What to do if you’ve made a mistake on your Self Assessment tax return

Whether you’ve made a mistake on your Self Assessment tax return or missed the filing deadline altogether, understanding what to do next can help limit stress, penalties and unnecessary costs

Below, Mike Parkes, technical director at GoSimpleTax, answers the most common questions beauty professionals ask about correcting errors and dealing with late submissions.

Advice if you’ve made a mistake on your Self Assessment Tax Return

Can you correct mistakes in a Self Assessment tax return after filing?
“If you realise you’ve made a mistake on your Self Assessment tax return, HMRC allows you to amend it up to 12 months after the filing deadline. For example, a 2023/24 return filed by 31 January 2025 can be corrected until 31 January 2026.”

Common mistakes
“Errors are common and can include , ticking the wrong boxes, entering figures in the wrong place, miscalculating totals, or failing to declare taxable income or expenses correctly. Some people also forget to include supplementary pages for certain income sources.”

How to make a correction
“Online returns can be amended after 72 hours, with HMRC updating the tax bill straight away. If the 12-month deadline has passed, you’ll need to write to HMRC to request a change. HMRC may also correct minor errors itself or ask for clarification.”

Penalties
“If you took reasonable care and made a genuine mistake, penalties are unlikely. Deliberate errors can lead to an investigation, a higher tax bill, and penalties.”

What to do now if you’ve missed the Self Assessment deadline for online filing

Mike Parkes, technical director at GoSimpleTax, said:

What to do if you miss the Self Assessment deadline
“If you miss the Self Assessment filing deadline, try not to panic. It’s a common issue and not a criminal matter. However, late filing does trigger automatic penalties.

"The initial fine is £100, with further penalties added the longer the return remains outstanding. These can increase significantly after three, six and twelve months.”

Appealing a late-filing penalty
“You may be able to appeal if you have a valid reason, such as a serious illness, bereavement, disability-related delays, or major technical issues when filing online.

"It’s best to submit your tax return as soon as possible before appealing. Appeals usually need to be made within 30 days of the penalty notice, and HMRC may ask for evidence.”

File your return as soon as you can
“Delaying only increases the amount you may owe. Filing promptly limits further penalties and brings the issue to an end, even if payment can’t be made straight away.”

Using estimated figures to stop penalties increasing
“If you’re struggling to finalise your figures, you can submit a return using reasonable estimates to stop further late-filing penalties. These can be updated later once accurate figures are available, as long as this is clearly stated on the return.”

Making future filings easier
“Self Assessment filing apps can help simplify the process next time. Tools such as GoSimpleTax guide users through their return and help reduce errors, with the option to have a return checked to ensure allowances and expenses are claimed correctly.”