Overpay Your Mortgage or Invest? The Numbers in 2026
With mortgage rates between 4.4% and 5.6% and the FTSE 100 around 10,500, the question of whether to overpay your mortgage or invest spare cash is one of the most common — and most nuanced — in...
title: "Overpay Your Mortgage or Invest? The Numbers in 2026" category: Housing & Mortgages date: 2026-06-29 tags: [mortgage, investing, overpayment, ISA, personal finance, FTSE 100] image: https://picsum.photos/seed/mortgage-overpay-vs-invest/2400/1350
With mortgage rates between 4.4% and 5.6% and the FTSE 100 around 10,500, the question of whether to overpay your mortgage or invest spare cash is one of the most common — and most nuanced — in personal finance. The maths depends on your specific rate, tax situation, and risk tolerance. Here is how to think through it.
The Basic Maths
Overpaying a mortgage at 4.5% gives you a guaranteed, tax-free return of 4.5%. This is because reducing your mortgage debt saves you interest you would otherwise have paid — and that saving is not subject to income tax.
Investing in a stocks and shares ISA or pension, by contrast, offers a potentially higher return but with risk. The FTSE 100 has historically returned around 7–8% per year on average over long periods, including dividends, but any individual year can be negative. In 2025, the FTSE 100 fell during periods of US tech selloffs and energy volatility. Year-to-date in 2026, it has traded mostly between 10,300 and 10,575.
If your mortgage rate is below 4%: Historically, the case for investing over overpaying has been strong, as expected long-run stock market returns typically exceed that level.
If your mortgage rate is 4–5%: It becomes much closer — and the decision hinges on tax treatment, risk appetite, and whether you have the discipline to actually invest the money.
If your mortgage rate is above 5%: Overpaying often wins on a pure numbers basis, because a guaranteed 5%+ risk-free "return" via interest saving is extremely hard to beat after adjusting for risk.
With five-year fixed rates currently starting around 4.5% and two-year fixes around 4.4%, most homeowners are in the middle range where this is genuinely not obvious.
The Tax Variable
Whether to invest in a pension or ISA changes the comparison significantly.
Pension: If you are a basic rate taxpayer, putting £100 into a pension costs you £80 (HMRC adds 20% tax relief). If you are a higher rate taxpayer, it costs £60 (you claim the extra 20% via Self Assessment). The boost from tax relief makes pensions highly efficient. Your money also grows free of income tax and capital gains tax inside the pension. On a risk-adjusted basis, pension investing wins over mortgage overpayment for most taxpayers — especially higher rate payers — provided you will not need the money until retirement.
Stocks and Shares ISA: Returns grow free of income tax and capital gains tax. You can invest up to £20,000 per tax year in 2026/27. The ISA wrapper removes the tax friction that would otherwise erode investment returns.
Outside a tax wrapper: Dividend income above £500 is subject to income tax, and capital gains above £3,000 per year are taxable. Investing outside an ISA or pension tilts the equation further towards mortgage overpayment.
The Psychological and Behavioural Case
For many people, the theoretical answer is "invest" — but the practical behaviour is to spend the money. An overpayment is automatic and certain: you make a bank transfer, the mortgage balance falls, the saving is permanent. Investment discipline is harder to maintain across market downturns.
If you are someone who would realistically invest the money in a low-cost global index tracker and leave it for 20 years, the maths probably favours investing (especially via pension or ISA). If you would dip into the investment pot or pick individual stocks, overpayment is often the wiser choice.
The Liquidity Question
A mortgage overpayment is typically illiquid — you cannot get that capital back easily if you need it. Some mortgages have an "overpayment reserve" you can draw on, but many do not. ISAs are fully flexible and accessible. Pensions are not accessible until age 57 (rising from 55 from 2028).
The practical recommendation for most people: Maintain three to six months of expenses in an easy-access savings account paying 4%+ first. Then maximise pension contributions (especially if you have employer matching — that is an immediate 100% return on your contribution). Then consider ISA investing. Only after these does mortgage overpayment typically win on a pure numbers basis.
International Perspective
US financial planning advice generally emphasises pension (401k) and ISA-equivalent (Roth IRA) maximisation before extra mortgage payments, given the tax advantages. Australian financial planners similarly prioritise superannuation contributions before extra mortgage repayments for most taxpayers. The UK logic is broadly similar.
A Simple Decision Framework
| Your situation | Lean towards |
|---|---|
| Mortgage rate above 5% | Overpay |
| Mortgage rate 4–5%, no employer pension match unclaimed | ISA or pension first |
| Higher rate taxpayer with pension headroom | Pension |
| Short time to mortgage end (< 5 years) | Overpay |
| High mortgage-to-value (> 80%) | Overpay — reduces LTV and future rates |
| Large emergency fund, stable income, long horizon | ISA investing |
Key Numbers
- FTSE 100 (June 2026): ~10,512
- Best 5-year fixed mortgage: 4.51% (HSBC)
- ISA annual allowance 2026/27: £20,000
- FTSE 100 historical annual return (incl. dividends): ~7–8%
- Pension access age: 57 from 2028
Sources
- Uswitch: Current mortgage rates
- MoneySavingExpert: Savings best buys
- Curvo: FTSE 100 historical performance
- GOV.UK: Individual Savings Accounts
Educational content only — not financial advice.