How offsetting works, and why no tax falls due on the cash

This page sets out how an offset mortgage reduces the balance you pay interest on, the tax rule that means nothing is owed on the cash, and how the result compares with leaving that money in a savings account.

Use the offset, overpay or save calculator Holding a large balance? See how deposit protection applies

Why offsetting doesn't create a tax bill

Income tax is charged on interest you receive. That's set out in the Income Tax (Trading and Other Income) Act 2005, section 369 (legislation.gov.uk, read 3 October 2026), which charges tax on interest received.

Cash sitting in an offset account doesn't earn interest. The lender reduces the balance your mortgage interest is worked out on each day. Because no interest is received, section 369 has nothing to charge tax on, and no tax arises on the saving.

No HMRC page covers offset mortgages directly

HMRC hasn't published guidance naming offset mortgages specifically. The position above follows from reading section 369 against the fact that offset cash never generates interest in the first place. Where your own arrangement is more complicated, such as a mix of accounts or a change partway through the year, a tax professional can confirm how the rule applies to your figures.

Weighing this against overpaying or saving

Once you've got the mechanism straight, the next question is usually how offsetting stacks up against overpaying the mortgage or keeping the cash in a savings account instead. The Understanding Offset, overpay or save guide works through that comparison, and the offset, overpay or save calculator runs it on your own mortgage balance, rate and savings figures.

Tax band	Savings allowance	Tax rate from April 2027	Offset cash, same band
Basic rateMay also qualify for the starting rate for savings if other income is low.	£1,000 tax-free	22% on interest above the allowance	Earns no interest, so none of this applies
Higher rate	£500 tax-free	42% on interest above the allowance	Earns no interest, so none of this applies
Additional rate	£0 — the allowance does not apply	47% on all savings interest	Earns no interest, so none of this applies

Allowance and rate figures are from GOV.UK and Finance Act 2026 s5, read 3 October 2026. Check your current band and the figures that apply before using them to weigh offsetting against saving. The offset, overpay or save guide sets out how to run that comparison on your own balance and term.

How the interest charge is worked out each day

An offset mortgage works by reducing the amount of your mortgage that interest is charged on, using cash you hold in a linked savings account. The lender does not charge interest on your full mortgage balance. It charges interest on the mortgage balance minus whatever sits in the linked offset account on that day.

The balance that counts

Say your mortgage balance is a certain amount and your offset savings are a separate, smaller amount. The lender does not treat these as two unrelated figures. It subtracts the savings from the mortgage and charges interest only on what is left. The money in the offset account stays yours and stays accessible; it simply stands between you and part of the interest charge for as long as it sits there.

Why the calculation runs daily

Mortgage interest on most residential deals accrues daily. The lender takes a snapshot of your mortgage balance and your offset balance each day, works out the difference, and applies that day's rate to it. The monthly figure you see on your statement is simply those daily charges added together over the month.

This means the offset pot does not need to sit still to be useful. A balance that is there for one week and gone the next still reduced the interest charged on the mortgage for exactly those days, no more and no less.

What happens when the pot moves

Cash held against a mortgage rarely stays flat, particularly if some of it is being set aside for a future payment. The daily calculation simply follows the balance wherever it goes:

While the pot is building up, each pound added reduces the balance that interest is charged on from that day onward.
While the pot is at its highest, the reduction in interest is at its largest.
Once part of the pot leaves, for example to cover a payment due on a fixed date, the mortgage balance that counts for interest rises again from that day.

Nothing about this requires the cash to be committed or locked away. The balance the lender uses in its daily sum is simply whatever is sitting in the offset account at the moment it runs the calculation, and it adjusts automatically as that figure changes.

Checking this against your own numbers

Once you can see how the daily sum works, the next question is whether offsetting your own balance beats overpaying with it or leaving it in a savings account. The offset, overpay or save guide sets out how to weigh those three options against each other, and the offset, overpay or save tool runs the comparison on your mortgage balance, your offset rate and your tax band.

Questions about offset cash, ISAs and tax bands

Does offset cash need to be declared on a tax return?

Offset cash earns no interest, so there is no interest received to declare under the rule in ITTOIA 2005 s369, which charges income tax on interest actually paid. Nothing appears on a tax return because nothing has been paid to you. If the same money sat in a savings account instead, the interest it earned there would need declaring once it used up your Personal Savings Allowance.

Can I hold an ISA alongside an offset mortgage?

Yes, but an ISA and a linked offset account are two different things. An ISA keeps its tax-free wrapper only while the money stays inside it, and moving it into a linked offset account to reduce mortgage interest takes it out of that wrapper. Whether that trade is worth it depends on the ISA rate against the mortgage rate it would offset, which you can work out on your own figures using the calculator. Check your mortgage terms for whether the linked account will even accept money transferred from an ISA before you move anything.

What happens to the tax position if my income tax band changes?

A change in tax band does not change anything about offset cash, because it earns no interest regardless of the band you are in. It does change the comparison against a savings account: interest above your Personal Savings Allowance is taxed at your marginal rate, so moving into a higher band makes keeping the same cash in a savings account more costly, and strengthens the case for offsetting or overpaying instead. The calculator takes your current band as an input, so rerunning it after a band change will show the updated comparison.

Does offset cash use up my Personal Savings Allowance?

No. The allowance covers interest you receive, and offset cash receives none, so none of it is used by holding money in an offset account. The full allowance stays available for interest earned elsewhere, such as a separate savings account held alongside the mortgage.

Does the size of the offset balance change its tax treatment?

No. The position under ITTOIA 2005 s369 depends on whether interest is received, not on how much cash is involved, so a balance of a few hundred pounds and one of tens of thousands are treated the same way. A large balance does raise a separate question worth checking, which is how much of it is protected if the lender fails. That is covered on the page about large balances and deposit protection.

Could this tax position change in future?

The position rests on current legislation, and HMRC has not published a page addressing offset mortgages directly, so it is read from the general rule on interest received. Legislation can change, as it is already set to do with the savings tax rates from April 2027. This is not tax advice, and anyone making a decision involving real money should check the current legislation at legislation.gov.uk and speak to a tax professional.

Where can I take this further?

This page sets out the tax position on its own, not which option fits your figures. The guide Understanding offset, overpay or save walks through how offsetting, overpaying and saving compare once your own balance and tax band are added in, and the offset, overpay or save calculator lets you run those figures directly, with the formula shown beside each result.