Overpaying Your Mortgage at 5.5% Is the Best Risk-Free Return in Britain — Forget Gilts

Every pound you overpay your mortgage saves you interest at your mortgage rate. If you're on a typical 2-year fix at 5.56%, that's a guaranteed 5.56% return on every overpayment. The Bank of England base rate sits at 3.75%, but mortgage lenders are pricing in geopolitical risk — the Iran conflict, oil volatility, and sticky inflation expectations have pushed fixed rates well above the base rate.

Gilts? The 10-year gilt yields roughly 4.8%. The 5-year gilt yields 4.45%. Both are lower than your mortgage rate. You'd be lending money to the government at a lower rate than you're borrowing from your bank. That's not clever investing — it's arbitrage running the wrong way.

For basic-rate taxpayers, gilt coupon income is taxed at 20% above the personal savings allowance of £1,000. A 4.8% gilt yield becomes 3.84% after tax for someone above the allowance. Your mortgage overpayment saving? Still 5.56%, untaxed.

If you're comparing investments, look at it this way: to beat a 5.56% mortgage overpayment with gilts, you'd need gilt yields above 6.9% for a higher-rate taxpayer (after 40% income tax on coupons). We haven't seen yields that high since 1998. For a complete breakdown of how gilts work, see our gilts guide.

People call gilts "risk-free" because the UK government won't default. But that's credit risk. You're still exposed to interest rate risk, and it's brutal.

If the BoE holds rates higher for longer — entirely plausible with oil at $93 and a ceasefire nobody trusts to hold — gilt prices fall. A 10-year gilt bought today at 4.8% would lose roughly 7-8% of its capital value if yields rose to 5.5%. You'd be sitting on a paper loss, watching your "safe" investment shrink while your mortgage balance stayed exactly where it was.

Overpaying your mortgage has no capital risk. Every pound reduces your balance. The interest saving is locked in the moment the payment hits your account. No mark-to-market losses, no yield curve surprises, no duration mismatch. Just debt, shrinking.

The FCA doesn't classify mortgage overpayments as investments because there's nothing to classify — it's the elimination of a known liability. That's as close to a free lunch as personal finance gets. If the mini-budget of September 2022 taught us anything, it's that gilt prices can move violently when markets lose confidence. Mortgage overpayments are immune to that kind of shock.

For context on how mortgage rates have been moving, see our mortgage guides hub. And for the bear case on gilts, our analysis on gilt capital risk makes the point in detail.

Run the numbers on a typical £250,000 mortgage over 25 years at 5.5%. Overpaying by £300 per month shaves roughly 7 years off the term and saves approximately £47,000 in total interest. That's £47,000 you never pay, sitting in your pocket instead of your lender's.

To generate £47,000 from gilts over the same period, you'd need gilt yields to stay above 4.5% for the entire time — while reinvesting all coupons, paying no dealing costs, and suffering no capital losses when rates move. The Bank of England's own projections suggest rates will continue falling through 2026, which means gilt prices will rise but new gilt yields will drop. Your overpayment return stays constant at your mortgage rate.

The compound effect matters too. As your mortgage balance shrinks faster through overpayments, you pay less interest each month, which means more of your regular payment goes toward principal. It's a virtuous cycle that accelerates over time. By year five, you're effectively earning above 5.56% on an annualised basis because of the compounding benefit.

Use our mortgage calculator to run the exact numbers for your situation.

The strongest case for gilts over overpayments is liquidity. You can sell a gilt tomorrow. Getting overpayments back requires remortgaging or selling your house.

But this argument contains its own refutation. If you need the money urgently, you're selling gilts into whatever market exists at that moment. If rates have spiked — as they did in September 2022 after the Truss mini-budget — you're crystallising a loss. Your "liquid" asset just cost you money.

Meanwhile, most lenders let you overpay up to 10% of your balance per year without penalty. That's £25,000 on a £250,000 mortgage. If you've got more than £25,000 in spare cash per year, you have a different kind of problem — and probably a financial adviser to help you solve it.

Keep a proper emergency fund in a high-interest savings account — MoneyHelper recommends 3-6 months of essential spending. Then overpay your mortgage with the rest. Don't confuse your emergency fund with your investment strategy. For the best current savings rates for your emergency fund, see our savings hub.

There's one scenario where gilts genuinely beat overpaying: if your mortgage rate is below gilt yields. Tracker mortgage holders on base rate plus 0.5% are paying 4.25%. Gilts at 4.8% offer a genuine spread.

But tracker rates move. The BoE has cut four times since August 2024, from 5.25% to 3.75%. If they cut twice more in 2026 — the market expects at least one — your tracker rate drops further while your gilts keep paying their coupon. The spread widens in your favour.

Except you're now betting on rates staying high enough for long enough to make the gilt trade worthwhile. That's a rate speculation, not a savings strategy. And if you're wrong — if rates rise because the Iran ceasefire collapses and oil hits $120 again — your tracker rate jumps and you're paying more on your mortgage while your gilts lose capital value. You've made both sides of the bet worse.

For the mortgage holders paying over 5% on fixed rates — which is most of you right now — this isn't even a debate. Overpay. For more on how mortgage rates are moving, see our mortgages hub.

The ceasefire announced today between the US and Iran could change this picture. Oil dropped from over $100 to $93 on the news, and if it holds, inflation pressures ease, the BoE can cut faster, and tracker rates fall further. But ceasefire deals in this conflict have broken down before — betting your financial strategy on geopolitics is not a plan. For a wider view on how the Iran situation affects your finances, see our coverage of the Iran oil impact.

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