How to Fill In a Self Assessment Tax Return
- Modified: 22 October 2025
- 11 min read
- Tax & VAT
Gabi Bellairs-Lombard
Author
Gabi is a content writer who is passionate about creating content that inspires. Her work history lies in writing compelling website copy, now specialising in product marketing copy. Gabi's priority when writing content is ensuring that the words make an impact on the readers. For Osome, she is the voice of our products and features. You'll find her making complex business finance and accounting topics easy to understand for entrepreneurs and small business owners.
Mosan Ali
Reviewer
Mosan Ali is our Accounting Manager based in the UK and has a wealth of knowledge of UK GAAP, VAT, and PAYE. With 12 years of experience crunching numbers and ensuring compliance, he keeps our financial reporting ship-shape. Think of Mosan as our blog's accounting guru. He carefully reviews our UK-focused content, ensuring it's accurate, up-to-date, and packed with helpful tips for UK businesses. Get your taxes right from day one with our informative blog posts.
Filing a Self Assessment tax return can seem daunting, especially if you’re self-employed, a landlord, or have untaxed income. Missing deadlines or entering incorrect details can lead to penalties, so understanding the process is crucial. This guide will walk you through everything you need to know about Self Assessment in the UK, from who needs to file to how to complete your return accurately and on time.
Key Takeaways
- Registration for Self Assessment with HMRC and timely submission of the tax return are essential to avoid late filing and payment penalties.
- Maintaining accurate and organised records of all income, expenses, and supporting documents is crucial for completing the tax return correctly and estimating the tax bill.
- Tax returns can be amended within 12 months after the deadline to correct mistakes, update provisional figures, or claim any entitled tax refunds.
What Is a Self Assessment Tax Return?
A Self Assessment tax return is a form HM Revenue & Customs (HMRC) requires individuals to complete if they have income that is not taxed automatically through the Pay As You Earn (PAYE) system. This includes income from self-employment, property rental, dividends, or foreign sources. The return allows HMRC to calculate your tax liability and ensure you pay the correct amount.
Completing a Self Assessment accurately helps avoid penalties and interest. Depending on your situation, you may need to file annually online or on paper, and HMRC provides guidance on which forms and sections are relevant for different types of income.
If you need assistance with filing your self assessment tax return and ensuring compliance with HMRC regulations, our team of tax professionals is here to guide you through the process and optimise your tax obligations. Contact us today!
Who Needs to File a Self Assessment Tax Return
You must file a Self Assessment tax return if, during the last tax year (6 April to 5 April), any of the following applied:
- You were self-employed as a sole trader and earned more than £ 1,000 before claiming any allowable expenses.
- You were a partner in a business partnership.
- You had to pay Capital Gains Tax on the sale or disposal of an asset that increased in value.
- You were liable for the High Income Child Benefit Charge and do not pay it through PAYE.
You may also need to file if you have any untaxed income, including:
- Rental income from property.
- Tips, commission, or other casual earnings.
- Income from savings, investments, or dividends.
- Foreign income.
Filing a return ensures HMRC can accurately calculate your tax liability and helps you avoid penalties for late or missing submissions.
If you’re not sure whether you need to file a tax return, chech HMRC. You must tell them by 5 October if you need to complete a return for the previous tax year and haven’t filed one before. Registering for Self Assessment on time helps avoid late filing penalties.
Registering for Self Assessment
If you need to complete a tax return and haven’t filed one before, you must register for Self Assessment with HMRC by 5 October following the end of the tax year (which runs from 6 April to 5 April). Registration notifies HMRC that you’ll be submitting a return and ensures you receive your Unique Taxpayer Reference (UTR).
Once registered, it’s important to keep accurate financial records— such as bank statements, invoices, and receipts — so you can complete your return correctly.
After registration, you can start preparing for payment:
- Estimate your tax bill early to avoid surprises.
- Set up regular payments to spread the cost and make budgeting easier.
- Submit your return early (any time after 5 April) to find out how much you owe well before the 31 January payment deadline.
How to File and Submit Your Self Assessment Tax Return
You can submit your Self Assessment tax return online or by sending a paper form. Most people prefer the online option as it’s faster, more secure, and includes automatic calculations.
- Online submission: File your return through your HMRC online account or with compatible commercial software.
- Paper submission: Download form SA100 (2025 version) or request a copy by post from HMRC. Some situations, such as filing for a partnership, trust, or estate, may require additional forms like SA970.
If you do not yet know your full profit for the tax year — for example, if your accounting period differs from the tax year — you can use provisional figures in your return. Make sure to tell HMRC that you’ve done this and update your return within 12 months once final figures are available.
Filing online allows you to check your tax calculation immediately and make changes easily if your figures later change.
If You No Longer Need to Send a Self Assessment Tax Return
If your circumstances change and you believe you no longer need to file a Self Assessment tax return, you must tell HMRC as soon as possible. This helps avoid penalties and ensures your record is updated before the next filing cycle.
When you might no longer need to file
You may not need to send a tax return if:
- You’ve stopped being self-employed
- You no longer rent out property
- You no longer need to pay the High Income Child Benefit Charge
If you’re unsure, you can use the online tool on GOV.UK to check if you still need to send a tax return.
How to tell HMRC
You can:
- Sign in to your HMRC online account and fill in the online form to close your Self Assessment account or remove yourself for a specific tax year
- Provide your National Insurance number and Unique Taxpayer Reference (UTR)
- If you cannot use the online form, contact HMRC by phone or post
HMRC will review your request and write to confirm whether you’re still required to send a tax return.
