A personal finance expert has explained what causes bigger tax demands, and what you need to do
12:48, 15 Jan 2026
For many taxpayers January comes with a shock: you check your Self Assessment calculation and think, “How can my bill be that high?” But in a lot of cases, a bigger bill doesn’t mean something’s gone wrong — it’s often down to how Self Assessment payments are scheduled.
For some taxpayers, the January deadline can include the amount due for this tax deadline, plus an advance payment towards next year’s bill - known as payments on account. Lee Murphy, Managing Director of The Accountancy Partnership, said:
“People often think they’re paying for one year, but January can include more than that — you could be paying what you owe for this tax return, and an upfront payment towards your next bill. That’s why the total can look higher than expected.
“If you’ve had a stronger year than last year, your tax will naturally be higher and it can also affect what you’re asked to pay going forward. Small changes can have a bigger impact than people realise. It’s worth double-checking your personal allowance position and whether anything has reduced it.”
He also said some 'small' mistakes can make a big difference, like missing allowable expense or reliefs, adding: “This is really common when people are rushing. If your records weren’t complete, you may have missed legitimate expenses and that can inflate the bill.”
And you can't overlook extra income. Lee said: " Side work, interest, dividends — it’s easy to forget smaller income streams until it’s time to add them to your tax return. Missing them can throw off the total and lead to surprises.”
Double-check that the information reported to HMRC by your employer (or contractor if you’re paying CIS tax) is accurate, and reflects what you have on your payslips - and what arrived into your bank account.
Lee said: “The best thing you can do before you pay is slow down for ten minutes and understand what the payment actually includes. Once you know what you’re paying for , you can plan instead of panicking.”