Self-employed

The Complete Guide for Self-Employed People in 2026/27

Step-by-step guide to completing a self-assessment tax return as a sole trader. Avoid costly mistakes and claim every allowance. Blue Tick explains.

If you are self-employed in the UK, completing a self-assessment tax return is one of the most important financial tasks you will face each year. Get it right and you pay exactly what you owe, claim every allowance available to you, and avoid HMRC penalties. Get it wrong and the consequences range from an unexpected tax bill to formal enquiries that take months to resolve.

This self-assessment tax return guide for UK 2026/27 covers everything a sole trader needs to know: who must register, what income and expenses to declare, how to calculate your tax, and how to avoid the most common errors. Whether you are filing for the first time or looking to sharpen your process, this guide gives you the full picture for the 2026/27 tax year.

In this guide:

  • Who needs to complete a self-assessment tax return
  • Key deadlines and what happens if you miss them
  • How to register with HMRC and set up your account
  • What income to declare and which expenses you can claim
  • How your tax bill is calculated, with a worked example
  • Common errors and how to avoid them

Who Needs to Complete a Self-Assessment Tax Return?

You must register for self-assessment if you were self-employed as a sole trader and earned more than £1,000 in a tax year. This £1,000 figure is the trading allowance. It is a gross threshold, not a profit threshold, so it applies to your total income before deducting expenses.

Beyond self-employment, you may also need to register for self-assessment if you have untaxed income that HMRC cannot collect through PAYE. Common examples include:

  • Rental income from property
  • Tips and commission not taxed at source
  • Income from savings, investments or dividends above your relevant allowances
  • Foreign income
  • Capital gains from the disposal of assets
  • The High Income Child Benefit Charge, where you or your partner earn over £60,000 and receive Child Benefit

Company directors are generally also required to file a self-assessment return. If you are unsure whether your circumstances require one, HMRC's online tool at gov.uk can confirm this based on your specific situation.

If you are newly self-employed, you must register with HMRC by 5 October following the end of the tax year in which you started trading. For the 2026/27 tax year, which ends on 5 April 2027, the registration deadline is 5 October 2027.

Key Deadlines and Making Tax Digital

How you file, and when, now depends on your income level. From 6 April 2026, HMRC introduced Making Tax Digital for Income Tax (MTD for ITSA), which replaces the traditional annual self-assessment return for those above the income threshold with a system of quarterly updates and a year-end final declaration.

If your qualifying income (self-employment plus property) exceeds £30,000 in 2026/27, MTD for ITSA is mandatory. You must keep digital records using HMRC-compatible software and submit four quarterly updates each year, followed by a final declaration. The quarterly deadlines for 2026/27 are:

  • 7 August 2026: Quarter 1 update (6 April to 5 July 2026)
  • 7 November 2026: Quarter 2 update (6 July to 5 October 2026)
  • 7 February 2027: Quarter 3 update (6 October to 5 January 2027)
  • 7 May 2027: Quarter 4 update (6 January to 5 April 2027)
  • 31 January 2028: Final declaration and payment of any tax owed

HMRC has confirmed it will not issue penalty points for late quarterly updates during the first year (2026/27), though penalties for late payment and late final declarations still apply.

If your qualifying income is between £30,000 and £50,000, MTD for IT will become mandatory from 6 April 2027. For the 2026/27 tax year, the traditional self-assessment process still applies, with the following key dates:

  • 5 April 2027: End of the 2026/27 tax year
  • 5 October 2027: Deadline to register for self-assessment if newly self-employed in 2026/27
  • 31 January 2028: Deadline to file your online return and pay any tax owed
  • 31 July 2027: Second payment on account for 2025/26 (if applicable)

If your qualifying income is below £30,000, traditional self-assessment continues for now. MTD for IT is planned to extend to those with income over £20,000 from April 2028.

Regardless of which regime applies, filing late or paying late triggers penalties. For traditional self-assessment, a single day late results in an automatic £100 fine. Under MTD, a points-based system applies to late quarterly updates, with a financial penalty once a threshold of points is reached.

How to Register and Access Your HMRC Account

If you have not filed a self-assessment return before, you need to register with HMRC before you can do anything else. To complete self-assessment for the first time, you go through two stages: registering for self-assessment, and then activating your Government Gateway account.

HMRC will post an activation code to your home address. This currently takes up to 10 working days, which means you should not leave registration until December or January if you are filing for the first time. Once activated, you can log in to your personal tax account at any time to complete and submit your return.

Self-employed individuals register online through the HMRC website. You will need your National Insurance number, contact details, and the date you started trading. Once registered, HMRC will send you a Unique Taxpayer Reference (UTR), a 10-digit number that identifies you for all future tax dealings.

What Income to Declare on Your Self-Assessment Return

The self-employed tax return centres on the Self-Employment pages (SA103), either the short version for simpler cases or the full version for more complex ones. Here you will record your trading income and allowable expenses to arrive at your taxable profit.

Your trading income is the total turnover from your self-employed work before any deductions. This is the gross amount invoiced or received during the relevant accounting period, not just what has cleared your bank account. HMRC generally uses a cash basis for sole traders with turnover below £150,000, which means you report income when received and expenses when paid.

