Guides / Mortgage Overpayment
Should You Overpay Your Mortgage?
Interest Savings, ERC Rules & How the Maths Works
Overpaying by even £100–£200 a month can cut years off a typical mortgage and save tens of thousands of pounds in interest. Use our calculator to see your exact numbers.
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A mortgage overpayment is any amount you pay above your required monthly repayment. Because mortgage interest compounds on your outstanding balance, even small regular overpayments reduce future interest charges every single month — and that saving grows larger over time. On a typical 25-year repayment mortgage of £200,000 at 5%, overpaying by just £200 per month saves over £24,000 in interest and cuts more than four years off the term.
How Mortgage Interest Works
On a repayment mortgage, your monthly payment covers two things: interest charged on the outstanding balance, and a capital repayment that reduces what you owe. In the early years, most of your payment goes towards interest because the balance is high. As the balance falls, an increasing proportion goes to capital.
Interest is calculated monthly on the outstanding balance. If you owe £200,000 at an annual rate of 5%, the interest for the first month is:
£200,000 × (5% ÷ 12) = £833 interest in month 1
Monthly repayment (25yr term): £1,169
Capital paid: £1,169 − £833 = £336
Next month's balance: £199,664 — and the interest charge drops slightly
This gradual process — called amortisation — is why overpaying early in the mortgage is particularly powerful. By reducing the balance now, you reduce the interest charged on every future month.
How Overpayments Save Interest
When you make an overpayment, the extra amount reduces your outstanding balance immediately. From that point on, interest is calculated on a lower figure — and that lower interest charge compounds month after month, year after year.
Example: £200,000 at 5%, 25 years
Base monthly payment: £1,169
Total interest without overpayment: £150,754
With £200/mo overpayment:
— Term reduces by ~4 years 3 months
— Total interest paid: ~£126,400
Interest saved: ~£24,350 — from just £200 extra per month
The compounding effect means the saving is much larger than the total extra cash you put in. Overpaying £200/month for ~21 years (reduced term) means you contribute about £50,000 extra — and save £24,000 in interest on top of that.
Early Repayment Charges Explained
If you are on a fixed-rate, tracker or discount deal, your lender can charge an Early Repayment Charge (ERC) if you overpay beyond their permitted threshold. ERCs exist because lenders lock in their funding costs when they set a fixed rate — if you repay early, they lose the interest they were counting on.
Key facts about ERCs:
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Free overpayment allowance: Most lenders allow 10% of the outstanding balance per year without any ERC. Some offer 20% or more.
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ERC rate: Typically 1–5% of the overpaid amount (the excess above your free allowance). Usually reduces each year of the fixed term.
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ERC period: ERCs only apply during the introductory deal period (e.g. 2-year or 5-year fix). Once you revert to the standard variable rate, you can usually overpay freely.
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Where to find your terms: Your mortgage offer letter sets out your specific ERC rate, free allowance and when the ERC period ends.
Even with an ERC, overpaying within the free 10% allowance is usually worthwhile. Our calculator lets you model both scenarios — with and without ERC — so you can compare the net saving.
When Overpaying Makes Sense
Overpaying is not always the right choice. Here is a simple framework:
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Clear high-interest debt first. Credit cards and personal loans typically charge 15–30%+ interest — far more than any mortgage rate. Pay these off before overpaying your mortgage.
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Keep an emergency fund. Financial advisers typically recommend 3–6 months of expenses in an accessible savings account before making overpayments.
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Compare your mortgage rate to savings rates. If savings accounts pay more than your mortgage rate, you might be better off saving. In practice, savings interest is taxable (above the Personal Savings Allowance) while mortgage interest is an unavoidable cost.
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Consider pension contributions. If your employer matches pension contributions or you are a higher rate taxpayer, pension saving often beats mortgage overpayment on pure numbers — but mortgages offer a guaranteed, risk-free return.
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Psychological value of being mortgage-free. Many people value the security of owning their home outright, which has genuine financial and mental wellbeing benefits that numbers alone do not capture.
Interest-Only Mortgages
On an interest-only mortgage, your monthly payment covers only the interest — your balance stays the same unless you make overpayments. Overpayments on interest-only mortgages directly reduce the principal, which in turn reduces your monthly interest charge immediately.
The maths is arguably even more compelling for interest-only mortgages: every pound overpaid reduces the balance on which interest is perpetually charged, and your required monthly payment falls (though most people keep paying the same amount, which accelerates repayment further).
Frequently Asked Questions
How does overpaying a mortgage save interest?
Mortgage interest is calculated on your outstanding balance each month. Every extra pound you pay reduces that balance, so less interest accrues in subsequent months. Over a 25-year mortgage, the compounding effect means small regular overpayments can save many times their value in interest.
What is an Early Repayment Charge (ERC)?
An Early Repayment Charge is a fee your lender can charge if you overpay beyond an agreed limit (typically 10% of the outstanding balance per year) during a fixed-rate deal. ERCs typically range from 1–5% of the overpaid amount and decrease as you approach the end of the fixed term.
How much can I overpay without an ERC?
Most lenders allow overpayments of up to 10% of the outstanding balance per year without any ERC. Some are more generous. Check your mortgage offer letter or call your lender to confirm your specific allowance.
Should I overpay my mortgage or invest instead?
This depends on your mortgage rate vs expected investment returns. If your mortgage rate is 5%, overpaying gives you a guaranteed 5% return (interest saved). Investment returns are higher on average but not guaranteed and come with tax implications. Many people choose to split the difference or clear the mortgage first for the certainty it provides.
Is it better to overpay monthly or in a lump sum?
Both work. A lump sum overpayment has an immediate impact on the balance and starts saving interest right away. Regular monthly overpayments are easier to sustain for most people and add up significantly over time. The total saving from both approaches is roughly equivalent if the total amount overpaid is the same.
Model your exact savings with ERC, repayment or interest-only support.
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