How to fill in a Self Assessment tax return: step by step (2026)

If you have never done one before, a tax return looks scarier than it is. For most people with a job and a side hustle, it takes an hour or two once your records are in order. Here is every step.

Before you start: what you need

• Your UTR, the 10-digit number HMRC posts to you after you register
• Your National Insurance number
• Your P60 from your job, or your final payslip of the tax year
• Your side hustle income and expenses for the tax year (6 April to 5 April)
• Any other income: savings interest over your allowance, rental income, dividends
• Bank details, in case you are owed a refund

Step 1: register for Self Assessment

If this is your first return, register on GOV.UK by 5 October after the end of the tax year. For side income earned between 6 April 2025 and 5 April 2026, the deadline was 5 October 2026.

Choose the option for self-employed or sole trader if your side income is from trading, like selling, freelancing or deliveries. HMRC then posts your UTR. Allow a couple of weeks.

Registered late? Do it now. You can still file by 31 January, and if you pay on time you are unlikely to get a penalty for registering late.

Step 2: sign in and start your return

Sign in to your HMRC online account, choose Self Assessment and start the return for the right tax year. The online return asks questions and only shows the sections you need.

Step 3: tell it what income you had

Early on, you tick the types of income you had. For a typical side hustler that is:

• Employment, for your job
• Self-employment, for your side hustle

Tick anything else that applies, such as UK property for rental income above the £1,000 property allowance.

Step 4: fill in your employment details

Copy these from your P60:

• Your employer's PAYE reference
• Pay from this employment
• Tax taken off

Getting these right matters, because the tax you have already paid is taken off your bill.

Step 5: fill in your self-employment section

Most side hustles can use the short version. You enter:

1. A description of the business, like "online reselling" or "freelance graphic design"
2. Your start date, if you started in the tax year
3. Turnover: everything you took in, before costs
4. Expenses: either your actual allowable expenses, or the trading allowance of £1,000. You cannot claim both.

If your turnover is £90,000 or less you can enter your expenses as one total. Keep a breakdown in your records anyway.

See what expenses you can claim. For mileage, use the flat rate: mileage calculator.

Step 6: check the calculation

Before you submit, HMRC shows how much tax and Class 4 National Insurance you owe. Compare it with an estimate:

Free toolSide hustle tax calculatorSee how much tax and National Insurance you owe on side income, and what to set aside each month.→

If the number looks very different, check you entered your P60 figures and your turnover correctly. A missing employment page is the most common reason for a shocking bill.

Step 7: submit and save a copy

Submit the return and download the calculation and your submission receipt. You can amend an online return within 12 months of the 31 January deadline if you spot a mistake.

Step 8: pay the bill

Pay by 31 January. You can pay by bank transfer, debit card or set up a payment plan if you cannot pay in one go.

If your bill is over £1,000 and less than 80% of your tax was taken at source, you may also have to make payments on account towards next year. The first is due on the same 31 January.

Deadlines and penalties

Miss the online deadline and there is an automatic £100 penalty, even if you owe no tax. After three months, £10 a day is added for up to 90 days, with more after six and twelve months. Late payment adds interest and further penalties. Source: GOV.UK: Self Assessment penalties.

Common questions

How long does a Self Assessment tax return take?

For a job plus a simple side hustle, often an hour or two, if your income and expenses are already totalled. Most of the time goes on gathering figures, not the form.

Do I need an accountant?

Not for a simple side hustle. An accountant is worth considering if you have several income sources, rental property with a mortgage, or you are unsure whether something counts as an expense.

What if I made a mistake on my tax return?

Sign back in and amend it. You have 12 months from the 31 January filing deadline to make changes.

Will Making Tax Digital replace this?

For people with self-employment and property income over the thresholds, partly. From April 2026 (over £50,000), April 2027 (over £30,000) and April 2028 (over £20,000), you send quarterly updates through software. Check with the MTD checker.