Stop Overpaying Your Mortgage: Your ISA Will Make You £94,000 Richer Over 20 Years

Run the numbers over a real mortgage term. Take a homeowner with a £250,000 mortgage at 5.15% over 25 years, contributing £500 a month extra.

Scenario A — Mortgage overpayment: You clear the mortgage roughly 10 years early, saving approximately £68,000 in total interest. Excellent. You feel safe. Your house is paid off sooner and you sleep well at night.

Scenario B — Stocks and shares ISA: That same £500 a month into a global equity tracker averaging 8% annually (conservative against the 10-year FTSE average of 9.5%) grows to approximately £162,000 after 20 years. You still have the mortgage, but your ISA pot dwarfs the interest saved.

The difference: roughly £94,000. That's not a rounding error. That's a deposit on a second property, a decade of retirement income, or your children's university fees — all sacrificed because "guaranteed returns" sounded reassuring.

The compound interest curve is relentless. In years 1-5, the gap between strategies is modest — about £23,000. By year 15, it's £96,000. The longer your time horizon, the more expensive the "safe" choice becomes. And a mortgage is, by definition, a long-horizon commitment. You can explore your own scenarios with our mortgage calculator.

Mortgage overpayment feels safe because humans are wired to hate debt. Behavioural economists call it "debt aversion bias" — we'd rather eliminate a liability than grow an asset, even when the asset grows faster. It's the same impulse that makes people pay off a 0% car loan early. It feels right. The maths says otherwise.

Consider what "guaranteed" actually means here. Your mortgage rate of 5.15% is guaranteed for the fixed period — typically 2 or 5 years. After that, you remortgage at whatever rate the market offers. The BoE base rate was 0.1% in 2021 and 5.25% in 2023. Nothing about mortgage rates is permanent. When you remortgage onto a lower rate, the return on your overpayments drops too.

Meanwhile, the FTSE 100 has never delivered a negative return over any 15-year rolling period in its history. Over 20 years, the worst rolling return was still positive. The stock market's "risk" is only real if you need the money tomorrow. Over a mortgage term? Equities aren't risky — they're the most reliable wealth-building tool we have.

The 2025 FTSE 100 return of 21.1% wasn't a fluke. It was the seventh-best annual performance on record. Missing years like that because you chose to overpay a 5% mortgage is an expensive form of caution. One year of equity returns can exceed the entire interest saving from years of mortgage overpayment.

Here's what the overpayment crowd never mention: you can always overpay your mortgage in July. Or October. Or next February. The 10% annual limit runs with your mortgage year, not the tax year.

Your ISA allowance of £20,000 expires on April 5 and never comes back. Every year you skip your stocks and shares ISA to overpay your mortgage, you permanently lose £20,000 of tax-free investment wrapper. Over 10 years, that's £200,000 of contributions that could have been growing tax-free — including all dividends, interest, and capital gains.

A higher-rate taxpayer with £50,000 in a general investment account pays 33.75% on dividends above the £500 allowance and 20% on gains above the £3,000 CGT allowance. Inside an ISA? Zero. The tax savings alone can be worth thousands per year on a mature portfolio.

This asymmetry is crucial. Mortgage overpayment flexibility is time-agnostic — you can do it whenever you have spare cash. ISA allowance is time-bound — miss it and it's gone forever. When two financial strategies compete for the same £20,000, you should always prioritise the one with the expiring deadline.

For a full comparison of ISA types, see our ISA guide. If you're new to investing, our beginner's ISA guide walks through the basics.

Once you overpay your mortgage, that money is gone. You can't withdraw it. If you lose your job, face an emergency, or spot an investment opportunity, your overpaid mortgage capital is locked in your walls — literally. Some lenders offer "offset" or "reserve" features, but these are uncommon and typically come with higher rates that erode the overpayment benefit.

An ISA is accessible. A stocks and shares ISA can be liquidated in 3-5 business days. Even the most cautious investor can hold a money market fund inside the ISA wrapper, earning 4.5%+ while keeping the capital available for any purpose.

This matters especially now. With the Iran war pushing up energy costs — bills forecast to rise by £332 a year from July — and UK borrowing costs hitting 2008 levels, economic uncertainty is elevated. The Bank of England has warned it may raise rates if the price shock persists. In this environment, having liquid, accessible savings is more valuable than having marginally less mortgage debt.

The mortgage overpayment evangelists assume a stable world where you'll never need that money for anything else. The world isn't stable. Keep your options open. Our savings guide covers the best options for maintaining an accessible cash buffer alongside your investment strategy. See our analysis on the case for mortgage overpayments as a guaranteed return. See our analysis on how mortgage overpayments work in practice.

This isn't about never overpaying. It's about doing things in the right order.

Before April 5, your priority should be:

1. Max your employer pension match — 100% return from day one, dwarfing any mortgage rate
2. Fill your stocks and shares ISA — £20,000 of tax-free growth that you lose forever if you skip it
3. Then overpay your mortgage with anything left, staying within the 10% limit

If you can only afford one, choose the ISA. The average stocks and shares ISA returned 6.2% annually over the past five years and 9.5% over ten years. Your mortgage rate is 5.15%. The expected return differential of 1-4 percentage points compounds dramatically over a 20-year mortgage term into the £94,000 gap shown above.

For those still unsure, consider the recent debate on our site about locking in fixed mortgage rates versus choosing trackers. The same principle applies here: the comfortable, obvious choice isn't always the smart one.

One more thing the overpayment fans won't tell you: when you eventually pay off your mortgage (whether early via overpayments or on schedule), you'll wish you'd been building an investment pot all along. A paid-off house is a roof over your head, not an income stream. An ISA is money that works for you — in retirement, in emergencies, whenever you need it. Visit our investing hub for guidance on building a diversified portfolio inside your ISA wrapper.

This article is for informational purposes only and does not constitute financial advice. You should seek independent financial advice before making any investment decisions.