Filing a Self Assessment tax return is how HMRC collects Income Tax and National Insurance on income that is not taxed at source. For most sole traders, freelancers and company directors, the process is far more manageable when broken into clear stages. This guide walks you through each step, from registering to submitting online and paying what you owe.

Who Must File a Self Assessment Return

Not everyone needs to complete a return, but you almost certainly do if any of the following apply during the tax year, which runs from 6 April to 5 April:

  • You were self-employed as a sole trader and earned above the trading allowance threshold
  • You were a partner in a business partnership
  • You received untaxed income from property, savings, investments or dividends above the relevant limits
  • You had to pay the High Income Child Benefit Charge
  • You earned income from abroad, or you are non-resident with UK income
  • You want to claim certain reliefs or report capital gains

Company directors often file too, particularly where they take dividends. If you are unsure, our self-employed and sole traders hub sets out the most common scenarios in more detail.

Registering with HMRC and Your UTR

If this is your first return, you must register with HMRC before you can file. Registration tells HMRC to expect a return and triggers issue of your Unique Taxpayer Reference (UTR), a ten-digit number you need every time you file.

The registration route depends on your situation:

Your situationHow to registerWhat you receive
New sole traderRegister for Self Assessment and Class 2 National InsuranceUTR and activation code
Not self-employed but with untaxed incomeRegister for Self Assessment onlyUTR and activation code
Partner in a partnershipRegister as a partner (the partnership registers separately)Individual UTR

Allow plenty of time, as the UTR and the Government Gateway activation code arrive by post and can take a couple of weeks. Register well before the deadline to avoid a last-minute scramble.

Information You Need to Gather

Good records make filing straightforward. Before you start, gather:

  • Your UTR and Government Gateway sign-in details
  • Records of all self-employment income and turnover
  • Business expense records and receipts
  • Details of any employment income and the tax already deducted, shown on your P60 or P45
  • Bank and building society interest, plus dividend vouchers
  • Records of any property income
  • Details of pension contributions and Gift Aid donations
  • Information on any capital gains or losses

Keeping these tidy throughout the year is far easier than reconstructing them in January. See our guide to sole trader bookkeeping basics explains a simple system that feeds straight into your return.

Completing the Main Return and Supplementary Pages

The main return (SA100) captures personal details and headline figures. On top of this, HMRC adds supplementary pages depending on the income you report:

  • SA103 for self-employment
  • SA104 for partnership income
  • SA105 for UK property
  • SA106 for foreign income
  • SA108 for capital gains

When you file online, the system asks a series of questions and only shows the sections that apply to you. Work through each section carefully, and double-check that figures match your records before moving on.

Claiming Expenses and Reliefs

You only pay tax on your profit, not your turnover, so claiming legitimate costs matters. Allowable business expenses typically include stock, equipment, premises costs, travel, and a share of home-office costs where you work from home.

You can also reduce your bill through reliefs such as pension contributions, Gift Aid and, for some businesses, capital allowances on equipment. Make sure you understand the difference between the cash basis and traditional accruals accounting, as this affects when income and costs are counted. For a fuller list of what you can deduct, see our business expenses and deductions hub .

Understanding Payments on Account

Many people are caught out by payments on account: advance instalments towards next year’s bill. If your Self Assessment liability is above the relevant threshold, HMRC asks you to pay half the estimated tax in January and the other half the following July, on top of any balancing payment for the year just gone.

This can make your first January feel expensive, because you settle the previous year and pay your first instalment together. Our guide to payments on account explained shows how the calculation works and how to budget for it.

Deadlines and Penalties

The key Self Assessment dates are predictable, so mark them in your calendar:

TaskDeadline
Register for Self AssessmentBy 5 October following the end of the tax year
File a paper return31 October
File online31 January
Pay any tax due (balancing payment)31 January
Second payment on account31 July

Missing the online filing deadline triggers an automatic fixed penalty, with further daily and percentage-based penalties the longer a return remains outstanding. Interest is charged on late tax, and late-payment penalties apply on top. Filing and paying on time is by far the cheapest approach.

Submitting Online

Filing through your HMRC online account is the standard route for most taxpayers. Before you press submit:

  1. Review each figure against your records
  2. Check the calculation HMRC produces, which shows your tax and National Insurance
  3. Confirm your payments on account for the coming year look right
  4. Save or print your submission receipt and the SA302 tax calculation

You can pay by bank transfer, debit card or Direct Debit. If you cannot pay in full, contact HMRC promptly, as a Time to Pay arrangement may be available. As Making Tax Digital for Income Tax is phased in, many sole traders will keep digital records and report quarterly using compatible software, so adopting a tidy digital system now is a sensible move. Our tax and VAT guides cover these changes in more depth.