Overpaying an Interest-Only Mortgage: Does It Work Differently?
On an interest-only mortgage your regular payment never touches the debt. Borrow £200,000 and you still owe £200,000 on the last day of the term, having paid £225,000 in interest along the way. Overpayments are the only thing that shrinks the balance, and how you handle the payment afterwards decides whether they save you thousands or tens of thousands. Here is how the maths differs from a repayment mortgage, and how to plan the endgame.
Published: 26 September 2026 at 09:00 · 7 min read
The Short Answer
Yes, it works differently: every pound you overpay comes straight off the capital, but the saving only compounds if you keep your monthly payment the same. On a £200,000 interest-only mortgage at 4.5% over 25 years, the payment is £750 a month. Overpay £200 a month and let the lender lower your interest payment each time, and you save about £33,600 and still owe £140,000 at the end. Keep paying £950 a month in total instead, and you save about £50,600 and owe just £89,400.
Pay £1,112 a month in total, the same as a repayment mortgage, and the loan clears on time with about £91,600 less interest than staying interest-only. The endgame is where interest-only borrowers need a plan: the whole balance is due on one day.
How Do Overpayments Work on an Interest-Only Mortgage?
On a repayment mortgage, each payment is part interest, part capital, and the capital share grows over time (we explain this in how much of your mortgage payment is principal). On an interest-only mortgage the split is fixed at 100% interest. Your monthly payment is simply:
Monthly payment = outstanding balance × annual rate ÷ 12
£200,000 × 4.5% ÷ 12 = £750
Overpay £10,000 and the balance drops to £190,000. Most lenders then recalculate your payment to £712.50, a cut of £37.50 a month. That gives interest-only overpayments a very simple saving rule: each pound saves your mortgage rate for every year left on the term. There is no compounding, because nothing else changes.
Here is how a £10,000 lump sum at the start compares across mortgage types, at 4.5% over 25 years:
| £10,000 lump sum on £200k | What happens next | Interest saved | Owed at end of term |
|---|---|---|---|
| Interest-only, payment recalculated | Payment falls to £712.50 | £11,250 | £190,000 |
| Interest-only, keep paying £750 | £37.50+ a month extra hits capital | £20,700 | £169,300 |
| Repayment, payment reduced | Payment falls £56 to £1,056 | £6,700 | £0 |
| Repayment, term reduced | Cleared 2 years 2 months early | £19,300 | £0 |
The interesting result is that an interest-only lump sum saves more than a repayment lump sum with the payment reduced. That is because the interest-only balance never falls on its own, so the £10,000 you removed would otherwise have attracted interest for the full 25 years. Timing still matters: the same £10,000 with 10 years left saves just £4,500. As with repayment mortgages, keeping the payment the same roughly doubles the benefit. Our guide to reducing the term or the payment covers why.
What Happens If You Overpay £100, £200 or £500 a Month?
Regular overpayments show the gap between the two approaches even more clearly. Same £200,000 at 4.5% over 25 years, where staying interest-only costs £225,000 in interest:
| Monthly overpayment | Payment recalculated: saved | Payment recalculated: owed at end | Total payment kept fixed: saved | Total payment kept fixed: owed at end |
|---|---|---|---|---|
| £100 | £16,800 | £170,000 | £25,300 | £144,700 |
| £200 | £33,600 | £140,000 | £50,600 | £89,400 |
| £362 | £60,900 | £91,400 | £91,600 | £0 (on time) |
| £500 | £84,100 | £50,000 | £119,000 | £0 (in 20 yrs 5 months) |
With the payment recalculated, your total outlay falls every month. After five years of £200 overpayments you would be paying £705 interest plus £200, or £905. Keep the full £950 going instead and the extra £45 also goes to capital, then a little more next month, and so on. To clear the whole loan with a flat overpayment while the payment is recalculated, you would need £667 a month; keep the total fixed and £362 a month does the same job.
Lenders handle this differently. Some let you set a fixed monthly payment above the interest due; others recalculate automatically and expect you to top up with a separate standing order. Either way, the practical rule is the same: whenever your interest payment falls, increase your overpayment by the same amount. Most fixed and tracker deals cap penalty-free overpayments at 10% of the balance a year, about £20,000 here, so even £500 a month is well inside the limit. Our 10% overpayment rule guide explains how lenders measure it.
Is It Better to Overpay or Switch to Repayment?
Look again at the £362 row: interest of £750 plus £362 overpaid is £1,112, exactly the repayment mortgage payment. Keeping your total payment at the repayment level is mathematically identical to switching. The difference is in the fine print:
- Switching to repayment locks in the discipline, and some lenders offer better rates on repayment deals. It may involve an affordability check, and once switched you cannot usually drop back to interest-only.
- Staying interest-only and overpaying keeps an escape hatch. If your income drops, you can fall back to £750 a month without asking permission. That flexibility is valuable if you are planning a Barista FIRE phase, parental leave or a career break.
- Part-and-part is a middle ground: move a slice of the loan to repayment (say £100,000) and keep the rest interest-only, with overpayments optional.
The risk with the flexible route is behavioural. An optional overpayment is the first thing to go when money gets tight, and an interest-only mortgage with no overpayments leaves the whole balance outstanding. If you choose flexibility, automate the overpayment on payday and treat it as a fixed bill.
