The Complete Guide to Self Assessment Tax Including Key Dates and Deadlines
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Every year, millions of people across the UK suddenly find themselves staring at an HMRC letter, wondering if they need to complete a tax return, and if so, where to begin. If that sounds familiar, not alone. The self assessment tax period trips people up constantly, not because it’s impossibly complicated, but because nobody ever sat down and explained it clearly. Thus, it is important to ask a professional tax expert for advice and to understand how self assessment tax works fully.
So let’s have a detailed look at it.
Summarise in :
Table of Content:
What Is Self Assessment, and the Need to Do It?
Self assessment is the system HMRC uses to collect income tax from people whose tax isn’t automatically deducted through an employer’s payroll. If you’re self employed, a sole trader, or you earn income from multiple sources, you’ll almost certainly need to complete a self assessment tax return each year.
But it’s not just for the self employed. You might also need to file a return if you earn over £100,000, receive rental income, have untaxed savings or investment income, or claim certain tax reliefs. If you’re unsure if this applies to you, it’s worth checking with HMRC directly. It’s far better to ask than to miss a deadline and face any unforeseen circumstances.
The UK Tax Year
One of the first things that confuses people is the UK tax year itself. Unlike a calendar year, the UK tax year runs from 6th April to 5th April the following year. So the 2024/25 tax year started on 6 April 2024 and ended on 5 April 2025.
Why these dates? It’s a quirk of history going back centuries, but you don’t need to understand the history; you just need to know the dates. Once you do, the rest of the self assessment timeline starts to make sense.
Important Dates You Cannot Afford to Miss
When it comes to the self assessment deadline, there are several key dates that matter. Missing them can result in automatic penalties. Let’s have a look at these:
- 5th October: If you’re new to self assessment and need to register for self assessment, this is your deadline. Miss it and you could face penalties even before you’ve filed anything. Self employed people and anyone else who needs to complete a tax return for the first time must register by this date.
- 31st October: If you’re submitting a paper tax return rather than filing online, your deadline is 31st October following the end of the tax year. Paper returns are increasingly rare, but some people still prefer them or have specific reasons for using a paper form. Just know that this deadline is earlier than the online one.
- 31st January: The online tax return deadline falls on 31 January, and it’s also the date by which you need to pay any tax you owe. Miss this and HMRC will start charging interest immediately, plus you’ll receive an automatic £100 fine; even if you don’t actually owe any tax at all.
Filing Your Return Online vs through Paper
Most people these days file a return online, and it’s the better option than juggling through paperwork. The online form is clearer, it calculates your tax bill automatically, and you have until 31 January rather than 31 October. Unless you have a specific reason to use a paper return, filing your online tax return is the simpler route.
That said, if you do go down the paper route, make sure your paper return is received by HMRC by 31st October. “I sent it” isn’t a defence if it arrives late.
Understanding Your Tax Bill
Once you’ve submitted your assessment tax return, HMRC will calculate how much tax you owe based on your income during that tax year. Your self assessment bill covers income tax and, if you’re self employed, Class 4 National Insurance contributions, which may include paying at the higher 40% tax bracket on part of your income if you earn above certain thresholds.
Here’s where people often get caught off guard: the payment on account system. If your tax bill is over £1,000 and less than 80% of your tax was deducted at source, HMRC will ask you to make advance payments towards your next tax return as well. These are split into two instalments, and small traders should also consider how the trading allowance for taxpayers interacts with their self assessment position:
- First payment on account: due 31st January (alongside your balancing payment for the previous year)
- Second payment on account: due 31st July
So in January, you might end up paying your previous year’s balance plus the first payment for the current year in one go. It catches a lot of people out in their first year of self employment.
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What If You've Overpaid?
Good news is that HMRC will refund you. If your tax position works out in your favour and you’ve overpaid tax, you can claim a tax rebate or tax refund. This often happens when your income drops compared to a previous year and the advance payments you made were based on higher earnings, or if HMRC has instead issued you with a simple assessment tax calculation that later needs to be corrected.
If you’re owed a refund, HMRC will usually process it fairly quickly, but for that you need to have submitted your return first before they can calculate whether you’re owed anything.
Common Mistakes to Avoid
A few things that regularly cause headaches:
Not knowing your tax code.
Your tax code affects how much income tax is deducted from any employed income you receive alongside self employment earnings. If it’s wrong, you could end up owing more tax than expected, so it’s worth understanding UK tax codes and their meaning in a bit more detail.
Forgetting previous year figures.
When calculating payments on account, HMRC uses your previous year’s tax bill as a reference point. If your income fluctuates significantly year to year, it’s worth understanding how this affects your advance payments and keeping an eye on current UK tax and business updates that might change the rules or thresholds.
Missing the registration deadline.
If you became self-employed during the tax year and haven’t yet told HMRC, you need to register for self-assessment by 5th October following the end of the tax year. Don’t wait; if you work with an accountant, they’ll typically handle this via their HMRC agent services account.
A Quick Example
Say you’re a self-employed individual who earned £35,000 in the 2024/25 tax year. After applying the personal allowance and relevant tax rates, let’s say you owe £5,500 in income tax and National Insurance. Since this is over £1,000, HMRC will also ask for a first payment on account for the following year, typically half of your current bill, so roughly £2,750, due at the same time in January.
That means your January payment deadline could be around £8,250 in total. Knowing this in advance allows you to plan, rather than being blindsided.
Conclusion
The self assessment tax period doesn’t have to be a source of dread. Once you understand the key dates, know whether you owe or are owed, and give yourself enough time to complete a tax return accurately, the whole process becomes far more manageable; especially if you lean on professional tax advisory services when things get complex.
The worst thing you can do is ignore it. HMRC isn’t known for letting things slide, and the penalties stack up quickly. Whether you’re a first-time filer or a seasoned sole trader, staying on top of your assessment deadlines is one of the most straightforward ways to protect your finances.
Mark those dates. File on time. And if in doubt, ask for help because a good accountant or HMRC directly can save you a lot of stress.
FAQs
What is the filing period for self assessment?
The filing period for an assessment return starts after the end of the tax year on 5 April. You can submit your tax return online anytime until 31 January, which is also when you must pay tax if you owe tax.
What tax year is it for self-assessment?
The self-assessment tax year runs from 6 April to 5 April the following year. Your tax return online will report income earned during this period, applying the relevant new tax rates set by HMRC.
What date do you have to pay your self-assessment tax?
The main deadline to pay tax is 31 January, when your assessment return is due. If you owe tax, missing this date results in penalties and interest on the outstanding amount.
What is the basis period for self-assessment?
The basis period is the timeframe HMRC uses to assess your taxable income for a tax year. For most people, it aligns with the tax year, ensuring your tax return online reflects accurate earnings under current new tax rates.
What dates do self assessment run from?
Self assessment runs from 6 April to 5 April each year in the UK. Within this period, you must track income, complete your assessment return, and pay tax by the January deadline if you owe tax.
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