You've found £200 a month in your budget. Two voices are arguing in your head. One says: throw it at the mortgage, you'll save thousands in interest and own the house sooner. The other says: invest it, you'll do better long-term, and you keep the flexibility. They're both right, and which one wins depends on a handful of factors that almost no online calculator captures.
If your expected investment return after tax is higher than your mortgage rate, investing wins. Simple in theory. Less simple when you put the actual numbers in.
UK mortgage rates in early 2026 sit around 4.25–5.00% for a typical 5-year fix. That's a guaranteed, risk-free, after-tax return on every pound you overpay. Beating it consistently from a Stocks & Shares ISA requires the long-run real return to land at the upper end of historical ranges — which it has, but with sizeable drawdowns along the way.
Historical UK equity returns (FTSE All-Share, dividends reinvested) have averaged around 5–7% real (after inflation) over rolling 20-year periods since the 1950s. Add 2.5% inflation and you get a nominal expectation of 7.5–9.5%. So on long horizons, equities have historically beaten 4.5% mortgage rates. The catch: they did it via decade-long sideways periods and a couple of 40% drops.
Sarah has a £200,000 mortgage at 4.75% with 25 years remaining. She's found £200/month spare. Two scenarios over the full 25 years:
By the time the mortgage is cleared in scenario one, the freed-up payment going into the same 6.5% ISA produces around £85,000 by year 25. So pure maths: Sarah ends up with about £85k of ISA money plus a paid-off mortgage, vs. £148k of ISA money plus a still-running mortgage. The investment route is ahead by perhaps £25–30k after factoring in the ongoing mortgage interest in the second scenario.
The above example assumes 6.5% every single year for 25 years. Real markets do not deliver that. They deliver +20%, then -8%, then +14%, then -32%, then +27%. The expected outcome is similar; the path is brutal.
Behavioural research consistently finds that people underestimate how they'll respond to a 30% drawdown. They sell. They take a break from contributing. They lock in losses. The maths edge of investing depends on you holding through the worst of it — exactly when overpaying the mortgage felt like the safer choice in retrospect.
Mortgage overpayments don't have this problem. The "return" is fixed and risk-free. Once made, the saving is locked in regardless of what happens next.
Almost no professional says "all to mortgage" or "all to investing." The middle path is so common because it acknowledges both the maths and the psychology: you take some of the guaranteed return and some of the long-run upside.
A classic split: overpay enough to clear the mortgage by, say, age 60, and put everything else in tax-advantaged investing (pension first if higher-rate, then ISA). It's not maximally optimal in any single dimension. It's robust to a wide range of futures, which is a much better property than optimal.
Want to see exactly how your numbers play out? Run the comparison on the mortgage tool.
Almost certainly not. With a fix that low, every pound you have is worth more in even a basic savings account at 4%+ — let alone in long-term investments. Hold the cheap mortgage, save aggressively, and decide what to do when the deal expires.
Most UK lenders allow up to 10% overpayment per year without an Early Repayment Charge. Above that, ERCs are typically 1–5% of the overpaid amount. Stay within the limit and ERCs are not a factor — see our overpayment calculator for details.
Materially, yes. On interest-only deals, the entire balance still needs to be cleared at term end. Investing into a wrapper specifically to clear it (an offset or repayment vehicle) becomes more attractive because the comparison is forced.
If you're a higher-rate taxpayer with unused pension annual allowance, pension contributions get 40% tax relief immediately — a return that's nearly impossible to beat with mortgage overpayments. For most higher-rate earners, pension > ISA > mortgage overpayment as the order of priority.
This article is general information about UK personal finance. It is not regulated financial advice and Pennywise Finance is not authorised by the Financial Conduct Authority. For decisions involving large sums or complex situations, please consult an FCA-authorised adviser.