What is an Interest Only Mortgage?

At Towerstone, we provide specialist property accountancy services for homeowners, landlords, and property investors. This article explains what you need to know to make informed decisions around this topic.

Interest only mortgages are often misunderstood. Some people see them as a clever way to keep monthly payments low. Others assume they are risky or no longer available. The truth is more balanced. An interest only mortgage is a legitimate mortgage product in the UK, but it works very differently from a standard repayment mortgage and it is only suitable in specific circumstances.

Understanding how an interest only mortgage works, who it is designed for, and what the long term implications are is essential before considering one. In this guide I will explain clearly and practically what an interest only mortgage is, how it differs from repayment mortgages, why people use them, how lenders assess them, and the main risks and benefits. This is written in UK English and reflects how the market actually operates today.

The simple definition of an interest only mortgage

An interest only mortgage is a type of mortgage where your monthly payments cover only the interest on the loan, not the capital itself.

This means that throughout the mortgage term, the amount you owe does not reduce. At the end of the term, the full original loan amount is still outstanding and must be repaid in one lump sum.

This is the key difference compared to a repayment mortgage, where each monthly payment gradually reduces both the interest and the capital until the loan is fully repaid.

How interest only compares to a repayment mortgage

With a repayment mortgage, your monthly payment is higher, but by the end of the term the mortgage balance should be zero.

With an interest only mortgage, your monthly payment is lower, but the debt remains unchanged and you need a separate plan to repay it later.

For example, if you borrow £200,000 on a repayment mortgage over 25 years, you pay interest and capital each month and aim to own the property outright at the end.

If you borrow the same £200,000 on an interest only mortgage, you pay only the interest each month and still owe £200,000 at the end of the term.

Why monthly payments are lower

Interest only payments are lower because you are not paying back the loan itself.

You are effectively renting the money from the lender and agreeing to repay it later.

This lower monthly cost is the main attraction, particularly for borrowers who want to manage cash flow or have irregular income.

However, lower monthly payments do not mean the mortgage is cheaper overall. In many cases, total interest paid over time can be higher.

What happens at the end of an interest only mortgage

At the end of the mortgage term, the lender expects full repayment of the loan.

If you cannot repay it, you may be forced to sell the property or refinance, assuming that is possible.

This is why lenders are very cautious about interest only mortgages. They want to know exactly how you plan to repay the capital.

Having no credible repayment strategy is one of the biggest risks with this type of mortgage.

Common ways people plan to repay the capital

When applying for an interest only mortgage, you usually need to declare a repayment strategy.

Common strategies include selling the property, downsizing to a cheaper home, using savings or investments, relying on pension lump sums, or repaying the loan with proceeds from another asset.

In the past, some people relied on endowment policies. These are now far less common due to past performance issues.

Lenders will assess whether your chosen strategy is realistic based on your circumstances.

Selling the property as a repayment strategy

One of the most common strategies is to sell the property at the end of the term and use the proceeds to repay the mortgage.

This can work where there is sufficient equity and where selling fits with long term plans.

However, it relies on property values remaining high enough and on your willingness to sell when the term ends.

If the property market is weak or your circumstances change, this strategy may be less reliable than expected.

Using savings or investments

Some borrowers plan to repay the mortgage using savings or investments built up over time.

This can include ISAs, shares, bonds, or other investment vehicles.

The risk here is investment performance. Markets fluctuate, and there is no guarantee that investments will be worth what you expect when the mortgage term ends.

Lenders may ask for evidence that investments already exist rather than relying purely on future growth.

Pension lump sums as a strategy

Using a pension lump sum to repay an interest only mortgage is common among older borrowers.

This can be viable where pension savings are substantial and retirement plans are clear.

However, this approach requires careful planning. Pension rules can change, tax may apply, and using a large lump sum reduces retirement income.

Lenders assess this strategy cautiously, especially where pension savings are not yet built up.

Interest only mortgages for residential buyers

Interest only mortgages are available for residential properties, but they are much harder to obtain than they once were.

Since the financial crisis, lenders have tightened criteria significantly.

Most lenders now restrict interest only mortgages to higher earners, borrowers with significant equity, or those with clear and credible repayment plans.

They are no longer a default option for first time buyers.

Typical eligibility criteria for residential interest only mortgages

While criteria vary by lender, common requirements include a higher minimum income, a lower loan to value ratio, often 50 to 75 percent, a strong credit history, and a proven repayment strategy.

Some lenders also restrict interest only mortgages to certain professions or financial profiles.

Because of these restrictions, many borrowers who want interest only mortgages find they are not eligible.

Interest only mortgages for buy to let properties

Interest only mortgages are much more common in the buy to let market.

Many landlords use interest only mortgages to maximise cash flow and manage rental yields.

In this context, the repayment strategy is often the sale of the property or refinancing in the future.

Buy to let lending is assessed differently, with a focus on rental income rather than personal income.

However, interest only buy to let mortgages still carry risks, particularly if property values fall or refinancing becomes difficult.

Why landlords favour interest only mortgages

Landlords often favour interest only mortgages because they keep monthly costs low, which can improve rental cash flow.

