If you have a mortgage and spare cash, the question is always the same: should you make an overpayment on your mortgage – or invest the money instead? The answer comes down to one comparison: your mortgage rate vs. your achievable after-tax investment return.
What Is a Mortgage Overpayment?
An overpayment is an additional lump-sum payment on your mortgage beyond your regular monthly payment. It immediately reduces your outstanding balance and therefore lowers the interest you pay on all future months. Most mortgage deals allow annual overpayments of 10% of the original loan without early repayment charges.
When Overpayment Makes Sense
The rule is simple: if your mortgage rate exceeds your achievable after-tax investment return, overpayment wins. At 4.5% mortgage rate vs. a 5.5% net ETF return, investing is marginally better – but not risk-free.
Comparison: Overpayment vs. ETF Investment ($10,000 over 10 years)
Overpayment at 4.5% rate: Interest saved approx. $5,400 – guaranteed, tax-free, zero risk
ETF investment at 7% (net ~5.5% after CGT): Growth approx. $7,100 – but with market risk and tax
Three Factors That Decide
- Mortgage rate: Above 4–5%, overpayment is hard to beat with safe alternatives
- Tax: Interest saved through overpayment is tax-free. Investment gains attract Capital Gains Tax (18–24% in the UK). This reduces the investment advantage.
- Risk: Interest saved through overpayment is guaranteed. ETF returns are historically good but volatile short-term.
When Investing Is Better
- Mortgage rate below 3% – historically cheap debt, market returns likely higher
- Long investment horizon (15+ years) – time smooths market volatility
- You have no emergency fund yet – build 3–6 months' expenses first
- ISA allowance unused – tax-free investment wrapper changes the maths significantly
The Smart Combination
Many financial advisers recommend a hybrid approach: secure your emergency fund → allocate part of surplus to overpayments → rest into ISA/ETF savings plan. This gives you guaranteed interest savings plus long-term market growth.