It depends on your lender and product. For example, Nationwide currently uses the original loan amount for many newer products, while Halifax uses the amount owed on 1 January where its standard allowance applies.

Your normal contractual mortgage payment is not an overpayment. Extra regular payments and lump sums normally count towards an overpayment allowance, subject to your lender’s terms.

It depends on the lender and product. Some allowances run by calendar year, while others reset around a mortgage or product anniversary. The calculator only shows a reset date where the lender rule or information you provide makes that date sufficiently clear.

Do not assume it does. Some lenders explicitly say unused allowance cannot be carried into the next allowance period. Check your own mortgage terms before trying to use an allowance from a previous year.

An Early Repayment Charge may apply to some or all of the amount above your fee-free allowance. The exact charge depends on your mortgage terms, so this calculator estimates the allowance rather than trying to calculate a product-specific ERC.

Yes, some do. For example, several lenders currently publish unlimited-overpayment rules for particular tracker or variable-rate products. The calculator checks your lender and mortgage type rather than assuming every mortgage has a 10% limit.

Full repayment can be treated differently from a partial overpayment. This calculator deliberately stops before giving a normal allowance result if you tell us the payment will clear the mortgage.

Different parts can have different rates or allowances. This version does not combine them because doing so could give a misleading result.