Overpay Your Mortgage or Invest in a Stocks & Shares ISA? The 2026 Maths

The Core Trade-Off

Every pound of spare monthly income put toward mortgage overpayment reduces the interest you'll pay over the life of the loan by an amount equal to your mortgage rate, compounded over the remaining term. That return is certain — it doesn't depend on markets, and it isn't taxed.

Every pound put into a Stocks & Shares ISA instead has the potential for a higher return over the long run (based on historical average market performance), but with no guarantee, and real risk of loss, especially if you need the money back within a short time horizon.

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Worked Example: £300/Month, 15 Years Left on the Mortgage

The investing route has a higher expected outcome at a 6% assumption versus a 4.5% mortgage rate, but that gap narrows or reverses entirely in a poor market decade, whereas the mortgage overpayment outcome doesn't change based on market conditions.

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Priority Order Most People Use

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Factors That Tilt the Decision

• Mortgage rate level: the higher your rate, the stronger the case for overpaying.
• Time horizon: a longer investment horizon gives markets more time to recover from downturns, strengthening the case for investing.
• Early repayment charge / overpayment allowance: most fixed mortgage deals cap penalty-free overpayments at 10% of the outstanding balance per year — check this before committing large lump sums.
• Risk tolerance: if market volatility would cause you to panic-sell an ISA investment during a downturn, the guaranteed nature of overpaying may suit you better regardless of the numbers.
• Flexibility needs: ISA money remains accessible if plans change; overpaid mortgage capital is generally locked into the property (though some lenders offer flexible/offset mortgages that preserve access).

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A Practical Middle Ground

Rather than treating this as binary, many households split spare income — for example, directing overpayments up to the penalty-free allowance and investing any surplus above that in an ISA. This captures the guaranteed benefit of overpaying up to the limit while still building an investment pot for the longer term, and can be rebalanced each year as mortgage rates and market conditions change.