How to File Self Assessment Tax Return

How to File Your Self Assessment Tax Return: The Definitive 2026 UK Guide

Published by LocalPage.uk Content Architecture Team | Last Updated: February 2026 | Region: UK-Wide

Completing a Self Assessment tax return is a fundamental obligation for millions of individuals across the United Kingdom. Whether you are a sole trader, a partner in a business, or a director with dividend income, navigating the complexities of HM Revenue and Customs (HMRC) requirements demands precision, punctuality, and an understanding of the ever-evolving fiscal landscape. In 2025-2026, the digital transition of the UK tax system continues to accelerate, making it more critical than ever to ensure your records are accurate and your submissions are timely.

12.1m UK taxpayers were required to file a Self Assessment return for the last tax year, with HMRC reporting that over 90% now choose to file online.

Determining Your Obligation to File Self Assessment

Identifying the Criteria for Taxable Income

The first step in any tax journey is establishing whether you actually need to file. In the UK, the threshold for "trading income" remains a key trigger. If you have earned more than £1,000 from self-employment before any expenses are deducted, you are legally required to register with HMRC. This applies regardless of whether this is your primary source of income or a "side hustle."

Complexity for Directors and High Earners

It is a common misconception that only the self-employed use Self Assessment. Company directors, even those paid via PAYE, often need to file if they receive dividends or have complex tax affairs. Furthermore, if your total taxable income exceeds £150,000, or if you or your partner receive Child Benefit and your individual income is over £60,000 (following the 2024-25 threshold changes), the High Income Child Benefit Charge will necessitate a return.

Verification via the GOV.UK Tool

If you are uncertain about your status, endeavour to use the official "Check if you need to send a Self Assessment tax return" tool on GOV.UK. This tool is updated annually to reflect changes in the Finance Act and is the most authoritative way to confirm your status.

The Mandatory HMRC Registration Process

Securing Your Unique Taxpayer Reference (UTR)

To file a return, you must first be registered. For new sole traders, this involves registering for Self Assessment and Class 2 National Insurance through the HMRC website. Once registered, HMRC will post a ten-digit Unique Taxpayer Reference (UTR) to your home address. This number is permanent and will be required for every interaction you have with the tax office for the rest of your professional life.

Setting Up Your Government Gateway Account

In 2026, the Government Gateway remains the primary portal for digital tax management. You will need to create a user ID and password, which then grants you access to your Personal Tax Account. This account allows you to view your tax code, check your state pension forecast, and, crucially, complete your Self Assessment return online.

Timeline for New Registrations

You must register for Self Assessment by 5 October in the second tax year of your business. For instance, if you started trading in the 2025-26 tax year, you must register by 5 October 2026. Failure to do so can result in "failure to notify" penalties, even if no tax is eventually due.

Essential Record-Keeping for the 2025-2026 Tax Year

Digital Bookkeeping and Making Tax Digital (MTD)

With the phased rollout of Making Tax Digital, HMRC increasingly encourages (and for many, mandates) digital record-keeping. Small businesses and landlords should utilise compatible software to track income and expenditure. According to the Federation of Small Businesses (FSB), businesses that adopt digital accounting tools save an average of 12 hours per month on administrative tasks.

Categorising Allowable Business Expenses

Understanding what you can claim is essential for optimising your tax position. Allowable expenses include office costs (stationary, phone bills), travel (fuel, parking, train fares), and financial costs (insurance, bank charges). If you work from home, you can claim a proportion of your heating, electricity, and Council Tax, or use HMRC’s simplified "flat rate" calculations to save time.

The Risk of Inadequate Records

HMRC has the power to check your records at any time. You must keep your records for at least five years after the 31 January submission deadline of the relevant tax year. In Northern Ireland, where cross-border trade with the Republic of Ireland is common, businesses must be particularly diligent in separating UK-based income from international earnings to ensure VAT and Income Tax are applied correctly.

5.6m private sector businesses operate in the UK, with 99.3% being SMEs

that rely on efficient Self Assessment processes to maintain cash flow.

Navigating the Self Assessment Deadlines

The Paper vs. Digital Divide

The deadline for paper tax returns is 31 October following the end of the tax year. However, paper filings are increasingly rare. The deadline for online returns is midnight on 31 January. For the 2025-26 tax year, your online return must be submitted by 31 January 2027.

