For many people, completing a Self Assessment tax return is simply part of the annual routine. But when it’s left too late or missed entirely, the consequences can be costly. Whether you’re self-employed, a company director, a landlord or someone with additional income to declare, it’s important to understand what’s required, when it’s due, and what happens if deadlines are missed.
The key Self Assessment deadlines
The Self Assessment deadline covers two separate obligations: submitting your tax return and paying any tax owed.
For most taxpayers, both must be completed by 31st of January following the end of the tax year (which runs from 6 April to 5 April).
There is also a second payment deadline of 31st of July for those who make payments on account.
While the system allows flexibility in how and when you prepare, the deadlines themselves are fixed and HMRC penalties begin automatically once they pass.
Who needs to complete a personal tax return?
Circumstances in which you may need to submit a Self-Assessment tax return, but are not limited to, include the following:
- Individuals who are self-employed or earn over £1,000 from self-employment
- Receive untaxed income such as rental income, dividends or overseas income
- Have capital gains to report
- Have complex income or tax relief claims that can’t be handled through PAYE
Even if you believe your tax affairs are straightforward, it’s worth checking whether you still fall within Self Assessment requirements. Missing the filing obligation can lead to penalties even where the tax due is minimal.
What happens if you file late?
HMRC issues late filing penalties automatically. The first penalty applies the day after the deadline and continues to increase the longer the return remains outstanding.
The standard penalty structure is:
- one day late: £100 fixed penalty
- after 3 months: £10 per day for up to 90 days (maximum £900)
- after 6 months: an additional £300 or 5% of the tax due (whichever is higher)
- after 12 months: a further £300 or 5% of the tax due (whichever is higher)
What happens if you pay late?
Late payment is treated separately from late filing. This means that you may face penalties for unpaid tax even if your return has been submitted.
HMRC can charge late payment penalties of:
- 5% of unpaid tax after 30 days
- a further 5% after 6 months
- another 5% after 12 months
In addition to penalties, interest is charged daily until the balance is paid in full. HMRC’s interest rate is variable and is linked to the Bank of England base rate.
Can you appeal penalties?
In some cases, yes. HMRC may cancel penalties if you can demonstrate a reasonable excuse, such as serious illness, bereavement, or unexpected technical issues that genuinely prevented submission.
However, HMRC expects you to act promptly once the issue is resolved — and you’ll usually need evidence to support your appeal.
Even where an appeal is possible, relying on this is risky. Penalties are often issued automatically, and it can take time to resolve disputes once they’re on your record.
How bk plus can help
How bk plus can help
Self Assessment can appear straightforward, but it becomes more complex when you factor in multiple income sources, property, dividends, capital gains or tax relief claims. And at this time of year, it’s easy for deadlines to creep up quickly.
If you’re an existing bk plus client, you can rest assured that your Self Assessment return is being managed with the upcoming submission deadline in mind. Your usual contact will be in touch if anything further is needed or if there are any updates relating to your tax position.
If you’re not currently a bk plus client, and you need support with your Self Assessment return, we’re here to help. We can provide clear guidance on what’s required and advise on the best next steps based on your circumstances. Our aim is to make the process clear, reduce the risk of avoidable costs, and ensure your return reflects your position correctly without last-minute pressure. Contact us for further information.