Should You Overpay Your Mortgage or Invest?

I get asked this question more than almost anything else. Someone has a spare £200 a month, or they have come into some money, and they want to know: do I throw it at the mortgage or put it into investments?

I have been on both sides of this. For most of my wealth-building years, I chose to invest rather than overpay the mortgage. And I stand by that decision. But the answer is not the same for everyone, and it depends on a few things that are worth thinking through properly.

The simple maths

At its core, this is a comparison of two numbers: your mortgage interest rate and your expected investment return.

If your mortgage rate is 5% and you overpay, you are effectively earning a guaranteed 5% return on that money. No risk. No volatility. That £1,000 overpayment saves you £50 a year in interest, compounding over the remaining term. It is the financial equivalent of a risk-free investment.

If you invest instead, history suggests a global index fund returns roughly 8-10% per year over the long term. But that is an average. Some years it is up 20%. Some years it is down 30%. Over a 20-year period, the odds are heavily in your favour. Over a 3-year period, anything can happen.

So the raw maths says invest, most of the time. But raw maths does not account for tax, risk, or the fact that you are a human being with emotions.

The tax angle

This is where it gets interesting, and where the UK system actually gives investors a significant edge.

If you invest inside a Stocks and Shares ISA, every penny of growth and every dividend is completely tax-free. In 2026, you can put up to £20,000 a year into an ISA. If your investments return 8% inside an ISA, you keep the full 8%. No income tax. No capital gains tax. Nothing.

Mortgage overpayments, by contrast, do not benefit from any tax wrapper. You are simply reducing a debt. There is no tax relief on the money you use to overpay (unless it comes via salary sacrifice arrangements, which is a different conversation).

This means the gap between the two options is wider than it first appears. A 5% mortgage rate is competing against a tax-free 8% return, not a taxable one. If you are a higher rate taxpayer investing outside an ISA, the picture changes, but most people have unused ISA allowance. Use it.

The risk angle

Overpaying your mortgage is a guaranteed return. If your rate is 5%, you are getting 5%. Full stop. No market crashes, no recessions, no watching your portfolio drop 25% in a month and wondering if you have made a terrible mistake.

Investing is variable. The long-term average is roughly 8-10%, but you do not get the average every year. You get wild swings that average out over time. If you need the money in 2 years, investing is a gamble. If you have 15 or 20 years, the probability of beating your mortgage rate is very high. But it is not certain.

I was comfortable with that uncertainty. Not everyone is.

The psychological angle

Here is the bit that spreadsheets cannot capture.

Some people cannot sleep with debt hanging over them. The idea of owing £180,000 to a bank, regardless of the interest rate, causes genuine anxiety. For those people, overpaying the mortgage is not just a financial decision. It is a mental health decision.

I have spoken to readers who paid off their mortgage in their early 40s and describe it as the most freeing moment of their financial lives. They know, mathematically, that they might have been better off investing. They do not care. The weight lifted off their shoulders was worth more than the marginal return difference.

If that is you, overpay the mortgage. Seriously. The “optimal” strategy is worth nothing if it keeps you up at night.

The hybrid approach

Most people do not need to pick one or the other. The approach I recommend to most people is:

Use your mortgage overpayment allowance first. Most lenders let you overpay up to 10% of the outstanding balance per year without early repayment charges. On a £200,000 mortgage, that is £20,000. Even a modest overpayment of £200 a month uses £2,400 of that allowance and can knock years off your term.
Put everything else into an ISA. Once you have made your overpayment, direct any additional savings into a Stocks and Shares ISA invested in a global index fund. This gives you the guaranteed return of mortgage reduction plus the higher expected return of equity investing, with the tax-free wrapper on top.

This way, you are reducing your debt, building your investment portfolio, and not putting all your eggs in one basket. It is the boring, sensible answer. It also happens to be the right one for most people.

The early retirement angle

If you are pursuing financial independence, this question has an extra dimension.

Your FIRE number is based on your annual expenses. If your mortgage is your biggest expense, paying it off dramatically reduces the amount you need invested to retire. A £1,000 monthly mortgage payment is £12,000 a year. At a 4% withdrawal rate, that is £300,000 less you need in your portfolio.

So paying off the mortgage does not just save you interest. It lowers the finish line. For some people, that is the faster route to financial independence, even if the raw investment returns would have been higher.

I went the other way. My mortgage rate was low (under 2% for most of the period), so the maths overwhelmingly favoured investing. Every spare pound went into ISAs and my SIPP. The gap between 2% guaranteed and 8-10% expected was too large to ignore.

If my mortgage rate had been 5 or 6%, I would have made a different choice. Context matters.

What I would do in 2026

With mortgage rates sitting at 4-6% for most people, the gap between the guaranteed return from overpaying and the expected return from investing has narrowed significantly compared to the era of 2% mortgages.

If your rate is above 5%, the case for overpaying is strong. You are locking in a guaranteed, risk-free return that is not far off the long-term stock market average.

If your rate is below 4%, the case for investing in a tax-free ISA is still compelling.

If your rate is between 4% and 5%, it genuinely comes down to temperament. Neither choice is wrong. The hybrid approach works well here.

The one thing I would not do is leave the money sitting in a current account doing nothing. Whether you overpay the mortgage or invest, you are building wealth. Doing neither is the only wrong answer.