If you keep a mortgage and a cash balance at the same time, that cash is doing a job whether you've chosen one for it or not. It can sit against your mortgage and reduce what you're charged interest on, it can go towards the mortgage itself as an overpayment, or it can sit in a savings account earning interest of its own. Each does something different to the interest you pay or earn, and each does something different to whether you can get the money back when you need it. The self-employed reader setting aside a balancing payment and the saver weighing up a remortgage deal are asking the same underlying question: where should this cash sit until I need it?
What offsetting does to your mortgage interest
An offset mortgage links a savings balance to your mortgage account, so interest is charged only on the difference between the two. If you owe £150,000 and hold £20,000 in the linked account, interest is calculated on £130,000. The mechanism and the daily calculation behind it are set out in full on how offsetting works, and why there is no tax to pay.
The cash in the linked account earns no interest of its own, so there's nothing for income tax to be charged on. The position rests on ITTOIA 2005 section 369, which charges income tax on interest received, and no HMRC page addresses offset mortgages directly. The reduction in your mortgage interest isn't income, so it isn't taxed as one.
Offset cash stays yours to withdraw. If the tax reserve you're holding needs to move to cover a bill, it comes straight back out, and your mortgage interest simply goes back up to reflect the lower balance sitting against it.
What overpaying does to your mortgage
An overpayment reduces the mortgage balance itself, permanently. That saves interest over whatever term is left, and for a cash balance you're confident you won't need again, it can save more than offsetting the same amount.
The trade-off is access. Once money has gone in as an overpayment, it generally cannot be recovered. Some lenders allow further borrowing or a drawdown facility, but that isn't the same as having the cash sit untouched, and it depends entirely on your own mortgage terms. Treat an overpayment as one-way unless your agreement states otherwise in writing.
That makes an overpayment a poor home for money with a known departure date, such as a tax reserve. It's a reasonable home for cash you're satisfied you won't need again.
What saving does with the same cash
A savings account pays you interest directly, and that interest is taxable income. The Personal Savings Allowance lets a certain amount through tax-free each year, and the starting rate for savings can shelter more for lower earners, with the amount sheltered falling as your other income rises. How much of your savings interest is actually taxed depends on your income tax band.
Savings tax rates are also changing. From April 2027, the rates applied to savings interest above the allowances rise, under Finance Act 2026 section 5. That matters most to anyone comparing offset against saving over a horizon that crosses that date, because the saving side gets more expensive while the offset side doesn't change at all.
Saving keeps your cash fully accessible, usually with no notice period at all.
Why the offset rate needs a fair comparison
Offset deals typically carry a higher interest rate than the same lender's equivalent non-offset product. That higher rate is the price of having the reduction in interest work automatically against whatever cash you hold.
The comparison that tells you anything useful is like for like, at the same lender: a fix against that lender's fix, a tracker against that lender's tracker. Comparing an offset rate against an unrelated best-buy rate from a different lender answers a different question than the one you're asking. Why the offset rate is higher, compared fairly sets out how to make that comparison before you put any figures into a calculator.
Where this goes wrong
- Comparing the offset rate against the market. A higher rate looks worse than it is if you're measuring it against the wrong baseline.
- Assuming an overpayment can be undone. Unless your mortgage terms say otherwise, it can't, and that's a hard thing to discover after the money has gone in.
- Overpaying with money that has a fixed departure date. Tax reserves, a deposit for a house move, a child's school fees: anything you know you'll need back belongs in offset or savings.
- Holding a large offset balance without checking deposit protection. If the lender fails, offset savings are treated as deposits. Large balances and deposit protection explains how that protection works and where the current limit is published.
The self-employed case: cash with a departure date
Money set aside for a January balancing payment, or for payments on account in January and July, sits in an unusual position. It needs to earn something while it waits, but it also needs to still be there, in full, on the date it's due to leave. The tax pot: cash that has to leave works through what offsetting does to that balance while it sits there, what overpaying with it would mean for your access to it, and what to check in your mortgage terms about minimum balances and withdrawal rights before you decide either way.
Working it out on your own figures
The comparison between the three depends on your mortgage balance, how much term is left, the offset rate against the same lender's equivalent rate, how much cash you hold, the savings rate available to you, your income tax band, and whether any of the cash has to leave on a known date. Change any one of those and the answer can change with it.
The calculator on this site takes all of those inputs and returns the interest saving from offsetting, the interest saving from overpaying, and the savings interest after tax, side by side in pounds over your own horizon. Beside each result it states what that option does to your access to the cash. Everything runs in your browser, and nothing you enter is stored or submitted. Go to the offset, overpay or save calculator to run it on your own numbers. Like any calculator, it can only work from the figures you give it, and a wrong input gives a wrong answer.
Before you act
- Check your mortgage terms for what counts as an overpayment and whether any of it can be withdrawn once made.
- Check the offset rate against the same lender's equivalent non-offset product.
- Check which UK lenders currently offer offset products if you're shopping around, using a reference such as the UK offset mortgage lenders list.
- Check where any large balance stands against deposit protection limits if it's held outside the offset account.
This site sets out the mechanics and runs the calculation on the figures you supply. It is not financial, mortgage or tax advice, and it isn't a substitute for a qualified independent mortgage adviser or a tax professional where your circumstances are complex.