Key takeaways
- Money offset against a mortgage earns the mortgage rate and is never taxed. Against a 4.82% offset tracker, a higher-rate taxpayer would need 8.03% gross from a taxed savings account to match it.
- A basic-rate taxpayer would need 6.03%, an additional-rate taxpayer 8.76%. The Bank of England put the effective rate on households' new time deposits at 4.30% in June 2026.
- Offset money is priced higher. Barclays listed a 2-year offset tracker at 4.82% and the plain 2-year tracker at 4.24% on the same sheet, dated 7 August 2026 — a 0.58 point premium.
- That premium sets a break-even offset balance of 25.9% of the mortgage at the higher rate, 42.0% at the basic rate and 23.6% at the additional rate.
- If the savings interest is already untaxed — inside the £1,000 Personal Savings Allowance or an ISA — the break-even is 111.5% of the loan, which is more than the loan.
Offset cash earns the mortgage rate, and no tax is charged on it
"Is an offset mortgage worth it?" usually collapses into a comparison of two rates: what the mortgage charges and what a savings account pays. That comparison is broken. Only one of the two numbers is taxed.
Here's the mechanism, in a lender's own words. Coventry Building Society's intermediary factsheet describes a client with a £100,000 mortgage and £20,000 in the linked account who will "Only pay interest on £80,000". The £20,000 earns nothing. It cancels interest instead.
Coventry's factsheet then states the tax consequence directly: "The money in the Offset savings account won't earn interest because it's offsetting the interest payable on the mortgage. But it also means there's no income tax liability on those savings and it doesn't count towards their Personal Savings Allowance." Yorkshire Building Society puts the same point in one sentence for retail customers: "Because you don't earn interest, you won't have to pay tax on it."
So a pound parked against the mortgage earns exactly the mortgage rate, and HMRC never sees the return. That makes the offset yield a net number — money already in your pocket. A savings rate is a gross number, taxed before it reaches you. To compare them you have to gross the offset up, and that conversion is where most of the interest in this question lives.
Grossing up: 4.82% offset is 8.03% for a higher-rate taxpayer
Start from a published product rate rather than a round number. Barclays' intermediary mortgage rate sheet, dated 7 August 2026, lists a "2 Yr Offset Tracker at BEBR +1.07% 4.82% £1,749 75%" — a two-year tracker at base rate plus 1.07 points, giving 4.82%, with a £1,749 product fee, up to 75% loan-to-value. Barclays doesn't expand "BEBR" anywhere in the sheet. The Bank of England held Bank Rate at 3.75% on 30 July 2026, and 3.75 plus 1.07 is 4.82, so the tracker is priced off Bank Rate.
The conversion is one division. The taxable-equivalent rate is the offset rate divided by one minus your marginal rate on savings income. GOV.UK sets those marginal rates for the 2026 to 2027 tax year, which runs from 6 April 2026 to 5 April 2027, at 20% on taxable income from £12,571 to £50,270, 40% from £50,271 to £125,140, and 45% above that.
Run 4.82% through it and you get 6.03% for a basic-rate taxpayer, 8.03% at the higher rate and 8.76% at the additional rate. The chart plots those three against what cash was actually earning. Nothing about that is clever. It's the same sum an adviser does for a corporate bond against a gilt, applied to a product almost nobody thinks of as an investment.
The same conversion works on rates that aren't a single lender's. The Bank of England put the effective interest rate on newly drawn mortgages at 4.35% in June 2026, and on the outstanding stock at 3.96%. Grossed up at the higher rate, those are 7.25% and 6.60%. If you already hold an offset and pay something like the stock rate, the cash inside it is doing the work of a 6.60% savings account.
The savings market isn't paying 8.03%. The average is 4.30%.
The reason the conversion matters is that the grossed-up numbers land outside the range of things you can actually buy.
The Bank of England's Money and Credit release for June 2026, published on 29 July 2026, reported the effective interest rate paid on individuals' new time deposits at 4.30%. That's the average rate on money newly locked away for a fixed term. The effective rate on the outstanding stock of sight deposits — instant-access money — was 1.65%.
So the higher-rate taxpayer's requirement of 8.03% sits 3.73 percentage points above the average rate on new time deposits. Nothing in the Bank of England's deposit data comes within reach of it. That gap, not any argument about discipline or flexibility, is the whole case for offsetting.
The catch: offset money is priced higher, and you pay for it on the whole loan
Yorkshire Building Society says so on its own product page: "the interest rate on an offset mortgage is likely to be higher than on a non-offset mortgage." Quantifying that premium is the part that decides the question.
The Barclays sheet lets you measure it cleanly, because it prices both products side by side on the same day, under the same terms. Against the offset tracker at 4.82%, the equivalent plain product is listed as "2 Yr Tracker at BEBR +0.49% 4.24% £999 75%". Same lender, same 7 August 2026 sheet, same two-year term, same 75% loan-to-value band, same absence of an early repayment charge. The difference is 0.58 percentage points on the rate and £750 on the fee. Barclays' Premier version, for customers with a qualifying banking relationship, is 0.15 points cheaper at 4.67%, so the premium isn't uniform even within one lender. The five-year offset tracker is listed at 4.85%.
The asymmetry is the point. The premium is charged on the entire mortgage balance. The benefit is earned only on the cash you actually park. Offset £1 against a £100,000 loan and you're paying 0.58 points more on £100,000 to earn 4.82% on £1.