Beyond trading income, your return should capture all other income sources in the relevant year:

  • Employment income (if you also have a job)
  • Rental income from property (covered on the UK property pages, SA105)
  • Savings interest above your Personal Savings Allowance
  • Dividends (if you also own shares)
  • Any other income not taxed at source

Omitting income is a common cause of HMRC enquiries. HMRC receives third-party data from banks, pension providers, and employers, and discrepancies between what these sources report and what appears on your return will be flagged automatically.

Allowable Expenses for Sole Traders

Once your income is established, you reduce it by your allowable business expenses to reach your taxable profit. An allowable expense is one that has been incurred wholly and exclusively for the purposes of your trade.

Common allowable expenses for sole traders include:

  • Premises costs (rent, utility bills, insurance for a business premises)
  • Use of home for work (calculated either using HMRC's flat rate or an actual cost method)
  • Office costs such as stationery, postage, and software subscriptions
  • Travel for business purposes (mileage, public transport, parking)
  • Subsistence (meals and accommodation on business trips, within reason)
  • Professional fees including accountancy, legal advice, and business insurance
  • Staff costs including wages, salaries, and employer National Insurance
  • Marketing and advertising costs
  • Equipment and tools (though larger capital items may need to be treated differently)

You cannot claim expenses that are personal, dual-purpose without a clear business element, or capital in nature (the latter may qualify for capital allowances instead).

Worked example: Sarah is a freelance graphic designer. In 2026/27 her total invoiced income is £48,000. She works from home five days a week, well above 101 hours per month, and claims the HMRC simplified expenses flat rate of £26 per month for that usage level, totalling £312 for the year. Her other allowable expenses, including software subscriptions (£600), professional fees (£800), travel (£1,200), and equipment (£1,500 claimed via the Annual Investment Allowance), come to £4,100. Her total expenses are £4,412. Taxable profit: £48,000 minus £4,412 = £43,588.

How Your Tax Bill Is Calculated

Once taxable profit is established, your tax and National Insurance are calculated as follows for 2026/27:

Income Tax:

  • Personal Allowance: £12,570 (no tax on this portion)
  • Basic rate (20%): on income between £12,571 and £50,270
  • Higher rate (40%): on income between £50,271 and £125,140
  • Additional rate (45%): on income above £125,140

National Insurance for self-employed individuals: Self-employed individuals pay Class 4 National Insurance on their profits. HMRC guidance sets Class 4 at 6% on profits between the Lower Profits Limit (£12,570) and the Upper Profits Limit (£50,270), and 2% above the Upper Profits Limit. Class 2 National Insurance is no longer separately paid by most self-employed people. For those with profits above the Small Profits Threshold, Class 2 is treated as having been paid automatically, protecting your National Insurance record without any additional payment being required. Those with profits below the Small Profits Threshold can choose to pay voluntary Class 2 contributions to protect their state pension entitlement.

Continuing the example above: Sarah's taxable profit is £43,588.

  • She has no other income, so her Personal Allowance of £12,570 is fully available.
  • Taxable income after allowance: £43,588 minus £12,570 = £31,018
  • Income tax at 20%: £31,018 x 20% = £6,203.60
  • Class 4 NI on profits above the Lower Profits Limit (approximately £12,570): £31,018 x 6% = £1,861.08
  • Class 2 NI: a flat weekly rate, collected through the return

Sarah's approximate total liability is around £8,100 before any payments on account already made.

Payments on account are advance payments towards the following year's liability. If your tax bill exceeds £1,000 and is not fully collected at source, HMRC requires you to make two payments on account: the first by 31 January (alongside settling the prior year balance) and the second by 31 July. Each payment is 50% of your prior year's liability.

Common Errors to Avoid

The following mistakes appear regularly when people complete self-assessment and are a frequent cause of penalties or underpayments:

Forgetting payments on account. Many first-time filers are caught out by owing three payments at once in January: the balancing payment for the prior year plus two payments on account for the current year.

Claiming disallowable expenses. Client entertainment, personal clothing (unless it is a uniform or protective equipment), and fines are not allowable. HMRC is alert to these categories.

Incorrect use of cash basis versus accruals. While cash basis is simpler, it is not always appropriate. If you have large amounts of stock or significant debtors, an accruals approach may better reflect your actual trading position.

Not claiming all available reliefs. The Marriage Allowance, pension contributions, Gift Aid donations, and the trading allowance are often overlooked. Each can reduce your bill materially.

Submitting after the deadline. Even a return showing no tax due attracts a £100 penalty if filed late. HMRC does not waive this automatically.

How Blue Tick Can Help

Completing a self-assessment tax return accurately requires a clear understanding of your income, your allowable expenses, and the reliefs available to you. Blue Tick works with self-employed individuals across a range of trades and professions, preparing tax returns that are accurate, compliant, and structured to minimise your liability within the rules. Head to our website and book a meeting now.

Final Thoughts

A self-assessment tax return is not simply a compliance exercise: it is an annual opportunity to ensure you are paying the right amount of tax and claiming every deduction you are entitled to. The key discipline is keeping accurate records throughout the year, understanding what income must be declared, and planning ahead so that deadlines and payment obligations do not come as a surprise. Engage a qualified accountant early and the process becomes straightforward rather than stressful.

This article is intended for general informational purposes only and does not constitute tax, legal, or financial advice. Tax rules are subject to change and their application will depend on your individual circumstances. You should always seek advice from a qualified professional before taking action. Blue Tick Accountants accepts no liability for decisions made on the basis of this content.