Overpaying earns your mortgage rate guaranteed and tax-free; investing has to beat it after fees, tax and nerve. How to compare the two properly — including what happens after the mortgage clears early.
The most-asked question in UK personal finance has a two-line core: overpaying your mortgage earns your mortgage rate, guaranteed, tax-free; investing the same money is expected to earn more — but isn't guaranteed to, and won't do it smoothly.
A fair comparison needs three honesty adjustments. First, the after-mortgage years: overpaying clears the loan early, and the whole freed payment can then be invested for the saved years — leaving this out flatters investing. Second, tax: inside an ISA the comparison is clean; outside one, dividend and capital gains tax drag the investing route, and at higher rates the drag is material. Third, your deal's terms: most fixes allow 10% a year penalty-free, with early repayment charges beyond it.
At mortgage rates around 4–5% against equity expectations of 5–7% after fees, investing carries a real but modest expected edge — bought with genuine risk: a bad decade can leave the invested route behind for years. That's why the honest answer is often deliberate diversification: some overpayment (a guaranteed return and falling fixed costs), some investing (the growth engine), with the split set by temperament as much as arithmetic. Two prior claims beat both: expensive debt, and an emergency fund.
Hundred Summers' overpayment calculator runs both routes properly — month-by-month amortisation on the mortgage side, the freed-payment years credited — and the full plan models the overpayment inside your whole lifetime picture, where the real competitor for the money is often the pension.