How Mortgage Interest Works

Mortgage interest is the cost of borrowing money to buy your home. It is charged as a percentage of your outstanding loan balance and, over the life of a 25-year mortgage, can easily exceed the original amount you borrowed. Understanding how it works — and how to reduce it — can save you tens of thousands of pounds.

How Mortgage Interest Is Calculated

Most UK mortgage lenders calculate interest daily on your outstanding balance, then collect it as part of your monthly payment. The formula behind every mortgage payment is:

Monthly payment = P × [r(1+r)ⁿ] ÷ [(1+r)ⁿ − 1]

Where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments. In the early years of a repayment mortgage, most of each payment covers interest; over time, more goes toward reducing your balance (the principal).

Worked Example: How Rate Affects Total Cost

The difference between a 4.5% and 5.5% rate on the same mortgage looks modest month to month — but compounds dramatically over 25 years.

A 1% higher rate costs an extra £147 a month — and £44,100 more in total interest over the full term. This is why securing the best available rate, even for a modest improvement, is worth considerable effort.

Fixed vs Variable Rates

When you take out a mortgage, you choose how your interest rate is set:

• Fixed rate: Your rate is locked for a set period — commonly 2, 5, or 10 years. Your monthly payment stays exactly the same regardless of what happens to the Bank of England base rate. At the end of the fix, you typically move to the lender's Standard Variable Rate (SVR) unless you remortgage.
• Tracker rate: Follows the Bank of England base rate plus a set margin (e.g., base rate + 0.9%). If the base rate rises by 0.25%, your rate rises by 0.25% immediately. Trackers often have no early repayment charge, making them attractive if you plan to move soon.
• Standard Variable Rate (SVR): The lender's default rate once a deal ends. SVRs are typically the most expensive option and are set at the lender's discretion — not directly tied to the base rate. Most borrowers remortgage before reaching their SVR.
• Discount rate: A set percentage below the lender's SVR for a fixed period. Your payment still varies if the SVR changes, but is always cheaper than the full SVR by the discount margin.

How Overpaying Reduces Total Interest

Because interest is calculated on your outstanding balance, every pound you overpay reduces the balance — and therefore the interest that accrues from that point forward. The savings compound over the remaining term.

Example: On a £200,000 mortgage at 4.5% over 25 years, the standard monthly payment is £1,111. Overpaying by just £200 a month (total: £1,311):

• Cuts approximately 5 years off the mortgage term
• Saves approximately £23,000 in total interest
• Means you own your home outright 5 years earlier

Most lenders allow overpayments of up to 10% of the outstanding balance per year without triggering an early repayment charge (ERC). Always confirm your limit before making a large overpayment.

Mortgage Interest and Tax Relief

For residential homeowners, mortgage interest is not tax-deductible. You cannot offset it against income for tax purposes — it is simply a personal expense.

For landlords, the rules changed significantly between 2017 and 2020. Prior to April 2017, landlords could deduct 100% of mortgage interest from rental income before calculating tax. This relief was gradually removed and replaced with a flat 20% tax credit on finance costs. Higher and additional-rate taxpayers who previously benefited from 40% or 45% relief now pay considerably more tax on their rental income.

How to Reduce Your Mortgage Interest Costs

• Make regular overpayments — even £50–£100 a month adds up over a 25-year term.
• Remortgage when your deal ends — moving onto an SVR is almost always more expensive than switching to a new deal.
• Increase your deposit — a larger deposit lowers your loan-to-value (LTV) ratio, which typically unlocks better rates. Going from 90% LTV to 85% LTV can cut your rate noticeably.
• Shorten your term — a 20-year term costs more monthly than 25 years, but significantly less in total interest.
• Use a mortgage calculator — model different scenarios (rate changes, overpayments, term adjustments) before committing.