This can make a property viable where a repayment mortgage would result in a monthly loss.

It also allows landlords to direct surplus cash into other investments or portfolio growth.

That said, it also means the debt remains outstanding and relies on long term property value growth.

The risks specific to interest only mortgages

The biggest risk is not having enough money to repay the capital at the end of the term.

Other risks include relying too heavily on rising property values, changes in lending criteria that make refinancing harder, rising interest rates that increase monthly payments, and the psychological risk of not reducing debt over time.

These risks are manageable, but only if they are understood and planned for properly.

Interest rate changes and affordability

Interest only mortgages are particularly sensitive to interest rate changes.

Because you are paying only interest, any rate increase affects the entire loan balance.

On a repayment mortgage, part of your payment goes towards reducing the balance, which gradually reduces exposure.

With interest only, the full balance remains exposed for the entire term.

This makes budgeting for rate rises especially important.

Interest only mortgages and negative equity

Interest only mortgages can increase the risk of negative equity if property values fall.

Because the loan balance does not reduce, any drop in property value directly affects your equity position.

If you need to sell or refinance during a downturn, this can create problems.

Repayment mortgages reduce this risk over time by paying down capital.

Why lenders are cautious about interest only mortgages

Lenders are cautious because interest only mortgages place more responsibility on the borrower.

Past experience showed that many borrowers did not adequately plan for repayment, leading to difficulties at the end of the term.

As a result, lenders now require stronger evidence, lower risk profiles, and more oversight.

This caution protects both lenders and borrowers, but it also limits availability.

Mixed mortgages as a compromise

Some borrowers choose a part repayment, part interest only mortgage.

This means part of the loan is repaid over time and part remains interest only.

This can reduce monthly payments compared to full repayment while still reducing overall debt.

It can be a sensible compromise for borrowers who want flexibility without taking on full interest only risk.

Interest only mortgages and retirement

Interest only mortgages are often used by older borrowers, but they need careful alignment with retirement plans.

Having a large mortgage balance at retirement can be stressful if income reduces.

Lenders often impose age limits or require repayment before or shortly after retirement age.

This is an area where assumptions about future income need to be tested carefully.

Can you switch from interest only to repayment

In many cases, yes.

Borrowers can often switch part or all of an interest only mortgage to repayment, subject to lender approval and affordability checks.

This flexibility can be useful if circumstances improve or priorities change.

However, switching later may increase monthly payments significantly, particularly if the remaining term is short.

What happens if you reach the end of the term without repayment

If you reach the end of an interest only mortgage term and cannot repay the capital, the lender will expect action.

Options may include selling the property, refinancing if possible, or negotiating a short extension in limited circumstances.

Lenders are not obliged to extend the term, and failure to resolve the situation can ultimately lead to repossession.

This is why relying on hope rather than planning is dangerous.

Interest only mortgages and total cost

While monthly payments are lower, the total cost of an interest only mortgage can be higher over time.

Because the capital is not reduced, interest is charged on the full balance for the entire term.

This means you may pay more interest overall compared to a repayment mortgage, even if rates are the same.

This trade off needs to be considered honestly.

When an interest only mortgage can make sense

Interest only mortgages can make sense where income is high but irregular, where significant assets exist to repay the loan, where short term cash flow flexibility is needed, or where the borrower has a clear long term plan.

They can also be appropriate for experienced landlords or high net worth individuals.

They are less suitable where affordability is tight or where there is no credible repayment plan.

When interest only mortgages are usually a bad idea

Interest only mortgages are usually a bad idea where the borrower is relying on future salary increases, speculative investment growth, or vague plans to repay.

They are also risky for first time buyers without assets, or for anyone uncomfortable with long term debt.

Using interest only purely to stretch affordability is particularly dangerous.

The role of professional advice

Interest only mortgages should always be discussed with a qualified mortgage adviser.

An adviser can assess eligibility, stress test interest rate changes, and help evaluate repayment strategies realistically.

This is not an area where generic advice or online calculators are sufficient.

Common myths about interest only mortgages

There are several misconceptions worth clearing up.

Interest only mortgages are not banned.
They are not automatically cheaper.
They are not only for the wealthy.
They are not inherently bad.

They are specialised products that require careful use.

A simple way to think about interest only mortgages

A useful way to think about an interest only mortgage is that you are separating the cost of borrowing from the act of repaying.

You pay for the use of the money now and promise to repay it later by some other means.

If you are confident in that later means and comfortable with the risk, it can work.

If not, it can cause serious problems.

Final thoughts from real world experience

So, what is an interest only mortgage.

It is a mortgage where you pay only the interest each month and repay the full loan at the end of the term. It offers lower monthly payments but carries higher long term responsibility.

In my experience, interest only mortgages work best for people who understand them deeply, plan carefully, and have strong financial buffers. They cause problems when they are used as a shortcut to affordability rather than a strategic choice.

An interest only mortgage is not about paying less, it is about paying differently. Whether that difference works for you depends entirely on your planning, discipline, and long term goals.

You may also find what is a variable rate mortgage and what to do with house deeds after mortgage paid off useful. For wider guidance, explore our mortgage guidance hub.