Overpay Your Mortgage or Invest? The Maths for 2026's Rate Environment

With average mortgage rates sitting around 5.63%–5.68% on new fixed deals, according to Uswitch, and Bank Rate held at 3.75% by the Bank of England, the old "invest, don't overpay" logic from the ultra-low-rate 2010s no longer applies automatically. Here's how to think through the decision properly.

The core comparison

Overpaying your mortgage is, in effect, a guaranteed, tax-free return equal to your mortgage rate. If your mortgage rate is 5.5%, every pound you overpay saves you 5.5% in interest you'd otherwise pay — with zero risk and no tax to consider, since mortgage interest saved isn't taxable income.

Investing in a Stocks and Shares ISA, by contrast, offers a variable, uncertain return that has historically outpaced mortgage rates over long periods (UK and US stock markets have delivered average annual returns in the high single digits to low double digits over multi-decade periods) — but with real risk of loss over shorter periods, particularly the 2-5 year horizons many mortgage-holders are actually comparing against.

Why the calculation has shifted

When mortgage rates were 1.5-2.5% (common through much of the 2010s and early 2020s), overpaying was a weak guaranteed return compared to plausible investment returns, so many advisers favoured investing spare cash instead. Now that new fixed mortgage rates average 5.63-5.68%, per Which?'s rate tracker, overpaying offers a guaranteed return that's competitive with, or better than, a cautious investor's expected return — without any market risk.

Factors that tip the decision

• Your mortgage rate specifically. If you're on a cheap legacy deal well below 4%, investing looks relatively more attractive than at today's average new-deal rates.
• Your time horizon. Mortgage overpayment guarantees its return regardless of when you need the money; investment returns are far more reliable over 10+ year horizons than over 2-5 years.
• Tax wrappers available to you. Unused ISA allowance (£20,000 for 2026/27, per Hargreaves Lansdown) or employer pension matching should generally be prioritised over mortgage overpayment, since employer pension contributions offer an immediate, guaranteed "return" via matching that no mortgage overpayment can beat.
• Overpayment limits. Most lenders cap penalty-free overpayments at around 10% of the outstanding balance per year — check your mortgage terms before committing extra cash.
• Emotional value of debt freedom. Some people rationally value the certainty and psychological relief of a smaller or paid-off mortgage more than a marginally higher expected return elsewhere — that's a legitimate factor, not just a "mistake."

A sensible order of priorities

1. Employer pension matching (free money, beats any mortgage rate).
2. High-interest debt (credit cards, personal loans) — almost always costs more than your mortgage rate.
3. Emergency cash buffer (3-6 months of expenses).
4. Split between mortgage overpayment and ISA/pension investing, weighted toward whichever offers the better guaranteed-vs-expected return once your mortgage rate is known.

International comparison

The UK's typical approach — comparing a fixed-rate mortgage against a Stocks and Shares ISA — has rough equivalents elsewhere. US homeowners often face 30-year fixed mortgages at rates that, in 2026, sit in a broadly similar mid-single-digit range according to Federal Reserve commentary, and weigh overpayment against a 401(k) or Roth IRA in much the same way. Australian borrowers, largely on variable-rate mortgages, tend to favour offset accounts — which achieve a similar guaranteed-return effect to overpayment while preserving access to the cash — a structure less common in the UK market.

Key Numbers

• Average 2-year fixed mortgage rate: 5.68%
• Average 5-year fixed mortgage rate: 5.63%
• Bank Rate: 3.75%
• 2026/27 ISA allowance: £20,000
• Typical penalty-free overpayment cap: around 10% of outstanding balance per year (check your specific lender terms)

Sources

• Uswitch: UK mortgage rates today
• Which?: best mortgage rates and deals
• Bank of England: June 2026 Monetary Policy Summary
• Hargreaves Lansdown: ISA allowance 2026/2027

Educational content only — not financial advice.