Mortgage overpayment vs ISA
Both reduce wealth leakage to interest or tax — but in opposite directions. Here's a clear comparison of overpaying your mortgage vs putting the same money into a cash or stocks-and-shares ISA.
Both reduce wealth leakage to interest or tax — but in opposite directions. Here's a clear comparison of overpaying your mortgage vs putting the same money into a cash or stocks-and-shares ISA.
You'll be mortgage-free 6 years 9 months earlier
and save £46,940 in interest.
| Without | With overpay | |
|---|---|---|
| Term | 25 years | 18 years 3 months |
| Total interest | £156,277 | £109,338 |
| You save | £46,940 · 6y 9m |
A lower interest rate could save you thousands more on top. See if you could remortgage to a better deal.
We may receive a commission if you remortgage through a partner broker. This never affects the rate you're offered.
Cash ISAs currently pay around 4–5% gross, tax-free. UK mortgage rates sit around 4–6%. If your mortgage rate is higher than your best cash ISA rate, overpaying wins — and you don't even need the ISA wrapper because mortgage 'return' is already tax-free.
Cash ISA wins only if your rate exceeds your mortgage rate, or if liquidity matters more than the small return difference.
Stocks-and-shares ISAs have a higher long-term expected return (~7% real historically) but with volatility and no guarantee. Over 20+ years they've usually beaten typical mortgage rates.
If you have a long horizon, can tolerate volatility, and have already capped your pension matching and built an emergency fund — investing in a stocks-and-shares ISA likely beats overpaying on expected return. The trade-off is risk and the loss of the guaranteed feeling of clearing the mortgage faster.
You don't have to choose. Many UK homeowners split: overpay enough to feel they're making progress (often £100–500/month), put the rest into an ISA for tax-free growth. The calculator below shows how much overpaying any monthly amount would save.
Important: This article is for general information and is not financial advice. Always speak to a qualified UK mortgage adviser before making decisions about overpayments, remortgaging, or your specific mortgage product.