- 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.
Knowing how to file a Self Assessment tax return matters the moment your untaxed income crosses HMRC’s thresholds, whether that’s freelance work, rental income, or a side business alongside a salary.
The process has four parts: checking if you need to file, registering for a Unique Taxpayer Reference, completing the return online, and paying what you owe. If you’d rather hand the whole thing over, Sleek’s Self Assessment service files returns for hybrid workers and sole traders.
How do you file a Self Assessment tax return in the UK?
Filing a Self Assessment tax return breaks into a sequence that rarely changes from year to year: confirm you’re in scope, tell HMRC you need to file, wait for your reference number, then report your income and pay.
The earlier you start, the more room you have to fix problems like a lost UTR or a Government Gateway lockout. Leaving it to late January is where most of the stress, and the penalties, come from.
Do you need to file a Self Assessment tax return?
Most people taxed entirely through PAYE don’t need to file a Self Assessment tax return, and HMRC actively removes people from the system when their affairs are simple. You generally need to file if your gross income from self-employment was more than £1,000 in the tax year, or you received untaxed income from property, dividends, or savings that HMRC can’t collect through your tax code.
The £1,000 figure is the current trading allowance threshold. A higher £3,000 reporting threshold has been announced but doesn’t take effect until the 2027/28 tax year, so for now £1,000 still applies. If you’re a freelancer weighing this up, our guide to Self Assessment for freelancers walks through the common triggers.
Some situations pull you in regardless of the £1,000 figure. You must file if you need to pay the High Income Child Benefit Charge, if you earned over £150,000 through PAYE, or if you want to claim certain reliefs or pay voluntary Class 2 National Insurance to protect your State Pension. When in doubt, HMRC’s online checker tool settles it in a couple of minutes.
What are the key Self Assessment deadlines you can’t miss?
Three dates govern the whole Self Assessment process. You must tell HMRC you need to file by 5 October following the end of the tax year, submit your online return by 31 January, and pay any tax owed by that same 31 January.
Paper returns carry an earlier 31 October deadline, though almost everyone now files online. If you also make payments on account, a second instalment falls due on 31 July. If you’re weighing up doing it yourself versus paying for help, our guide on what accountants charge for Self Assessment sets out the typical costs.
How do you register for Self Assessment and get a UTR?
Registering for Self Assessment is how HMRC issues your Unique Taxpayer Reference, the 10-digit number you need before you can file anything. You register online through GOV.UK, choosing the route that matches your situation: sole trader, not self-employed, or partner.
HMRC then posts your UTR, which can take up to 10 working days, or longer if you’re abroad. Because of that lag, registering close to the 5 October deadline is risky. Our step-by-step guide to registering for Self Assessment covers each screen in order.
Before you start, have a few things ready: your National Insurance number, details of the business or income source, and the date it began. If you’ve filed before but can’t find your UTR, it appears on previous returns and HMRC letters, or inside your online tax account, so check there before requesting a new one.
How do you file your Self Assessment return step by step?
Filing the Self Assessment return itself is a guided online form once you have your UTR and a Government Gateway account. Sign in, select the relevant tax year, and HMRC tailors the sections based on your answers about income type.
Work through each section in order, enter your figures, claim your allowable costs, and the system calculates the tax owed before you submit.
- Sign in to your Government Gateway account and open the Self Assessment service.
- Select the tax year you are filing for.
- Complete the income sections that apply to you, using your records.
- Enter allowable expenses or claim the trading allowance, whichever suits.
- Review HMRC’s calculation, then submit and note your confirmation reference.
You can save a partly finished return and come back to it, which helps if you’re still chasing a figure from a bank or platform.
Set aside roughly 20 to 30 percent of your untaxed income in a separate account as you earn it. The 31 January bill is far less painful when the money is already waiting.
What expenses can you claim on a Self Assessment return?
Claiming allowable expenses on a Self Assessment return reduces the profit you pay tax on, so it pays to get this right. Sole traders can deduct costs incurred wholly and exclusively for the business, from software and equipment to a proportion of home and travel costs.
Alternatively, you can claim the £1,000 trading allowance instead of itemising, which is simpler if your costs are low. For the full breakdown of what qualifies, see our guide to Self Assessment expenses, and to see how profit maps to what you owe, check the current UK tax brackets for freelancers.
If you want to estimate the bill itself, our guide to how much tax you’ll pay when self-employed is a good starting point.
How does filing a Self Assessment return change under Making Tax Digital?
Making Tax Digital for Income Tax is now in effect, having gone live on 6 April 2026 for sole traders and landlords with qualifying income over £50,000. Those affected keep digital records and send HMRC quarterly updates through compatible software, rather than relying on a single annual return.
The threshold drops to £30,000 from April 2027 and £20,000 from April 2028, so the group in scope keeps widening. A final declaration after the tax year replaces the traditional return, with the first MTD return covering 2026/27 due by 31 January 2028. Our explainer on MTD for Income Tax covers who needs to act and when.
If your income sits below £50,000, the familiar annual return still applies for now, but switching to digital record-keeping early makes the eventual move smoother.
How Sleek helps with filing your Self Assessment tax return
Self Assessment trips people up on the small things: a missed registration date, a forgotten income source, or a January deadline that arrives faster than expected. Sleek prepares and files your return, checks your expenses are claimed correctly, and keeps you on the right side of HMRC, including the shift to Making Tax Digital. You can also pair filing with full accounting services if you’d rather hand over the books entirely.
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.
How much does it cost to file a Self Assessment tax return?
Filing it yourself directly with HMRC is free. If you use an accountant, a straightforward sole trader return typically costs between £150 and £300, rising with complexity such as multiple income sources or capital gains.
Sleek prices Self Assessment as a fixed fee so there are no surprises, which helps if you’d rather not pay by the hour.
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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.
