Overpaying a Fixed-Rate Mortgage: A Worked Example (UK)

Many UK lenders allow some level of overpayment during a fixed-rate period, but the rules vary significantly from lender to lender, and getting this wrong can trigger a charge that cancels out the interest you were trying to save.

This is a worked example with typical allowances and questions to ask your lender. For the full rules, including what happens if you go over your allowance and when your fixed rate ends, see can you overpay a mortgage during a fixed rate period?

Typical Overpayment Allowances

Many lenders allow up to 10% of the outstanding balance per year without penalty during a fixed-rate deal, but this varies by lender. That allowance usually resets every 12 months from your mortgage anniversary, not the calendar year — an important distinction if you're timing a lump sum.

Exceeding your lender’s allowance (often 10%) is where Early Repayment Charges (ERCs) come in. The charge varies by lender and deal, so check your mortgage offer. It is often higher earlier in a fixed deal and steps down as you approach the end of the fixed period.

Why Fixed Rates Need Careful Planning

Overpaying aggressively during a fixed period can reduce your financial flexibility or lead to unexpected costs if you misjudge your lender's specific allowance. Two things catch people out most often:

• Assuming the 10% allowance is universal. Some lenders offer more generous allowances (occasionally up to 20%), while others are stricter. This is lender-specific, not a fixed industry rule — always check your mortgage offer document or ask directly.
• Not distinguishing lump sums from regular overpayments. Some lenders treat a single large lump sum differently from the same amount spread across monthly overpayments, even though the total is identical.

A Worked Example: £300 Monthly Overpayment

On a mortgage with a £200,000 balance, a 10% annual allowance permits up to £20,000 in overpayments per year without triggering an ERC. A consistent £300 monthly overpayment totals £3,600 a year — within a 10% allowance, while still reducing your balance and the interest charged on it over the life of the fixed deal.

As an illustration, on a £200,000 balance at 4.5% with 25 years left, £300 a month could save about £47,709 in interest over the whole term and end the mortgage about 97 months (8 years 1 month) sooner, assuming the rate stays the same and ignoring fees and early repayment charges. Treat this as an estimate.

Small, consistent overpayments like this are generally the safer approach precisely because they rarely come close to the annual cap, unlike a single large lump sum that needs checking against your allowance before you commit to it.

Questions to Ask Before Overpaying

Before increasing your overpayments during a fixed-rate period, it's worth confirming with your lender:

• Does my overpayment allowance reset annually, and on what date?
• Are lump sum overpayments treated differently from regular monthly ones?
• Will my overpayments reduce my mortgage term, or can I choose to reduce my monthly payment instead?
• What is the exact ERC percentage that applies if I exceed my allowance, and does it decrease as my fixed period progresses?

If you're unsure of any of these, a mortgage broker or your lender's customer service team can confirm the specific rules attached to your deal before you make changes — the general guidance above is a starting point, not a substitute for checking your actual mortgage terms.

Try the Numbers Yourself

Run your own overpayment scenario, including checking how it compares against a typical 10% allowance, using our free mortgage overpayment calculator.

Frequently Asked Questions

What happens if I go over my lender's overpayment allowance?
You'll typically be charged an Early Repayment Charge on the amount that exceeds your allowance, usually calculated as a percentage of the excess. This can reduce or even eliminate the interest savings from overpaying.

Does the overpayment allowance carry over if I don't use it?
Generally not: with many lenders, an unused allowance doesn't roll over into the following year. It's a "use it or lose it" annual limit.

Can I overpay a fixed-rate mortgage every single month?
Yes, and doing so in smaller, regular amounts is often the easiest way to stay within your annual allowance without needing to track a single large lump sum against the cap.