenders count as rent

The rent the lender uses may not be the rent you expect. If the lender's figure is lower than yours, your loan shrinks with it. Lenders typically rely on one of the following:

• A valuer's rental assessment. This is the most common source and is often conservative.
• A letting agent's estimate. Some lenders accept a written letter from a local agent.
• An existing tenancy agreement. This may be used if the property is already let.

For a sense of whether your rental assumptions are realistic in the local market, our guide to rent affordability by postcode and income is worth a read. Realistic rent is the foundation of every number in this process.

How Section 24 Changes the Picture

What Section 24 actually does

Section 24 is the tax rule that limits how individual landlords can claim mortgage interest. Since April 2020, individual landlords of residential property can no longer deduct finance costs from rental income when working out taxable profit. Instead, they receive a basic-rate tax credit of 20% on those costs.

For a basic-rate taxpayer, the effect is often modest. For a higher-rate or additional-rate taxpayer, it can raise the tax bill substantially. Rental profit can also push some landlords into a higher tax band.

Here is a simplified illustration that ignores the personal allowance and other income to keep things clear. Suppose rent is £20,400, running costs are £2,000 and mortgage interest is £12,000:

• Under the old rules, taxable profit would be £6,400.
• Under Section 24, taxable profit is £18,400, because the interest is not deducted.
• A higher-rate taxpayer would pay 40% on £18,400, which is £7,360, then subtract a 20% credit on £12,000, which is £2,400.
• The net tax is about £4,960, compared with about £2,560 under the old approach.

That is roughly £2,400 more tax every year on a single property, or £24,000 over a decade. The government explains how the calculation works, with case studies, in its official guidance listed in the Sources section.

Why lenders care about your tax band

Lenders know Section 24 reduces the after-tax cash left over for higher-rate landlords. That is why many apply a higher ICR, such as 145%, to higher-rate individual borrowers. They want a bigger rent cushion for people who will pay more tax on that rent.

As a result, your tax position can influence your borrowing power. A basic-rate taxpayer might face a 125% ICR on the same property where a higher-rate taxpayer faces 145%, which in our earlier example is a difference of around £41,000. Remember too that adding rental profit to your salary may push you across a tax threshold, and lenders may consider this when deciding which ICR applies.

Personal Name or Limited Company?

How company ownership changes the rules

Limited companies are not caught by Section 24 in the same way. A company can generally treat finance costs as a business expense, and it pays corporation tax on profit rather than income tax. This is one reason many landlords consider buying through a company.

Lenders often apply a lower ICR to company applications, commonly around 125%. That can allow a larger loan on the same rent, and for a higher-rate taxpayer the difference can be significant. However, borrowing through a company is not automatically cheaper or easier. Company mortgage rates and fees can differ, and lenders often ask directors for personal guarantees.

Weighing the trade-offs

Structure should never be chosen on borrowing power alone. Moving existing properties into a company can trigger costs, and the tax picture is complex. Consider these points when comparing personal and company ownership:

• Borrowing capacity. Company applications can attract a lower ICR, which may support a bigger loan.
• Tax treatment. Companies deduct finance costs, while individuals face Section 24 restrictions.
• Extracting profit. Taking money out of a company through salary or dividends can create further tax.
• Transfer costs. Moving an existing property can mean stamp duty and capital gains tax.
• Mortgage costs. Rates, fees and lender choice may differ for company products.

Costs hidden beneath the headline numbers can wipe out the benefit of a bigger loan. Our guide to landlord hidden costs and rental yield walks through what to budget for.

Practical Ways to Increase What You Can Borrow

Levers you can pull

If the ICR is holding your loan back, you have more options than you might think. Some change the numbers directly, others change how the lender views them, and a few cost nothing at all. Here are the most common approaches:

1. Put in a larger deposit, so the loan you need sits within the rent-based limit.
2. Choose a longer fixed rate where the lender assesses at the pay rate.
3. Look at lenders with a lower ICR for your tax band or structure.
4. Target properties with stronger rent relative to their price.
5. Improve the property in ways that lift the achievable rent and the valuer's assessment.

You do not need to use all of these. As Priya's example shows, a single change of product is often enough to close the gap.

Choosing the right property from the start

The cheapest fix is to avoid the problem before you make an offer. Check the likely rent and run the ICR sums before you become attached to a property. A quick calculation takes around fifteen minutes and can save weeks of wasted effort and several hundred pounds in valuation fees.

