Overpaying a mortgage is a risk-free return equal to the mortgage rate: £10,000 paid off a 4.5% loan saves £5,530 of interest over ten years, the same as growing it to £15,530. Invested at 6% inside an ISA the same £10,000 becomes £17,908; outside an ISA with the growth taxed at 20% it becomes £15,981. Investing wins on those assumptions, by a margin that disappears if returns fall below 4.5% in an ISA or 5.6% in a taxed account. The decision therefore turns on the mortgage rate, the tax wrapper available, and how much certainty is worth to you; a pension contribution with tax relief usually beats both.
The comparison
| £10,000 over 10 years | Value at the end | Gain |
|---|---|---|
| Overpay a 4.5% mortgage | £15,530 of interest avoided and capital cleared | £5,530, guaranteed |
| Invest at 6% in a stocks and shares ISA | £17,908 | £7,908, not guaranteed |
| Invest at 6% outside an ISA, growth taxed at 20% | £15,981 | £5,981, not guaranteed |
| Invest at 6% outside an ISA, growth taxed at 40% | £14,233 | £4,233, not guaranteed |
Overpaying earns exactly the mortgage rate with no volatility and no tax, because avoided interest is not income. Investing has to beat that rate after tax and charges, and over ten years a diversified equity fund has usually done so but not always. The break-even return is the mortgage rate itself inside an ISA, and the mortgage rate divided by one minus the tax rate outside one: 5.6% for a basic rate taxpayer and 7.5% for a higher rate taxpayer against a 4.5% mortgage. The overpay or invest calculator runs any amount, term, mortgage rate, return and tax rate.
What overpaying actually saves
| £200,000 mortgage at 4.5% over 25 years | Cleared early by | Interest saved |
|---|---|---|
| £100 a month extra | 42 months | £21,142 |
| £10,000 lump sum in year one | 26 months | £19,300 |
| £30,000 in an offset savings account | 45 months | £50,438 |
Most fixed-rate deals allow overpayments of 10% of the balance a year without an early repayment charge, and the charge on anything above that, typically 1% to 5% of the excess, wipes out the benefit. Overpayments should reduce the balance immediately rather than sitting as a credit, and the term should be kept the same so the monthly payment falls only if you ask; keeping the payment and shortening the term is what produces the savings above. The mortgage overpayment calculator shows the effect of any monthly or lump sum amount, and our mortgage repayment guide gives the underlying costs by rate and term.
Offset mortgages
An offset mortgage links a savings account to the loan and charges interest only on the difference, so £30,000 of savings against a £200,000 balance means interest on £170,000. The savings stay accessible, earn the mortgage rate tax free in effect, and clear the £200,000 loan 45 months early if the payment is kept level, saving about £50,000 of interest over the term. Offset rates are usually a little higher than the best standard deals, so the arrangement suits people with large cash balances they want to keep available, such as the self-employed holding tax money, rather than those with small savings. The offset mortgage calculator works the saving for any balance, rate and savings figure.
The pension alternative
A pension contribution attracts tax relief at your marginal rate, so £10,000 in a pension costs a basic rate taxpayer £8,000 and a higher rate taxpayer £6,000 of take-home pay, an immediate 25% or 67% uplift before any investment return, and salary sacrifice adds the National Insurance saving. Against that, the money is locked until 55, rising to 57 in 2028, and is taxed on the way out beyond the 25% tax-free share. For anyone not yet using their employer’s matching contribution in full, the pension beats both overpaying and an ISA. The pension versus ISA calculator compares the two wrappers at your tax rates, and the compound interest calculator shows how a return compounds over any period.
A sensible order
Clear expensive debt first, since a credit card at 24.9% or a personal loan at 9% costs more than any mortgage; then hold three to six months of spending in accessible savings; then take all the employer pension matching available. After that the choice between the mortgage and an ISA is a matter of rate and temperament. At mortgage rates around 2%, as many people fixed in 2021, investing was clearly better; at 4.5% to 6% the gap narrows to the point where the certainty of a guaranteed return and a shorter mortgage is a reasonable preference, particularly within a few years of retirement or where the fixed deal is ending and the new rate will be higher.
Common questions
Is it better to overpay my mortgage or invest? Investing tends to win on expected return when the mortgage rate is well below long-run investment returns and an ISA is available; overpaying wins on certainty, and beats investing whenever returns fall short of the mortgage rate.
What return do I need to beat overpaying? More than the mortgage rate inside an ISA or pension, and more than the mortgage rate grossed up for tax outside one: about 5.6% for a basic rate taxpayer and 7.5% for a higher rate taxpayer against a 4.5% mortgage.
How much does overpaying £100 a month save? On £200,000 at 4.5% over 25 years, £21,142 of interest and 42 months off the term.
Should I pay into my pension instead? Usually yes, up to the level that captures full employer matching and your marginal tax relief, because the relief is an immediate uplift no mortgage rate can match. The trade-off is that the money is locked away.
Can I overpay without a penalty? Most fixed deals allow 10% of the balance a year. Above that an early repayment charge of 1% to 5% applies until the fixed period ends.
What is an offset mortgage? A mortgage where savings held with the lender reduce the balance on which interest is charged, keeping the cash accessible while it saves interest at the mortgage rate tax free.
Information, not financial advice. Figures are from the site’s calculators using illustrative rates of 4.5% for the mortgage and 6% for investment returns, which are not guaranteed; tax on investment growth depends on the wrapper and your circumstances. Take regulated advice before making large financial decisions.