Should I overpay my mortgage, save or invest?
Overpaying reduces future mortgage interest. Saving or investing keeps money outside the home, where it could earn interest or grow. Which leaves you with more depends on the mortgage cost and the return after tax and fees, while access to the money and investment risk matter alongside the calculation.
What happens to the extra money?
An overpayment reduces the mortgage balance, leaving less debt to charge interest on. If you keep making the same normal payment, more of each later payment goes towards clearing the loan. The benefit builds as the balance falls.
Putting the money into savings leaves the mortgage on its original repayment path, while the savings pot earns interest. To compare the two, we need to count the cash alongside the remaining debt at the same point in time.
Investing follows the same idea, with an uncertain outcome. Growth could exceed the interest avoided by overpaying, but the investment could also lose value. A higher assumed return makes the calculation look better without making that return any more likely.
How do the rates compare?
Overpaying a mortgage at 5% has a benefit roughly equivalent to earning 5% after tax on that money, before any overpayment charges. It comes from interest you avoid paying. That gives you a useful starting point for comparing it with savings interest or investment growth after tax and fees: in our example, 3% savings earn less, while an assumed 7% investment return could earn more, with the risk that it won’t materialise.
With a fixed-rate mortgage, you can think of overpaying as securing that interest saving for the rest of the fixed period. The debt reduction lasts, but the rate of interest you avoid follows your mortgage rate. If that rate changes, so does the comparison; today’s 5% is not a return locked in for the whole mortgage term.
What does £300 a month change?
Keep the starting mortgage at £270,000 with 25 years left, then change what happens to the spare money each month. The £10,000 reserve stays untouched in every route; it is part of this example, not a suggested amount for everyone.
Investment returns are uncertain and you could lose money. This shows a steady assumed return; real returns fluctuate, and their timing changes the result. Try a negative return as well as a positive one.
Suppose the mortgage has 25 years left at 5%, with a normal monthly payment of about £1,578. There is another £300 available each month, which can go towards overpayments or into savings earning 3% AER. We’ll compare both routes after five years.
The £10,000 reserve stays untouched in both cases, so it doesn’t affect the difference. Both rates remain constant for this illustration, with no fees or tax on savings interest. These are assumptions, not current offers or forecasts, and £10,000 is not a suggested reserve for everyone.
Both bars use the same money scale, starting at zero. The overpayment bar shows the reduction in debt compared with making normal payments only. The savings bar excludes the existing reserve.
The overpayments reduce the mortgage by £20,402 more than normal payments alone would have done. Of that, £18,000 is your extra money going in; the remaining £2,402 comes from interest you no longer have to pay.
Saving the same £18,000 earns £1,374 instead. That puts overpaying £1,028 ahead after five years when we compare savings minus debt, with the home and existing reserve held equal.
| Overpay | Save | |
|---|---|---|
| Mortgage remaining | £218,765 | £239,167 |
| New cash savings | £0 | £19,374 |
Overpaying has the higher calculated value here, while saving leaves another £19,374 in cash. Money paid into a standard mortgage cannot usually be taken back out on demand, so those positions offer different flexibility.
Before overpaying, check your lender’s penalty-free overpayment allowance and any early repayment charges. These can reduce or outweigh the interest saved and are not included in this comparison.
When could saving or investing come out ahead?
If savings earn more after tax than the mortgage costs over the same period, saving can produce the higher result. That comparison needs to account for how interest is calculated: a savings AER includes compounding, whereas the quoted mortgage rate may use a different convention.
Rates can also change. A savings account might become less competitive, or the mortgage rate might rise when a fixed deal ends. Any tax on savings interest reduces what you keep; the amount depends on your income, allowances and account type.
What else affects the choice?
Access to cash matters if you might need the money for repairs or a gap in income. Savings accounts have different withdrawal rules, while flexible or offset mortgages may offer access that a standard mortgage does not.
Overpayment limits and early repayment charges can change the result. Lenders also differ in how they adjust payments afterwards. Our example keeps the normal payment unchanged; reducing it would change the outcome. The mortgage-term guide explains how payment size affects the time it takes to clear the loan.
An investment needs enough growth after tax and fees to outweigh the mortgage interest it leaves you paying. Even where that happens in an illustration, an uncertain investment return is different from avoiding interest on a debt. Other debts and pension contributions may also matter before deciding what spare money is available.