Overpay Mortgage or Invest? What the Numbers Say
Verdict
At 5.5%, overpaying your mortgage is the stronger move right now.
Confidence: Medium
Break point: This holds while your rate stays above the investment return and horizon is under 2 years.
The rate decision
Overpaying only loses if after-tax returns clear the mortgage hurdle by enough to justify volatility.
With a mortgage rate of 5.5%, overpaying your mortgage becomes the superior financial strategy, as it effectively eliminates debt at a guaranteed return equivalent to the interest rate. This 5.5% acts as a hurdle rate, meaning any investment must exceed this rate to justify the risk; with an expected investment return of 7.0%, the gap of 1.5 percentage points indicates that overpaying the mortgage yields a more secure and immediate financial benefit. By prioritizing mortgage repayment, you not only save on interest payments but also achieve a risk-free return that outpaces the investment alternative. Therefore, directing funds towards mortgage overpayment is the more prudent choice in the current financial landscape.
The rate backdrop
Rate rises since 2021 mean the case for overpaying is materially stronger than when mortgage costs were ultra-low.
Since 2021, the Bank of England's rate rises have significantly impacted UK mortgage borrowers, with floating rates increasing by 4.5 percentage points and 2-year fixed rates rising by 2.75 percentage points, while 5-year fixed rates have seen a more modest increase of 0.75 percentage points. This environment of elevated interest rates makes overpaying your mortgage a compelling strategy, particularly at a current rate of 5.5%, as it allows borrowers to reduce their principal faster and minimize future interest payments, effectively countering the higher costs associated with variable and short-term fixed rates. By overpaying, borrowers can lock in savings that outweigh the benefits of holding onto cash in a high-rate climate, where the cost of borrowing is significantly elevated.
Worked example
Assumptions (illustrative): £200,000 mortgage · 5.5% rate · £400/month spare · 7.0% assumed return
| Year | Overpay saving | Invest profit | Who is ahead |
|---|---|---|---|
| Year 1 | £132 | £157 | Invest profit ahead by £25 |
| Year 2 | £528 | £672 | Invest profit ahead by £144 |
By year 2, investment profit is ahead by £144.
Risk-adjusted verdict: The 1.5pp gap does not clear the 2.5pp risk margin — overpaying is the risk-adjusted winner despite invest leading on raw numbers.
Raw numbers show invest ahead — but the risk margin accounts for investment volatility, sequence-of-returns risk, and the guaranteed nature of mortgage interest saved.
When this flips
This flips only when returns must exceed 8.0% (= 5.5% mortgage rate + 2.5pp margin). The minimum horizon constraint is 2 years.
What to do next
| Your situation | Action | Why |
|---|---|---|
| Deal ending within 2 years | Overpay now | Reducing balance before refinancing lowers the new rate tier |
| Rate above expected return | Overpay first | 2 years is not enough runway for investing to compensate for volatility |
| Uncertain income | Preserve liquidity | You need cash flexibility before your deal expires |
| Mixed case | Overpay to next rate tier, hold remainder | Target a balance that unlocks a better LTV band at refinancing |
Sources and provenance
- boe_mpr_2026_02.pdf
- boe_mpc_2026_03.pdf
Data as of: 2026-04-01
