Interest-Only Mortgages Explained UK — How They Work.

What is an interest-only mortgage?

With an interest-only mortgage, your monthly payment covers the interest your lender charges on the outstanding loan and nothing more. The capital balance — the amount you originally borrowed — stays the same throughout the entire mortgage term. At the end of the term, you must repay the full capital in one lump sum.

For example, if you borrow £300,000 on a 25-year interest-only mortgage at 4.5%, your monthly payment would be around £1,125. After 25 years you would still owe the full £300,000. On a repayment mortgage at the same rate and term, the monthly payment would be roughly £1,667, but the loan would be fully cleared by the end of the term.

Because the capital is never reduced, you need a separate plan — known as a repayment vehicle or repayment strategy — to accumulate enough money to clear the debt when the mortgage matures. Lenders will ask for evidence of this plan at the application stage and may review it periodically during the term.

Who qualifies for an interest-only mortgage?

Residential interest-only mortgages are no longer widely available to all borrowers. Following the Mortgage Market Review (MMR) in 2014, the FCA tightened the rules around affordability and repayment strategies, and most high-street lenders now reserve interest-only deals for higher-income applicants with substantial deposits and a demonstrable repayment plan.

Typical criteria include a minimum household income of £75,000 to £100,000 (depending on the lender), a maximum loan-to-value of 75% (meaning you need at least a 25% deposit or equivalent equity), and a credible repayment strategy such as investments, savings, or the planned sale of a property. Some lenders also set a minimum loan size, often £150,000 or more.

The rules are different for buy-to-let mortgages. Most BTL lenders offer interest-only as standard because the property itself is expected to be the repayment vehicle, either through sale at the end of the term or through portfolio refinancing. If you are a landlord, our guide to buy-to-let mortgages explains the specific criteria.

Interest-only vs repayment: how they compare

The fundamental difference between these two mortgage types comes down to what your monthly payment actually covers. With a repayment mortgage, each payment includes both interest and a portion of the capital, so the loan is gradually paid off over the term. With interest-only, your payment covers just the interest, and the capital remains untouched until the end.

On a £250,000 mortgage at 4.5% over 25 years, the repayment option costs around £1,389 per month and results in zero debt at the end. The interest-only option costs around £938 per month but leaves you still owing the full £250,000 after 25 years. The total interest paid on the interest-only mortgage is also significantly higher because the balance never decreases.

Repayment strategies lenders accept

When you apply for an interest-only mortgage, your lender will want to see a realistic plan for how you intend to repay the capital at the end of the term. The days of vague promises about future windfalls are long gone — lenders now require specific, documented evidence of your chosen strategy. Some will accept a combination of approaches, which can strengthen your application.

The most commonly accepted repayment strategies include ISAs and investment portfolios, sale of the mortgaged property (mainly for buy-to-let), sale of another property, pension lump sums, endowment policies, and regular savings plans. Each strategy has its own advantages and limitations, and the right choice depends on your financial circumstances, risk tolerance, and investment timeline.

Advantages and risks of interest-only mortgages

Interest-only mortgages are neither inherently good nor bad — their suitability depends entirely on your financial situation, discipline, and long-term plans. The lower monthly payments can be a genuine advantage for certain borrowers, particularly those with irregular income or those who can invest the difference productively. However, the risks are significant if your repayment strategy falls short.

The main advantage is cash flow flexibility. The difference between an interest-only and a repayment mortgage payment on a typical loan can be several hundred pounds a month, which you can redirect into higher-returning investments or use to manage periods of variable income. For buy-to-let investors, interest-only payments maximise rental yield and are tax-deductible as a business expense.

Switching from interest-only to repayment

Many borrowers who started on an interest-only mortgage eventually decide to switch to a repayment basis — either because their circumstances have changed, their repayment strategy has not performed as expected, or they simply want the security of knowing the mortgage will be fully paid off by the end of the term. The good news is that switching is usually straightforward, though your monthly payments will increase.

You can switch by asking your current lender to change your mortgage from interest-only to repayment (this is often a simple administrative change), or you can remortgage to a new lender on a repayment basis. If you remortgage, you have the opportunity to shop the whole market for a better rate at the same time. A broker can compare both options and advise on which saves you more in the long run.

The earlier you switch, the more manageable the increase in payments will be. If you have 20 years left on your term, spreading the capital repayment over that period results in a much smaller monthly increase than if you switch with only 10 years remaining. It is also worth considering whether making voluntary overpayments alongside your interest-only payments could be a stepping stone towards full repayment.