The break-even balance: 25.9% of the loan at the higher rate, 42.0% at the basic rate
There's a balance at which the two effects cancel, and it's worth writing out. Offsetting wins when the premium on the whole loan is smaller than the extra return on the offset cash. In symbols, when the offset balance divided by the mortgage balance exceeds the rate premium divided by the gap between the offset rate and your after-tax savings rate.
Using the Barclays pair and the 4.30% deposit rate, the after-tax savings rate is 3.44% for a basic-rate taxpayer, 2.58% at the higher rate and 2.37% at the additional rate. That gives break-even offset balances of 42.0%, 25.9% and 23.6% of the mortgage.
Yorkshire Building Society's own customer example puts £30,000 against a £100,000 mortgage, so interest is charged on £70,000. That's 30.0% of the balance. On the pricing above, a household holding that position clears the higher-rate break-even of 25.9% and misses the basic-rate one of 42.0%. Two people with identical balance sheets, opposite answers, because of a tax band.
The £750 fee moves the break-even further than the tax band does
Rate premiums are quoted in points and fees are quoted in pounds, which makes them easy to compare badly. Convert the fee.
The £750 fee difference, spread over a two-year deal, is £375 a year. On a £100,000 mortgage that's another 0.375 percentage points, taking the effective premium from 0.58 to 0.955 points. The higher-rate break-even moves from 25.9% to 42.6% of the balance. That is a slightly bigger shift than the one between tax bands, which runs from 25.9% at the higher rate to 42.0% at the basic rate.
Because the fee is fixed and the premium is proportional, the arithmetic runs the other way as the mortgage grows. On a large loan the £375 a year barely registers; on a small one it dominates. Any break-even figure quoted without a mortgage size attached is missing half the calculation.
The objection that does the most damage: your savings may already be untaxed
The strongest case against everything above doesn't dispute the arithmetic. It points out that the tax the conversion assumes may not be payable at all.
GOV.UK's guidance on tax on savings interest sets a Personal Savings Allowance of £1,000 for basic-rate taxpayers, £500 for higher-rate taxpayers and £0 at the additional rate. There's also a starting rate for savings of up to £5,000, which disappears entirely once other income reaches £17,570. And ISA interest sits outside the system altogether: "Savings in tax-free accounts like Individual Savings Accounts (ISAs) and some National Savings and Investments accounts do not count towards your allowance." Our guide to UK dividend and savings tax works through how those allowances stack against income.
Rerun the break-even with a tax rate of zero and the answer changes completely. The offset rate of 4.82% then beats an untaxed 4.30% by only 0.52 points, against a 0.58 point premium charged on everything. The break-even balance comes out at 111.5% of the mortgage, which is more than the mortgage. It cannot be reached.
At £1,000 of allowance and a 4.30% rate, roughly £23,000 of savings fits inside the basic-rate allowance before a penny of tax is due. A basic-rate taxpayer whose cash sits below that line gets no tax benefit from offsetting at all — and still pays the premium. This is the case where the product is sold hardest and works least.
What this comparison cannot tell you
Every rate here is one lender's sheet on one day. Barclays' document says the rates "are correct at the time of going to print", and offset premiums move product by product and lender by lender. The 0.58 point figure is a measurement, not a constant. Reprice it against whatever pair is actually in front of you.
The 4.30% deposit rate is an average of new time deposits, not a rate you're guaranteed to be offered, and it moves monthly. A specific fixed-rate bond or a competitive easy-access account can sit above or below it, which shifts every break-even in this piece.
Two further limitations sit outside the arithmetic. The first is protection. Coventry's factsheet notes the Financial Services Compensation Scheme "protects your client's eligible deposits with Coventry Building Society up to a total of £120,000". Yorkshire Building Society is blunter about what happens in a failure: "If Yorkshire Building Society were to fail, funds held in an Offset savings account would not be offset against the linked Offset mortgage account." A large offset balance is a concentrated deposit with one institution, which is the same question our note on FSCS protection limits works through for ordinary savings.
The second is the ceiling. Coventry's document states that if the offset balance exceeds the mortgage, "Your client won't receive any benefit or interest on the excess amount". The offset return is capped at the loan balance and then falls to zero, which no savings account does.
None of this covers the liquidity question either. Offsetting and overpaying produce similar interest savings, but an overpayment can't be reversed. Coventry's factsheet says as much: "an overpayment can't be withdrawn after it's been paid into the mortgage account". That is the whole difference between the two routes: offsetting buys the same interest saving and leaves the money where you can reach it. Whether either of them beats putting the cash into the market is a separate question, and our guide to mortgage overpayment versus investing works the hurdle rate out at each tax band. Where cash is genuinely an emergency reserve, the comparison in our piece on where emergency cash sits matters more than the last few basis points of yield.
What would change the conclusion
Three things would move it, and none of them is the mortgage rate on its own.
If offset premiums compressed toward zero, the break-even balance would collapse with them, and the product would be close to free optionality. The 0.58 points measured here is what one large lender charged on 7 August 2026, not a structural constant, and it is the single figure most worth re-checking.
If the savings market outran mortgage pricing — if the 4.30% on new time deposits rose above the 4.82% on offset money — the offset return would stop being the higher gross number, and only the tax shield would be left to argue with. At a 0% effective tax rate, that argument already fails.
And if your marginal rate on savings changes, so does the answer, without you touching anything. A pay rise across £50,270 halves the Personal Savings Allowance from £1,000 to £500 and lifts the conversion from 6.03% to 8.03%. The offset product doesn't reprice when that happens. The question of whether it's worth holding does.
The number worth tracking isn't the mortgage rate. It's the share of the loan your cash actually covers, measured against the break-even the premium implies — 25.9% at the higher rate on these figures, and further away than most people assume at the basic rate.