A short checklist can keep you on track:

• Estimate the realistic monthly rent using three or four comparable local listings.
• Check the property's gross yield against your target.
• Run the ICR at 125%, 145% and 150% to see your range.
• Compare your result with the LTV-based loan.
• Add up the deposit you would need for any shortfall.

Questions to ask a broker

A broker who specialises in buy-to-let can help you match a property to a lender early, and their knowledge of lender criteria often saves more than any fee they charge. A first call usually takes around twenty minutes. These questions will help you get the most from it:

1. Which ICR will lenders apply to me, given my tax band and ownership structure?
2. Which products assess affordability at the pay rate rather than a stressed rate?
3. What rent figure are lenders likely to accept for this property?
4. Do you charge a fee, and if so, how much and when is it payable?
5. Will any agreement in principle involve a soft or a hard credit search?

Common worries, answered

Many landlords delay getting advice because of a few understandable concerns. Most of them are easier to resolve than people expect:

• Will checking my numbers hurt my credit score? Running your own figures or using a calculator has no effect on your credit file, and many lenders use a soft search for an agreement in principle. Ask before anything is submitted.
• Are there hidden fees? Some brokers are paid only by the lender, while others charge a flat or percentage fee. A reputable broker will tell you in writing before you proceed.
• Am I locked in if I choose a longer fix? You can usually leave early, but early repayment charges often apply. Check the charge schedule so you know the cost of changing your mind.

Common mistakes to avoid

Many borrowing disappointments come from the same few errors. Spotting them early keeps your plans realistic:

• Assuming the LTV figure is the loan you will receive.
• Using your own rent estimate instead of the valuer's figure.
• Ignoring how your tax band affects the ICR applied.
• Applying to a lender before checking its criteria.
• Forgetting that product choice changes the stress rate used.
• Budgeting for the mortgage but not for fees, voids and repairs.

Each of these is avoidable with a little preparation. Most are easy to fix before an application, and expensive to fix afterwards.

Dates worth keeping in mind

Timing matters more than many landlords realise, particularly when tax and structure are involved. These dates are worth noting now rather than later:

• 5 April each year. The end of the tax year is the natural point to review your tax band and ownership plans with an accountant.
• April 2027. Making Tax Digital for Income Tax is due to extend to landlords and sole traders with qualifying income above £30,000, adding new digital record-keeping requirements.
• Your current fixed-rate end date. Lenders often let you secure a new deal several months in advance, so start comparing products well before your rate expires.

If any of these fall within the next few months, it is worth running your numbers this week rather than waiting.

A step-by-step approach before you apply

Putting everything together, a sensible order of work looks like this, and most people can complete it in under an hour:

1. Work out your tax band and whether you will borrow personally or through a company.
2. Estimate realistic rent for the property from local listings.
3. Calculate the maximum loan at the ICR and stress rate that are likely to apply.
4. Compare that figure with the LTV limit and take the lower one.
5. Decide how much deposit you need and whether the numbers still stack up.

Following these steps in order stops you building a plan on a loan figure that does not exist. It also means that when you do speak to a broker or lender, you arrive with realistic expectations and sensible questions.

Conclusion

A buy-to-let mortgage that is smaller than expected is rarely a sign that something has gone wrong. It is usually the interest coverage ratio, the stressed rate and your tax position doing exactly what they are designed to do. The lowest result from the lender's tests sets your limit, and the rent is often the weakest link.

The good news is that you can plan around it. Check the rent early, understand which ICR is likely to apply to you, compare products that use different stress rates, and think carefully about ownership structure with proper advice. Running the numbers will not affect your credit score, a good broker will be upfront about any fees, and you can weigh early repayment charges before committing to a longer fix. To see how your own figures play out, try the UK Buy-to-Let Mortgage Engine · ICR + Section 24. For the bigger picture on affordability and costs, read our guides on mortgage stress test preparation, hidden landlord costs and rent affordability by postcode.

This article is general information, not financial or tax advice. Lender criteria and tax rules change, so always check current terms and speak to a qualified adviser before you commit.

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Sources

• GOV.UK: Changes to tax relief for residential landlords, how it's worked out including case studies
• GOV.UK: Renting out a property