The essential guide to Self Assessment
Completing a Self Assessment tax return is a legal requirement for millions of sole traders, freelancers and higher earners — but the rules aren’t always straightforward.
This comprehensive guide sets out exactly who must register for Self Assessment, how the registration and filing process works, and the critical HMRC deadlines you need to plan for. It also explains how to prepare your records, submit your return accurately, calculate what you owe and avoid late filing penalties.
Whether you’re new to self-employment or reviewing your obligations for the year ahead, this advice provides a clear, step-by-step overview of the Self Assessment process so you can meet your tax responsibilities with confidence.
Completing a Self Assessment tax return is a legal responsibility for millions of sole traders, freelancers and higher earners. It is not simply an annual form filling exercise. It is how HMRC collects Income Tax when it is not deducted automatically through Pay As You Earn. If you work for yourself or receive income that is not taxed at source, you are responsible for declaring your earnings, claiming allowable expenses and ensuring the correct amount of tax is paid.
For anyone running their own business, understanding how Self Assessment works is a core part of staying compliant and protecting your income.
Who must complete a Self Assessment tax return
You will usually need to file a Self Assessment tax return if you earn income that is not taxed automatically. This commonly includes sole traders, freelancers and contractors. It also applies to landlords with rental income, people receiving dividends or investment income, and those earning untaxed income such as tips or commission.
From the 2025 to 2026 tax year onwards, employees earning more than £150,000 through Pay As You Earn must also submit a Self Assessment tax return.
If you are unsure whether the rules apply to you, it is important to check the official HMRC guidance. Filing when you do not need to creates unnecessary administration. Failing to file when required can result in penalties and interest.
Self Assessment deadlines you need to know
The United Kingdom tax year runs from 6 April to 5 April. The key dates that follow are fixed and missing them can be expensive.
You must register for Self Assessment by 5 October in your second tax year if you are newly self employed.
Paper tax returns must be submitted by 31 October following the end of the tax year.
Online tax returns must be submitted by 31 January. Any tax owed is also due by 31 January.
Although January is the final deadline, you can submit your return much earlier. Sole traders can file as soon as the tax year ends. Employees can file once they receive their P60. Filing early gives you clarity over your tax bill and allows you to plan ahead with confidence.
How to register for Self Assessment
If you are filing for the first time, you must register with HMRC through the GOV.UK website. You will need your National Insurance number, the date you started trading if you are self employed, and details about the type of work you do.
Once registered, you will receive a ten digit Unique Taxpayer Reference. You will also set up a Government Gateway account, which you use to complete and submit your tax return online.
Registration can take up to twenty working days, so it is important not to leave it until close to the deadline.
How to submit your Self Assessment tax return
Most people now complete their Self Assessment tax return online. You log in to your Government Gateway account, enter details of your income and expenses, review your tax calculation and submit.
If you choose to file a paper return, you must complete form SA100 and send it to HMRC by 31 October. If submitting by post, allow enough time for delivery and keep copies and proof of postage.
Before you start, make sure you have your Unique Taxpayer Reference, your National Insurance number, records of untaxed income, details of taxed income such as your P60, evidence of allowable business expenses, and information about pension contributions or charitable donations. Good record keeping throughout the year makes the process significantly easier.
Why filing your tax return early helps
Leaving your Self Assessment tax return until the end of January adds pressure at the busiest point of the year. Filing early means you know what you owe, can manage your cash flow properly and avoid last minute errors.
For the self employed, staying ahead of your tax responsibilities is part of running a resilient and sustainable business.
What happens if you file your Self Assessment late
If your Self Assessment tax return is submitted late, HMRC will issue an automatic £100 penalty, even if no tax is due. After three months, additional daily penalties may apply. Further charges are added at six and twelve months, and interest is charged on unpaid tax.
If you are at risk of missing the deadline, submit your return as soon as possible and contact HMRC if there is a genuine reason for the delay. Acting quickly limits further penalties.
Can you correct a mistake on your tax return
If you discover an error after submitting your Self Assessment tax return, you can usually amend it. Online returns can be corrected up to twelve months after the original 31 January deadline. It is always better to amend mistakes promptly rather than leave incorrect information on your record.
Accurate and up to date records make both filing and correcting a return much more straightforward.
What to do if you cannot pay your tax bill
If you are struggling to pay your Self Assessment tax bill in full, do not ignore the problem. HMRC may agree to a Time to Pay arrangement, which allows you to spread payments over a period of time.
You should still submit your tax return by 31 January, even if you cannot pay immediately. Then contact HMRC as soon as possible to discuss your circumstances. Early communication is far better than allowing penalties and enforcement action to escalate.
Understanding payments on account
Many self employed people are required to make payments on account. These are advance payments towards your next Self Assessment tax bill. If your previous tax bill was more than £1,000 and most of your tax was not collected at source, you will usually need to make them.
Payments on account are based on your previous year Income Tax and Class 4 National Insurance bill. They are split into two equal instalments. The first is due on 31 January and the second on 31 July. Each instalment is normally half of your previous year tax bill.
If your actual tax liability is higher, you will make a balancing payment the following January. If it is lower, you will receive a refund or have the excess credited against future payments.
Understanding deadlines, penalties, payment plans and payments on account is essential for managing cash flow as a sole trader or freelancer. Staying informed and acting early helps you remain compliant and in control of your business finances.
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