"Always overpay your mortgage" is common advice — but is it always right? The answer depends on your mortgage rate, your tax situation, and what you would do with the money instead. This guide helps you make a tax-informed decision.
The guaranteed return of mortgage overpayment
Overpaying your mortgage gives you a guaranteed, risk-free return equal to your mortgage interest rate. If your rate is 4.5%, every extra pound you pay saves you 4.5% in interest — after tax, because you are paying with post-tax money and the saving is tax-free.
The comparison: ISA investing
A Stocks & Shares ISA historically returns approximately 7–8% annualised (long-term average, not guaranteed). All growth is tax-free. So the question is: does a 7% expected (but uncertain) return beat a 4.5% guaranteed return?
Key difference: investment returns are variable. In any given year you might gain 20% or lose 15%. Mortgage overpayment is guaranteed every single time.
The pension comparison
Pension contributions get tax relief that dramatically boosts the effective return. A higher-rate taxpayer contributing £1,000 via salary sacrifice only loses ~£580 from their take-home. The £1,000 then grows tax-free. This makes pensions almost always better than mortgage overpayment from a pure financial perspective — but you cannot access the money until age 57+.
When overpaying wins
- Your mortgage rate is higher than expected after-tax investment returns
- You value the guaranteed reduction in debt and monthly outgoings
- You are close to paying off the mortgage and want the psychological benefit
- You have already maxed your ISA and pension allowances
- Your emergency fund is fully funded
When investing wins
- Your mortgage rate is low (under 3%) and investment horizons are long (10+ years)
- You have unused ISA or pension allowance
- Tax relief on pensions makes the effective cost much lower
- You are comfortable with investment risk
The hybrid approach
Many people do both: pension contributions up to employer match, then ISA, then mortgage overpayment with anything left. This maximises tax efficiency while also reducing debt.
See your tax relief on pension contributions
Use the pension tax relief calculator to see the effective cost of contributing to a pension vs. using that money for mortgage overpayment. The main calculator shows your overall marginal rate.