Interest-Only Mortgages Explained: How They Work in the UK.

What is an interest-only mortgage?

Every monthly payment on an interest-only mortgage goes towards the interest your lender charges on the outstanding loan. That is all it covers. The capital balance, meaning the amount you first borrowed, stays exactly where it started for the entire term. Then, when the term ends, the whole capital falls due in one lump sum.

Picture borrowing £300,000 over 25 years at 4.5%. On interest-only, the payment works out at roughly £1,125 a month, and 25 years later you would still owe every penny of that £300,000. Switch the same loan to a repayment basis, same rate and same term, and the payment climbs to about £1,667 a month. The trade-off is that the debt has vanished by the end.

Because the capital never shrinks, you need a plan running alongside the mortgage, a repayment vehicle or repayment strategy, to build up enough money to settle the debt when the mortgage matures. Lenders want to see evidence of that plan when you apply. Some will also check in on it from time to time as the years go by.

Who qualifies for an interest-only mortgage?

Walking into any lender and asking for residential interest-only is no longer an option. The Mortgage Market Review (MMR) of 2014 saw the FCA tighten the rules around affordability and repayment strategies. As a result, most high-street lenders now reserve interest-only for higher-income applicants who hold substantial deposits and can show a repayment plan.

Typical criteria include a minimum household income somewhere between £75,000 and £100,000, depending on the lender, and a maximum loan-to-value of 75% (the size of your mortgage set against the property value). That means at least a 25% deposit, or the same again in equity if you already own. You will also need a believable repayment strategy, whether that is investments, savings, or the planned sale of a property. On top of all this, some lenders apply a minimum loan size, often £150,000 or more.

Buy-to-let plays by its own rules. Most BTL lenders offer interest-only as standard, since the property is expected to be the repayment vehicle, either sold when the term ends or refinanced within a portfolio. Landlords will find the specific criteria in our guide to buy-to-let mortgages.

Interest-only vs repayment: how they compare

The real difference between these two mortgage types comes down to what your monthly payment actually buys. With a repayment mortgage, each payment covers the interest plus a slice of the capital, so the loan shrinks steadily year after year. With interest-only, the payment covers interest alone, and the capital sits untouched right up to the end.

Take a £250,000 mortgage at 4.5% over 25 years. On a repayment basis it costs around £1,389 a month and leaves you debt-free at the finish. Interest-only pulls the payment down to about £938 a month, yet you still owe the full £250,000 after 25 years. There is a sting in the tail too: you pay far more interest overall on interest-only, because the balance never comes down.

Repayment strategies lenders accept

Apply for an interest-only mortgage and your lender will expect a realistic plan for repaying the capital when the term ends. A vague hope of some future windfall no longer cuts it. Lenders now want specific, documented evidence of the strategy you have chosen, and many will accept a combination of approaches, which can actually strengthen your application.

The strategies lenders see most often include ISAs and investment portfolios, selling the mortgaged property (chiefly for buy-to-let), selling a different property, pension lump sums, endowment policies, and regular savings plans. Each one carries its own strengths and limits. The right choice depends on your finances, how much risk sits comfortably with you, and how many years you have to save.

Advantages and risks of interest-only mortgages

An interest-only mortgage is neither good nor bad on its own. Whether it suits you comes down to your finances, how disciplined you are with money, and what your long-term plans look like. Lower monthly payments really do help some borrowers, particularly those with irregular income or anyone who can put the difference to work in investments. There is a flip side, and it is a serious one: if your repayment strategy falls short, you are left exposed.

Cash flow is the headline advantage. On a typical loan, the gap between an interest-only and a repayment payment can reach several hundred pounds a month, money you can steer into higher-returning investments or fall back on when income dips. For buy-to-let investors, interest-only payments lift the rental yield and count as a tax-deductible business expense.

Switching from interest-only to repayment

Plenty of borrowers who started out on interest-only decide, further down the line, to move onto a repayment basis. For some, their circumstances have shifted. For others, the repayment strategy has not grown the way they hoped. And some simply want the certainty of a mortgage that is fully clear by the end of the term. The switch itself is usually straightforward, though your monthly payments will rise.

You can ask your current lender to change the mortgage from interest-only to repayment, which is often a simple administrative adjustment, or remortgage to a new lender on a repayment basis. Remortgaging has the added benefit of letting you shop the whole market for a sharper rate at the same time. A broker can compare both routes and tell you which one saves you more over the long run.

The sooner you switch, the gentler the jump in payments. Leave 20 years on the clock and the capital spreads across a long enough stretch to keep the monthly rise modest. Switch with only 10 years to run and that same capital is squeezed into far fewer payments. It is worth asking, too, whether voluntary overpayments alongside your interest-only payments could serve as a stepping stone towards clearing the capital.