Guide · Mortgages & Property
Making Mortgage Overpayments: How Much Interest Can You Save?
An extra £200/month on a typical UK mortgage saves tens of thousands of pounds in interest and knocks years off the term. Here is exactly how much, whether the 10% limit applies to you, and how to weigh it against investing.
How overpayments reduce interest
Mortgage interest is calculated on the outstanding balance. Every pound you overpay reduces that balance immediately, and the interest saving compounds through the life of the mortgage — because a lower balance means lower interest each month, which means a larger share of future payments reduces the principal, which reduces the balance further.
Even small regular overpayments produce disproportionate savings because they reduce the balance early in the mortgage, when the outstanding debt — and therefore the interest being charged — is at its highest.
The impact of different overpayment amounts
Starting scenario: £200,000 mortgage, 4.5% interest rate, 25-year term, regular monthly payment of £1,111.
| Monthly overpayment | Years saved | Interest saved |
|---|---|---|
| £50/month | 1 year 11 months | £5,432 |
| £100/month | 3 years 6 months | £10,179 |
| £200/month | 6 years 4 months | £18,224 |
| £500/month | 11 years 9 months | £32,415 |
| £1,000/month | 15 years 6 months | £43,821 |
Assumes rate stays constant (it will not). Interest savings are approximate and based on overpayment starting from day one of the mortgage.
The 10% annual overpayment limit
Most fixed-rate mortgages cap penalty-free overpayments at 10% of the outstanding balance per year. On a £200,000 mortgage, that is £20,000 per year — more than enough for most overpayers. Anything beyond triggers an Early Repayment Charge.
The 10% limit resets each year on the anniversary of the mortgage or each calendar year, depending on your lender. If you are approaching the limit, consider whether it is worth waiting until the next year to continue overpaying rather than triggering an ERC.
Overpay vs invest: the decision framework
The guaranteed, tax-free return from overpaying equals your mortgage rate. At 4.5%, overpaying "earns" 4.5% risk-free. Compare that to:
- A Cash ISA at 4.5% AER — roughly equivalent, but ISA retains flexibility.
- A Stocks & Shares ISA at ~7% expected — likely beats overpaying in expected value, but with equity volatility risk.
- Pension contributions with 40% tax relief — effectively 6.7%+ risk-free return, well above the mortgage rate for higher earners.
The practical recommendation for most people: ensure you get any employer pension match first, keep a fully-funded emergency fund, then split the surplus between ISA and mortgage overpayments. Avoid over-concentrating in one or the other.
Frequently asked questions
How much can I overpay on my mortgage?
Most fixed-rate mortgages allow overpayments of up to 10% of the outstanding balance per year without penalty. Overpaying more triggers Early Repayment Charges (ERCs), which can be 1–5% of the amount overpaid. Tracker and variable-rate mortgages often have no overpayment limits. Check your mortgage terms or call your lender before making a large overpayment.
Does overpaying shorten the term or reduce monthly payments?
It depends on how your lender applies the overpayment. Most lenders default to reducing the remaining term (keeping monthly payments the same). Some allow you to choose to reduce the monthly payment instead. Reducing the term saves significantly more interest — because the balance reduces faster, compound interest charges are lower for longer.
Is it better to overpay the mortgage or invest the money?
If your mortgage rate is below the expected return from investing (e.g. mortgage at 4.5%, expected equity return 7%), investing theoretically wins. But the mortgage return is guaranteed and tax-free (no CGT on reduced interest), while investment returns are uncertain. Many people split the excess — some to overpayment, some to ISA — for a blended approach. If you have high-interest debts, clear those first.
What is a mortgage offset account?
An offset mortgage links your savings account to your mortgage. The bank calculates interest on the mortgage balance minus your savings balance. If you owe £200,000 and have £30,000 in the linked savings account, you only pay interest on £170,000. The savings earn no interest themselves but 'earn' the mortgage rate tax-free, which often beats cash savings rates — especially for higher rate taxpayers.