How to File Your First Self Assessment Tax Return

New sole traders and first-time filers can work through registration, record keeping and submission in order, with the deadlines and pitfalls flagged at each stage.

Summary

• Register with HMRC by 5 October following the end of the tax year in which you met the Self Assessment criteria, even if you owe no tax.
• The online return and the tax bill are both due by 31 January after the tax year ends; paper returns must reach HMRC by 31 October.
• Keep bank statements, receipts and your P60 from the start; cash basis accounting is the default for self-employed people from April 2024.
• If you registered for Self Assessment before but did not file last year, you may need to reactivate your account before filing.
• From April 2026, Making Tax Digital for Income Tax starts replacing Self Assessment for many self-employed people and landlords with qualifying income over £50,000.

Filing a Self Assessment tax return for the first time can look like a pile of paperwork with no obvious starting point. Split into stages, it is a manageable job: check whether you need to file, register with HMRC, keep records, then submit the return before the deadlines. This guide works through each stage in order.

Step 1: Check whether you need to file

Self Assessment is HM Revenue and Customs’ (HMRC) system for collecting tax on income that is not taxed at source. Most employment income is handled through Pay As You Earn (PAYE), with employers deducting tax before you are paid. Common situations within the criteria for the 2026/27 tax year include:

• self-employment, including gig economy work, unless income before expenses is within the annual £1,000 trading allowance
• property income above certain reporting limits
• claiming tax relief on employment expenses over £2,500
• capital gains tax to pay that has not been paid in-year
• total taxable income of £150,000 or more before tax
• the high income child benefit charge

The list is not exhaustive. You can fall within the criteria even if you owe nothing. Failing to register when required can bring a failure to notify penalty where tax is due. GOV.UK has a tool that checks your position.

From April 2026, Making Tax Digital for Income Tax began replacing Self Assessment for many self-employed people and landlords, starting with those who had qualifying income over £50,000 in the 2024/25 tax year. Check whether the rules apply to you.

Step 2: Register with HMRC

Register by 5 October following the end of the tax year in which you met the criteria. The tax year runs from 6 April to 5 April. Registration is a one-off: HMRC sends a notice to file each year until you leave Self Assessment, so you never re-register annually.

The route depends on your circumstances. Self-employed people register as self-employed; anyone else who needs to file, for example because of property income, completes form SA1 online or on paper. HMRC then issues a unique taxpayer reference (UTR) and sends a formal notice to file.

Step 3: Set up your sign-in details

You sign in to HMRC online services with either a Government Gateway user ID or GOV.UK One Login details, which use an email address and password. If you have never used HMRC online services, choose “Create new sign in details”, even if you already use One Login elsewhere. HMRC posts an activation code after a first registration, so allow time for it to arrive.

Step 4: Keep records from day one

HMRC expects you to keep records such as bank statements and receipts so you can fill in the return correctly. The documents that matter most:

• your P60, showing pay, tax deducted and your employer’s PAYE reference
• your P11D, if your employer issues one
• accounting records or bank statements covering the full tax year

A missing P60 can be requested from your employer or HMRC, and a replacement activation code can be requested if the original is lost.

Step 5: Work out your allowable expenses

Allowable expenses are running costs you set against business income before working out taxable profit; keep a receipt for every claim. From April 2024, cash basis accounting is the default for self-employed businesses and partnerships with trading income, meaning income and costs are recorded when money is actually received or paid. Some businesses can opt for the accruals method instead. Cash basis is simpler to run, but it is worth checking which method fits before you file.

Step 6: Fill in the return section by section

File any time on or after 6 April following the end of the tax year; filing early means knowing what you owe well before the payment deadline.

For employment income you need the salary received, the tax taken off at source and your employer’s PAYE reference, all shown on your P60. If you are a limited company director, tax is due on salary that was due in the tax year, regardless of when it was paid.

For self-employment, add up all your business income for the year and enter it alongside your expenses; work from records, not estimates.

Step 7: Deadlines, payments and penalties

Missing a deadline brings a penalty, and registering after 5 October can also trigger one. HMRC suggests estimating what you owe and setting up weekly or monthly payments to spread the cost; our guide to easy-access vs fixed-rate savings sets out where to keep the money until it is due. Depending on your circumstances, you may also be asked to make advance payments towards the following year’s tax, known as payments on account.

Common first-time mistakes to avoid

• Missing the 5 October registration deadline by assuming registration happens at filing time.
• Filing through an old account without reactivating it, which can delay the return.
• Entering income and expenses from memory instead of from receipts and statements.
• Using a P60 from the wrong tax year for employment figures.
• Assuming there is nothing to file because no tax is owed.

Next steps

• Budgeting for UK Household Bills: building the regular set-aside that covers a January tax bill alongside household costs.
• Workplace Pensions and Auto-Enrolment: how payroll deductions work if you take employed income alongside self-employment.

Questions readers ask