How Should You Plan the Interest-Only Endgame?
The balance is due in full on the final day. Lenders must check you have a credible repayment strategy and contact you about it during the term, but the plan is yours. The shorter the time left, the bigger the monthly effort needed. Take £150,000 interest-only at 4.5% with 15 years to go (payment £562.50):
- Keep paying £862.50 (+£300): about £73,100 still owed at the end.
- Keep paying £1,062.50 (+£500): about £21,800 owed.
- Keep paying £1,147.50 (+£585): cleared on time — the repayment equivalent.
Overpaying is not the only way to fill the gap. Common UK repayment strategies, often used in combination:
- Investing in a Stocks & Shares ISA and repaying from the pot at the end. Investing £362 a month for 25 years builds about £161,500 at 3% growth, £200,200 at 4.5% and £250,900 at 6%. You only beat overpaying if returns exceed your mortgage rate, and a market fall just before the term ends can leave you short.
- Pension tax-free cash: from age 55 (57 from April 2028) you can usually take up to 25% of your pension tax-free, capped by the £268,275 lump sum allowance. Check the term ends after you can access it. See our retiring with a mortgage guide.
- Selling or downsizing, which relies on having enough equity and somewhere cheaper to go.
- Extending the term or a retirement interest-only (RIO) mortgage, which keeps the debt running and is usually repaid when you die or go into long-term care.
For FIRE, an outstanding interest-only balance means your investments must fund £9,000 a year of mortgage interest (on £200,000 at 4.5%) forever, or a large lump sum at a fixed date. That can add £225,000 to your FIRE number at a 4% withdrawal rate. Clearing the balance first often makes early retirement far easier. The MoneyHelper guide to repaying an interest-only mortgage and the FCA’s advice for interest-only borrowers set out what to do if you are worried about a shortfall.
What about buy-to-let interest-only mortgages?
Most buy-to-let mortgages are interest-only, and the tax treatment changes the maths. Since April 2020, individual landlords can no longer deduct mortgage interest from rental profits; instead they get a tax credit worth 20% of the interest. That makes the net cost of a 4.5% buy-to-let loan about 3.6% for any taxpayer, so overpaying earns a lower effective return than on your own home, and investing in an ISA or pension may compete more strongly. See HMRC’s guidance on tax relief for residential landlords.
Frequently Asked Questions
Can you overpay an interest-only mortgage?
Yes. Most UK interest-only mortgages allow overpayments on the same terms as repayment mortgages, typically up to 10% of the balance each year without an early repayment charge during a fixed or tracker deal, and without limit on a standard variable rate. Every pound overpaid comes straight off the capital, because your regular payment only covers interest. Check your lender's allowance and how it treats overpayments before paying extra.
How much does overpaying an interest-only mortgage save?
If your lender simply lowers your interest-only payment after each overpayment, every pound saves your mortgage rate for each remaining year: a £10,000 lump sum at 4.5% with 25 years left saves £11,250 of interest and cuts the payment by £37.50 a month, and £200 a month saves about £33,600 and leaves £140,000 owed instead of £200,000. If you keep paying the same total each month, the savings compound: the same £200 a month saves about £50,600 and leaves £89,400 owed.
Is it better to overpay an interest-only mortgage or switch to repayment?
Mathematically, keeping your total monthly payment at the repayment level is the same as switching. On £200,000 at 4.5% over 25 years, paying £1,112 a month (£750 interest plus £362 overpayment) clears the loan on time and saves about £91,600 of interest versus staying interest-only. Switching formally guarantees the discipline and may need an affordability check; overpaying keeps the flexibility to drop back to £750 if your income falls, which suits Barista FIRE or career breaks.
What happens at the end of an interest-only mortgage if you cannot repay?
The full balance is due on the final day of the term. If you cannot repay it, options include selling or downsizing, using savings, ISAs or a pension tax-free lump sum (from age 55, rising to 57 in April 2028), extending the term, switching to repayment, or a retirement interest-only mortgage if you are older. Lenders must contact interest-only borrowers about their repayment plan, and the FCA expects them to treat you fairly, but repossession remains possible if no solution is agreed. Plan at least 10 years ahead.
Should I overpay my interest-only mortgage or invest in an ISA to repay it?
Overpaying earns a guaranteed, tax-free return equal to your mortgage rate. Investing the same money in a Stocks & Shares ISA only comes out ahead if returns beat that rate. Investing £362 a month for 25 years builds about £200,200 at 4.5% (exactly enough to clear £200,000), £250,900 at 6% and £161,500 at 3%. A mix of the two, overpaying enough to reduce the risk while investing the rest, is a common UK FIRE compromise.
Work Out Your Own Numbers
Our mortgage calculators model repayment mortgages, which is exactly what you need to test the “keep the total payment fixed” strategy or a switch to repayment. Enter your interest-only balance, rate and remaining term to see the repayment-level payment you would need, then compare it with your current interest-only payment:
- Mortgage Overpayment Calculator — see the interest saved and new mortgage-free date from a lump sum or regular overpayment once you are paying above the interest
- Principal Payback Calculator — compare a standard repayment schedule with paying extra principal each month, side by side
For the wider question of where spare cash should go, read pay off the mortgage or invest and how overpayments affect your FIRE date.
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