- Individuals with untaxed income must file a Self Assessment tax return each tax year and meet strict HMRC deadlines.
- Online filing requires a UTR, Government Gateway access, and complete income and expense records for the year.
- Late filing or payment triggers automatic penalties and interest, with limited options to amend or defer payments.
A self assessment tax return is how HMRC collects Income Tax from individuals with income not fully taxed through PAYE. This includes sole traders, freelancers, landlords, company directors, and anyone with untaxed income. Each tax year follows a fixed filing cycle, with strict registration, submission, and payment deadlines that apply every year.
If you want the process handled end to end, Sleek manages registration, calculations, submissions, and HMRC correspondence through our Self Assessment tax return service. Otherwise, understanding the filing requirements, timelines, and information HMRC expects is essential to staying compliant and avoiding penalties.
Filing a Self Assessment tax return involves confirming whether you need to file, gathering income and expense records, submitting the return online through HMRC, and paying any tax due by the deadline. The process repeats annually and errors or delays automatically trigger penalties, interest, or compliance follow-ups.
Who needs to file a self assessment tax return?
You must file a self assessment tax return if HMRC cannot collect all your tax automatically.
You usually need to file if you are:
- Self employed with income over £1,000.
- A company director with income outside PAYE.
- A landlord receiving taxable rental income.
- Earning untaxed income from dividends or savings.
- Earning over £100,000 in total income.
- Issued a notice to file by HMRC.
HMRC reviews this obligation each tax year. Changes in income can start or stop the requirement.
File early once the tax year ends on 5 April. Early submission confirms your tax bill sooner, flags payments on account in advance, and removes last-minute deadline risk if HMRC queries your return.
How do I register for self assessment?
For a full explanation of how to register for self assessment, read our step-by-step guide on how to register for self assessment.
What income must be reported on a tax return?
Your tax return must include all taxable income for the year, even if some tax was already deducted.
This commonly includes:
- Self employed trading income.
- PAYE income above thresholds.
- Dividend income.
- Savings interest.
- Property income.
- Capital gains.
- Foreign income.
HMRC cross-checks returns against third-party data. Missing income often leads to follow-up queries.
What is the self assessment deadline?
The self assessment deadline follows the UK tax year, which runs from 6 April to 5 April.
Date | What happens |
5 October | Register for Self Assessment if filing for the first time |
31 October | Paper tax return deadline |
31 January | Online filing deadline and tax payment due |
31 July | Second payment on account due |
Missing any deadline triggers automatic penalties and interest.
What do you need before filing your tax return?
Before you start your self assessment, HMRC expects your records to be complete.
You will need:
- Your Unique Taxpayer Reference.
- Your National Insurance number.
- Government Gateway login details.
- Income records for all sources.
- Expense records where applicable.
If you’re not sure where to find your UTR Number, we have a guide on that too.
How do you file a self assessment tax return online?
Most people file online using HMRC’s digital service.
- Log in to your HMRC account and select Self Assessment.
- Complete the SA100, the main tax return form.
- Add supplementary sections if prompted, such as self employment or property income.
- Review HMRC’s calculation and submit the return.
HMRC sets out the official online process on its page for filing a Self Assessment tax return.
How is your tax bill calculated?
HMRC calculates your tax bill after submission based on your declared income and reliefs.
The calculation takes into account:
- Total taxable income.
- Allowable expenses.
- Income tax bands and rates.
- National Insurance contributions.
- Tax already paid.
If payments on account apply, HMRC includes them automatically in the calculation.
How do you pay your self assessment tax bill?
Payment is usually due by 31 January following the end of the tax year.
You can pay HMRC using:
- Online or telephone banking.
- Debit or corporate credit card.
- Direct Debit.
- A Budget Payment Plan for advance payments.
HMRC lists accepted methods and processing times on its page for paying your Self Assessment tax bill.
What happens if you file late or make a mistake?
HMRC applies penalties automatically if you miss the self assessment deadline, even if no tax is owed. Interest applies to late payments from the day after the deadline.
If you submit incorrect information, you can amend your return within the permitted window through your HMRC account. Corrections update your tax bill immediately.
We go into a lot more detail in what sort of fines you can expect from HMRC and more in our ultimate guide to penalties and fines.
How Sleek handles your self assessment tax return
Sleek manages the Self Assessment process as a repeatable annual system. Income collection, expense review, SA100 preparation, submission, and HMRC communication are handled each tax year, with deadlines tracked automatically.
This removes last-minute filing risk and ensures your tax return, tax bill, and ongoing compliance stay aligned year after year.
Disclaimer: The preceding information is not legal advice. This content is aimed to provide general guidance. For more formal or legal advice, contact Sleek directly.
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FAQs on how to file a self assessment tax return
Can I file a self assessment tax return myself?
Yes. HMRC allows you to file your own self assessment tax return online using your Government Gateway account. You are responsible for entering accurate figures, completing the SA100, and submitting it by the deadline.
If you are unsure about the process or want to avoid errors, many people compare this with using professional support. See how much accountants charge for self assessment to understand the difference.
When can I submit my self assessment tax return?
You can submit your self assessment tax return as soon as the tax year ends on 5 April. Online filing remains open until the 31 January deadline. Filing early confirms your tax bill sooner and reduces deadline risk, even though payment is still due later. HMRC aligns this process with the wider tax calendar each year, which you can see in UK tax year dates.
Can I amend my self assessment tax return after submitting it?
Yes. HMRC allows amendments to a submitted self assessment tax return within the permitted amendment window, usually up to 12 months after the filing deadline. Amendments are made online and replace the original figures automatically. Any change updates your tax bill immediately. This applies whether the correction increases or reduces the tax you owe.
Do I need to file if I am employed and self employed?
Yes, in many cases. If you are employed through PAYE and also earn more than £1,000 as self employed income, you must file a self assessment tax return. HMRC combines both income sources to calculate your final tax position. This is common for freelancers with mixed income, which is explained in can you be employed and self employed.
What happens if I no longer need to file a tax return?
If your circumstances change and you no longer meet HMRC’s criteria, you must inform HMRC rather than simply stopping. HMRC may continue issuing notices to file until records are updated. Ignoring a notice can still trigger penalties. This often applies when people stop trading, which is covered in stop being self employed.
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How long do I need to keep self assessment records?
HMRC requires you to keep self assessment records for at least five years after the 31 January filing deadline for the relevant tax year. Records include income, expenses, and supporting documents. HMRC can request these during compliance checks. Keeping organised records also makes future filings faster and more accurate.
Does Making Tax Digital replace Self Assessment?
No. Making Tax Digital does not fully replace Self Assessment. It changes how certain self employed individuals and landlords report income by requiring digital records and regular updates. An end-of-year submission still applies for those within scope. You can see how this works in practice in Making Tax Digital guide for small businesses and MTD for Income Tax.