Payment Deadlines and "Payments on Account"

Submission is only half the battle; payment must also reach HMRC by 31 January. Furthermore, if your tax bill is more than £1,000, you will likely be asked to make "payments on account." These are two advance payments towards your next tax bill, due on 31 January and 31 July. This can create a significant cash flow challenge for new businesses in their second year of trading.

Penalties for Late Submissions

HMRC applies an immediate £100 penalty if your return is even one day late. After three months, daily penalties of £10 begin to accrue, up to a maximum of £900. By staying ahead of these dates, you avoid unnecessary costs that can hamper micro-business growth.

Step-by-Step: Completing the Online Return

Filling in the "Main Section"

The return begins with personal details and a series of "Yes/No" questions regarding your income sources during the year. This includes interest from UK banks, dividends from shares, and any state benefits. It is vital to answer these accurately to ensure the correct "supplementary pages" are generated.

Understanding Supplementary Pages

Depending on your income, you will need to complete additional sections. The 'Self-employment' section (SA103) is for sole traders, while 'Property' (SA105) is for those receiving rental income. If you have capital gains from selling assets like property (other than your main home) or stocks, the SA108 page must be completed.

Regional Variation in Income Tax

While HMRC manages tax for the whole UK, the Scottish Government and the Welsh Government have the power to set their own Income Tax rates. If you live in Scotland, you will pay the Scottish Rate of Income Tax (SRIT), which features different bands (such as the Starter, Intermediate, and Top rates) compared to England and Northern Ireland. The system automatically calculates this based on your postcode, but you should verify your "tax regime" is correctly identified on your return.

Calculating Your Tax Liability and Reliefs

Utilising the Personal Allowance

For the 2025-2026 tax year, the standard Personal Allowance remains at £12,570. This is the amount of income you can earn before you start paying Income Tax. It is important to note that this allowance is tapered for those earning over £100,000, reducing by £1 for every £2 earned above that threshold.

Claiming Tax Reliefs and Pensions

You can reduce your tax bill through various reliefs. Contributions to a registered UK pension scheme are particularly effective, as the "grossed up" amount can extend your basic rate tax band, allowing more of your income to be taxed at 20% rather than 40%. Similarly, Gift Aid donations to charities allow you to claim back the difference between the basic rate and higher rate of tax.

National Insurance Contributions (NICs)

Self Assessment also calculates your National Insurance. Class 4 NICs are calculated as a percentage of your profits. Recent legislative changes have sought to simplify the NICs system for the self-employed, so ensure you review the final calculation screen carefully to understand how your contribution to the state pension and benefits system is being processed.

Regional Considerations: From Belfast to Bristol

Guidance for Businesses in Wales and Scotland

In Wales, the Welsh Revenue Authority (WRA) handles Land Transaction Tax, but Income Tax remains within the Self Assessment system. Business Wales provides extensive bilingual support for startups. In Scotland, Scottish Enterprise offers guidance on how the distinct tax bands affect local recruitment and business investment strategies.

The Windsor Framework and Northern Ireland

For businesses in Northern Ireland, the interface between UK tax and EU VAT rules (via the Windsor Framework) can be complex. If your self-employment involves the movement of goods, you must ensure your tax return reflects any specific VAT accounting required for NI-EU trade, which differs from the Great Britain-EU model.

Support from Local Authorities and LEPs

Across England, Local Enterprise Partnerships (LEPs) and Growth Hubs provide regional grants and advice. Whether you are in the "Northern Powerhouse" or

the "Midlands Engine," local support can often help bridge the gap between filing a return and scaling your operations.

Common Mistakes and How to Avoid Them

Inaccurate Revenue Reporting

Mistyping figures is the most common cause of HMRC enquiries. Always double-check your bank statements against your invoices. Professional services firms often make the mistake of reporting income when an invoice is raised rather than when it is paid; however, small businesses with a turnover of £150,000 or less can choose "cash basis" accounting to simplify this.

Forgetting the Student Loan Repayment

If you have a student loan, HMRC will collect repayments through Self Assessment based on your income above the relevant threshold. Forgetting to tick the student loan box can lead to an unexpected bill later in the year when HMRC reconciles your data with the Student Loans Company.

The Danger of "Estimated" Figures

While you can use estimated figures if final data isn't available, you must mark them as such and provide the actual figures as soon as possible. Repeated use of estimated figures without valid reason can trigger a formal audit.

Working with Professionals: Accountants and Agents

When to Appoint an Accountant

As your business grows, the complexity of your return increases. While 4.2 million UK businesses are micro-operations, many find that the fee of a Chartered Accountant is offset by the tax savings they identify. An accountant can also act as your "agent" with HMRC, meaning they can speak to the tax office on your behalf.

