HMRC issued penalties to over a million taxpayers who missed the January Self Assessment deadline last year, and late filing is only the most visible mistake.
Plenty of returns arrive on time but wrong, and wrong returns cost money in overpaid tax, penalties, or an enquiry letter eighteen months later. These are the ten errors we see most often, roughly in the order they happen.
1. Registering Too Late, or Not at All
If you had untaxed income in the 2025/26 tax year, you needed to tell HMRC by 5 October 2026. New sole traders, landlords with their first rental income, and higher earners hit by the Child Benefit charge miss this constantly, because nobody sends you a form. You have to know to ask. Set the reminder the day the income starts, not the following autumn.
2. Leaving Filing Until Deadline Week
Late January is when HMRC's systems slow down, accountants stop taking new work, and you discover your Government Gateway password doesn't work. Activation codes for new accounts arrive by post and take up to ten days. The full list of filing and payment dates sits in our Self Assessment deadlines guide for 2026/27. File in November and January becomes just another month.
3. Forgetting Income Sources
Bank interest, dividends from a share portfolio, a few weeks of freelance work, rental income from a house share, crypto disposals. HMRC receives data from banks, platforms and letting agents, and its systems match that data against returns. Omissions get noticed, and "I forgot" is not a defence that reduces a penalty.
4. Claiming Expenses You Can't Support
Round numbers with no receipts behind them are an enquiry trigger. The rule for the self-employed is that costs must be wholly and exclusively for the trade, and estimates only survive if the records behind them exist. Keep digital copies as you go; reconstructing a year of receipts in January doesn't work.
5. Claiming Both the Trading Allowance and Expenses
You can deduct the £1,000 trading allowance or your actual expenses, never both. If your real costs exceed £1,000, claim the costs. Below it, take the allowance. People tick both boxes every year and HMRC's software bounces the return or, worse, quietly accepts it and queries it later.
6. Ignoring Payments on Account
The first year your bill exceeds £1,000, HMRC usually wants one and a half times the amount by 31 January: the year's tax plus a 50% advance on the next. Thousands of first-time filers budget for the bill they can see and get flattened by the one they can't. Check whether payments on account apply to you before January, while there's still time to save for it.
7. Using the Wrong Figures From Employment
Copying a March payslip instead of the P60, missing the P11D benefits figure, or forgetting a job you left in May. Employment figures feed straight into your tax calculation, and mismatches with what your employer reported are the easiest error for HMRC to spot automatically.
8. Missing the High Income Child Benefit Charge
If your income tops £60,000 and your household claims Child Benefit, some of it comes back through your return. Taxpayers who don't realise this file a clean-looking return and receive a bill with interest two years later. The taper runs to £80,000, so even modest pay rises can pull you in.
9. Paying Late Even After Filing on Time
Filing and paying are separate deadlines that happen to share a date. Interest starts the day after 31 January, and a 5% surcharge lands on tax still unpaid at the start of March. If you can't pay in full, set up a Time to Pay arrangement with HMRC before the deadline rather than after; it stops surcharges, though not interest. Walking through the whole process, from registration to payment, is what our complete Self Assessment guide is for.
10. Doing It All Yourself When the Return Got Complicated
A straightforward employment-plus-a-little-interest return is fine to self-file. Once you add rental property, foreign income, capital gains, or a limited company alongside self-employment, mistakes get expensive fast. An accountant's fee is itself tax-deductible for the self-employed, and a professional review typically finds more than it costs. That's the everyday work of our personal tax and Self Assessment services.
Avoiding All Ten at Once
Nearly every mistake on this list comes from starting too late. Register early, keep records as the year runs, and treat October as your private filing deadline. If this year's return already looks messier than you'd like, request a callback from ASPIRE UK and have an ACCA-qualified accountant take it off your desk before the January rush.
