Should you overpay your mortgage? The maths, and when it backfires

Overpaying the mortgage has a good reputation. Pay a bit extra each month, the thinking goes, and you save a fortune in interest and clear the debt years early. Mostly true, but "a bit extra" hides a lot of detail, and there are real situations where overpaying is not the best use of spare cash at all.

The mechanics are worth understanding before you set up a standing order, because the size of the saving depends heavily on when you pay and how long is left on the loan.

What a small monthly overpayment actually does

Take a fairly ordinary mortgage: £200,000 outstanding, a 4.75% rate, 25 years left, repayment basis. The standard monthly payment on that is £1,140.23, and left untouched it will cost £142,070 in interest over the full term.

Now add just £200 a month on top, with the lender shortening the term rather than the payment. The mortgage clears in 18 years and 11 months instead of 25, knocking just over six years off the end. Total interest drops to £103,053. That is a saving of £39,017 for an extra £200 a month, which works out at roughly £45,400 paid in to save £39,017 in interest plus finish six years sooner. Not free money, but a genuinely strong trade if you have the spare income and no better use for it.

The reason it works so well is compounding running in reverse. Mortgage interest is charged on the balance each month, so every pound that comes off the balance early stops generating interest for every month that follows. Pay the same extra pound in year 24 instead of year one and it saves almost nothing, because there is barely any term left for it to work on.

Lump sum or monthly, and why it matters

Say instead of the monthly overpayment you had a £10,000 lump sum, perhaps from a bonus or an inheritance, and put it straight onto the same £200,000 mortgage on day one. The term shortens by 27 months, about two years and three months, and total interest falls from £142,070 to £121,025, a saving of £21,045.

Pound for pound, the lump sum is the more efficient move, because the whole £10,000 starts saving interest immediately rather than building up gradually like the monthly version. But most people do not have a spare £10,000 sitting around, and £200 a month is a habit you can actually sustain. In practice the two are not really in competition. If you have both a lump sum and some headroom in your monthly budget, doing both inside your lender's fee-free allowance is usually the strongest option of all.

When overpaying is the wrong call

None of this means overpaying always wins. A few situations flip the answer.

If you locked in a very cheap fixed rate a few years back, say 1.5% or 2%, and savings accounts are paying more than that after tax, you are mathematically better off leaving the money in savings or an ISA rather than overpaying. It feels less satisfying than watching the mortgage balance fall, but the numbers do not lie. Check the actual rates rather than going on gut feeling, because the gap can be larger than people expect.

There is also the question of what else the money should be doing first. An employer pension match is close to free money and usually beats any mortgage saving by a wide margin. No emergency fund is a real risk, because overpaying ties cash up in the house, and getting it back out again means a remortgage or a sale, neither of which happens quickly if the boiler dies in February. And going over your lender's fee-free overpayment allowance, typically 10% of the balance a year, means an Early Repayment Charge that can easily wipe out the interest you were trying to save.

My own rule of thumb: emergency fund first, then any pension match going spare, then high-interest debt if you have any, and only after all of that does the mortgage get the leftover. It is a fairly conservative order, but it avoids the situation where someone has thrown every spare pound at the mortgage and then has to put a car repair on a credit card.

Telling your lender what the money is for

One small thing that trips people up: when you make an overpayment, tell the lender explicitly that it is a capital reduction, not just an extra payment held on account. Some lenders apply it automatically, others hold it as a payment in advance unless you say otherwise, which does nothing for your interest bill. A two-minute phone call or a note on the online portal sorts it, and it is worth checking your next statement to confirm the balance actually moved.

This guide is general information, not financial advice. Figures are illustrative and your own mortgage terms and rates may differ, so check your numbers before making a decision.