If you’ve already told HMRC that your self-employment has ended but still receive return requests, it’s worth following up to make sure your status has been fully updated.
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Deadlines for Filing and Payment
HMRC must receive your Self Assessment tax return and any payment you owe by the official deadlines. Missing these dates can result in penalties and interest, so it’s best to plan ahead.
Key tax year dates
The latest tax year runs from 6 April 2024 to 5 April 2025. If you still need to file for an earlier year, submit your return as soon as possible to limit penalties.
Registering for the first time
If you haven’t filed a Self Assessment before, you must register with HMRC by 5 October 2025. If you register after that date, HMRC will send a letter or email setting a new deadline, usually three months from the notice.
Even with an extended filing deadline, the payment deadline of 31 January 2026 still applies.
Paper tax return deadline
Paper returns must reach HMRC by 31 October 2025. You can send them any time after 6 April 2025, and filing early gives you more time to budget for your tax bill.
Online tax return deadline
Online returns must be submitted by 31 January 2026. To pay through your tax code, submit by 30 December 2025.
Paying your tax bill
You need to pay any Self Assessment tax owed by 31 January 2026. If you make payments on account, the second instalment is due by 31 July 2026.
Special filing deadlines
Trustees of registered pension schemes or non-resident companies must submit a paper return by 31 January 2026.
Partnerships with a company as a partner: online returns within 12 months of the accounting date, paper returns within 9 months.
Submitting a Self Assessment tax return can be done online, by paper, or through commercial software, depending on individual circumstances. Even when profits are estimated, provisional figures can be used, and adjustments can be made within 12 months of the deadline to ensure accurate reporting. Early submission provides time to plan payments, set up instalments, and avoid interest on underpaid tax.
Penalties and Interest for Late Returns
HMRC can charge penalties if you:
- Send your tax return late
- Pay your tax bill late
You can estimate your penalty for late Self Assessment returns and payments on HMRC’s website.
Late registration
If you register for Self Assessment after 5 October 2025 and do not pay your full tax bill by 31 January 2026, you may face a ‘failure to notify’ penalty. This penalty is calculated based on the unpaid amount and will be issued within 12 months after HMRC receives your tax return.
Late tax return
The late filing penalties are:
- £ 100 initial penalty immediately after the deadline
- After 3 months, additional £ 10 per day, up to a maximum of £ 900
- After 6 months, further 5% of tax due or £ 300, whichever is greater
- After 12 months, another 5% of tax due or £ 300, whichever is greater
All partners in a partnership will be charged if the partnership return is late.
Late payment
If you pay your tax late, penalties of5% of the unpaid tax apply at:
- 30 days
- 6 months
- 12 months
Interest is also charged on the outstanding amount. Paying promptly can help avoid additional costs.
Responding to penalties
You must pay the penalty within 30 days of the notice. If you have a reasonable excuse, you can appeal against the penalty.
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How to Change Your Self Assessment Tax Return
If you spot a mistake after submitting your Self Assessment tax return, don’t worry — you can update it. Depending on the changes, you might need to pay extra tax or could be due a refund.
Updating your return within 12 months
You can make corrections within 12 months of the Self Assessment deadline. For example, for the 2023–2024 tax year, you’d usually need to make changes by 31 January 2026. If you miss this window or need to update an earlier year, you’ll need to write to HMRC.
- Online returns: Wait 3 days after filing, then log in to your HMRC account. Go to Your tax account → Self Assessment account → More Self Assessment details → At a glance → Tax return options, choose the tax year, make your corrections, and file again. Updates are usually reflected straight away, including any changes to what you owe or refunds due.
- Paper returns: Download or request the SA100 form, mark each page ‘amendment’, include your name and Unique Taxpayer Reference (UTR), and send it to HMRC, so they will process your changes and update your bill. Refunds are usually paid directly into your bank account if you provide your details.
- Commercial software: Contact your software provider for guidance. If the software cannot make amendments, HMRC can assist directly.
Changing a return after 12 months
For changes more than 12 months after the deadline, you must write to HMRC. Include:
- Tax year being corrected
- Why you think you overpaid or underpaid
- The amount overpaid or underpaid
- Your signature
If claiming overpayment relief, also include a declaration confirming the information is accurate and note any previous appeals related to the same payment. Keep evidence of tax payments in case HMRC requests it, and specify how you’d like any refund to be paid.
How Osome Can Help
Getting started with Self Assessment can be straightforward with the right support. Osome can help you register for Self Assessment with HMRC and prepare all the information you’ll need to complete your tax return accurately. We organise your income and expense records, summarise your business activity, and ensure your figures are ready for submission.
We can also guide you through updating your return if you need to make corrections after submission. With our support, you can stay organised, reduce errors, and save time, making the Self Assessment process much smoother and less stressful.
Summary
Filing a Self Assessment tax return is essential for many self-employed individuals, partners in a business, or anyone with untaxed income. You must register with HMRC, keep accurate records, and submit your return by the deadlines — 31 October for paper returns and 31 January for online submissions. Penalties apply for late filing or payment, but returns can be amended within 12 months if needed. With careful preparation and timely submission, you can manage your tax obligations efficiently and avoid unnecessary fines.
Gabi is a content writer who is passionate about creating content that inspires. Her work history lies in writing compelling website copy, now specialising in product marketing copy. Gabi's priority when writing content is ensuring that the words make an impact on the readers. For Osome, she is the voice of our products and features. You'll find her making complex business finance and accounting topics easy to understand for entrepreneurs and small business owners.
