Self Assessment tax return: the complete 2026/27 guide

Who needs to file a Self Assessment return?

Most people who are employed and taxed through PAYE never file one. You usually need to if you are self-employed, a partner in a business, a landlord, a company director with extra income, or you have untaxed income, capital gains or foreign income. You may also need to file if you receive Child Benefit and your income is above the threshold. If your self-employed income is only small, the £1,000 trading allowance can mean you do not need to report it.

If you are unsure, our guide on how to file a Self Assessment tax return walks through the basics, or you can ask us for a quick check.

Key Self Assessment dates for 2026/27

How to file your return, step by step

1. Register with HMRC and get your Unique Taxpayer Reference (UTR).
2. Gather your records: income, expenses, bank interest, dividends, rental income, pension payments and employment details.
3. Work out your profit using the trading allowance or your actual expenses, whichever is better.
4. Complete and submit the return through your HMRC account or approved software.
5. Pay your tax by 31 January and make any payments on account.

Payments on account explained

Payments on account are advance payments towards next year’s bill. Each is half of your previous year’s bill, due on 31 January and 31 July. You normally make them if your last Self Assessment bill was over £1,000 and less than 80% of your tax was collected at source. For example, with a £3,000 bill you pay £3,000 plus a £1,500 first payment on account on 31 January 2027, then £1,500 on 31 July 2027. If your income will fall you can ask HMRC to reduce them. Read more in our guide to payments on account.

Penalties for filing or paying late

• £100 immediately if the return is late, even if you owe nothing.
• £10 a day after three months, up to £900.
• Interest on unpaid tax, plus further penalties if payment is very late.

Allowances and reliefs to check

• Personal allowance of £12,570, tapered above £100,000.
• £1,000 trading allowance and £1,000 property allowance, as an alternative to claiming actual expenses.
• Pension contributions, which can extend your basic rate band.
• Mileage at 45p for the first 10,000 business miles and 25p after.

Worked example: a London freelancer’s first return

A freelance designer in London has £45,000 of turnover and £9,000 of allowable expenses, so her profit is £36,000. For 2026/27 her income tax is £4,686 and Class 4 National Insurance is £1,406, a total of £6,092, or about £508 a month to set aside. In her first January she pays that balance plus a first payment on account of £3,046 (half the bill), and then a second payment of £3,046 the following July. That is why many people are surprised by the size of their first January payment.

Records you need to keep

• Sales invoices and income records, including cash and card takings.
• Receipts, bills and bank statements for every expense you claim.
• Mileage logs and details of any home-working costs.
• Pension, Gift Aid and bank interest statements.

Keep records for at least five years after the 31 January submission deadline for the tax year.

Checklist before you file

1. Confirm you have your UTR and HMRC online account access.
2. Total income and expenses by category.
3. Decide between actual expenses and the £1,000 trading allowance.
4. Check other income such as dividends, rent, interest and pensions.
5. Estimate the bill and set the cash aside.

What happens after you file

HMRC calculates your bill and shows it in your online account. If you spot an error you can usually amend the return within 12 months of the 31 January filing deadline. If you overpaid, HMRC repays the difference to your bank account. Keep a copy of the filed return and your calculation.

Self Assessment for London sole traders and freelancers

The tax rules are the same across the UK, but London costs can matter. Co-working desks, studio rent and public transport for business trips are typically allowable. Congestion Charge and ULEZ payments on business journeys can usually be claimed, but fines cannot. We help self-employed people in every London borough, from Croydon to Camden, online and with a fixed fee. Browse our self-employed accountants by borough.

Common mistakes to avoid

• Missing income, such as small side earnings or interest.
• Claiming personal costs as business expenses.
• Forgetting payments on account, which makes the January bill much larger.
• Not keeping records for at least five years.
• Filing late because of a missing UTR. Register early.