The Role of the FCA and Professional Bodies

When hiring a tax advisor, ensure they are members of a recognised body such as the ICAEW, ACCA, or CIOT. While tax preparation isn't regulated by the Financial Conduct Authority (FCA) in the same way as financial advice, using a qualified professional ensures you are protected by professional indemnity insurance and high ethical standards.

Digital Security and the ICO

If you handle your own data or use third-party apps, you must comply with UK GDPR. The Information Commissioner’s Office (ICO) provides guidelines for small businesses on how to securely store financial records. Data breaches involving tax information are treated with extreme severity.

Final Review and Submission

Using the "View Your Calculation" Feature

Before hitting 'Submit', the online system provides a full breakdown of your tax and NI. Ensure this matches your own expectations. If the figure looks wildly different from your projections, go back through the supplementary pages to find the discrepancy.

Receiving Your Submission Receipt

Once submitted, you will receive a confirmation message and a submission reference number. Take a screenshot or print this page. It is your proof that you have met your legal obligation for the year. HMRC will then update your Personal Tax Account within 72 hours to show the amount due.

Amending a Return

If you realise you have made a mistake after submitting, do not panic. You can usually amend a Self Assessment return for up

to 12 months after the 31 January deadline. Simply log back into the Government Gateway and select 'Amend your return'.

"When is the 2026 tax return deadline?"

The deadline for filing your 2025-26 Self Assessment tax return online is 31 January 2027. If you prefer to file on paper, the deadline is much earlier: 31 October 2026.

"How much do I need to earn to register for Self Assessment?"

You must register with HMRC if your gross income from self-employment (before expenses) exceeds £1,000 in a single tax year.

Frequently Asked Questions

What happens if I miss the 31 January deadline?

If you miss the deadline, you will receive an automatic £100 penalty. This applies even if you have no tax to pay or have already paid your tax. Further penalties accrue at 3, 6, and 12 months. If you have a "reasonable excuse" (such as a death in the family or a serious illness), you can appeal the penalty via the HMRC website.

Can I pay my tax bill in instalments?

Yes, through a "Time to Pay" arrangement. If you owe less than £30,000 and are within 60 days of the payment deadline, you can often set up a payment plan online without calling HMRC. You will be charged interest on the outstanding balance, but it prevents more severe debt collection actions.

Do I need to report income from selling on Vinted or eBay?

Only if you are trading for profit. From 2024, digital platforms are required to report seller data to HMRC. If you are just selling old personal items for less than you bought them, this is not taxable. However, if you buy items to resell and your total revenue exceeds £1,000, you must register for Self Assessment.

Is the process different in Scotland?

The filing process is identical, but the tax calculation is different. Scotland has more tax bands (Starter, Basic, Intermediate, Higher, Advanced, and Top). The HMRC system will apply these automatically based on your "S" prefix tax code, which denotes Scottish residency.

How do I get a UTR number if I've lost mine?

You can find your Unique Taxpayer Reference (UTR) on previous tax returns, payment reminders from HMRC, or within your Personal Tax Account on the GOV.UK website. If you cannot find it, you must contact the Self Assessment helpline; HMRC will post it to your registered address for security reasons.

Do I have to file if I'm a sole trader but made a loss?

If you have registered for Self Assessment, you must file a return even if you made a loss. Reporting a loss is actually beneficial, as you can "carry back" the loss to offset tax paid in previous years or "carry it forward" to reduce future tax bills.

What is a 'Payment on Account'?

It is an advance payment towards your next year's tax bill. Each payment is half of your previous year's tax bill. These are due on 31 January and 31 July. You can ask HMRC to reduce these if you know your income will be significantly lower in the coming year.

Can I claim for my home office?

Yes. You can calculate the proportion of your actual bills (heating, lighting, rent) based on the number of rooms and time spent working, or use simplified expenses (£10 to £26 per month depending on hours worked) if you work more than 25 hours a month from home.

Do I need to keep paper receipts?

No, digital copies (scans or photos) are acceptable to HMRC, provided they are legible. Using a bookkeeping app is the most efficient way to store these.

Ensure you have a backup system in place to comply with the five-year record-keeping rule.

I'm moving from NI to England - does my tax change?

Your Income Tax rates will remain the same as the rates in England and Northern Ireland are currently aligned. However, you must update your address with HMRC immediately to ensure your tax records are correctly attributed to your new local authority and regional tax office.