Common Self Assessment mistakes that change your bill
Most Self Assessment errors aren't dramatic. They're small slips that either cost you money you didn't need to pay, or trigger a penalty you could have avoided. Here are the ones that come up again and again.
1. Missing the deadline
File online late and there's an automatic £100 penalty, even if you owe nothing. Miss it by longer and daily penalties and interest pile on. The deadlines don't move, so put them in the calendar: see the key dates.
2. Forgetting payments on account
The classic first-year shock. If your bill is over £1,000, HMRC asks you to pay towards next year's tax in advance, in two instalments. New sole traders often budget for the tax they owe and then get hit with an extra 50% on top in January. Understanding payments on account before they land is the single best way to avoid a cash-flow crisis.
3. Under-claiming expenses
Every legitimate cost you forget is profit you're taxed on unnecessarily. People routinely miss home-working costs, mileage, software subscriptions and bank charges. Our guide to allowable expenses lists the main categories worth checking.
4. Over-claiming, or claiming the wrong things
The opposite error is just as costly if HMRC checks. Client entertaining, everyday clothing and the personal share of mixed-use costs aren't allowable. Claiming them can turn a routine return into an enquiry.
5. Leaving out income
All taxable income belongs on the return, not just your main trade: savings interest, dividends, a bit of rental income, income from a side gig. HMRC receives data from banks, platforms and other sources, so gaps get noticed. If you're unsure whether something counts, it's safer to include it and check.
6. Forgetting allowances and reliefs
Marriage Allowance, higher-rate pension relief, Gift Aid and the trading and property allowances all get missed. Higher-rate pension relief is a common one. The basic-rate relief is automatic, but the extra relief for higher-rate taxpayers has to be claimed through the return.
7. Simple number errors
Transposed digits, a figure in the wrong box, mixing up gross and net. These are easy to make when you're rushing at 11pm on 31 January, which is exactly why filing earlier, when you can check calmly, prevents so many of them.
8. Not keeping records
You don't send receipts with the return, but you must be able to produce them if asked, generally for five years after the filing deadline. No records means no way to defend a claim if HMRC queries it.
9. Assuming last year's numbers still apply
Rates and allowances change. The Dividend Allowance and Class 4 rate have both moved recently. A plan that was right two years ago can be wrong now, which is why the estimator keeps separate figures for each year rather than reusing one set.
The theme
Almost all of these come down to two habits: file early enough to check your work, and understand the calculation well enough to spot when a number looks wrong. Do those two things and most of the list takes care of itself.