Self Assessment Tax Return: UK Deadlines and Steps

Quick answer: A Self Assessment tax return reports income HMRC has not already taxed, such as sole trader profits. Register by 5 October after your first tax year, file online by 31 January (paper by 31 October), and pay by 31 January. Payments on account may also be due on 31 January and 31 July.

If you have started working for yourself in the UK, the Self Assessment tax return is the yearly form that turns your invoices and expenses into an Income Tax and National Insurance bill. The process is manageable once you know the dates, but the timing catches people out: you pay for the year after it ends, often alongside an advance payment for the next year, so a first bill can be much larger than expected.

This guide covers who needs to file, the key deadlines for the 2025 to 2026 return, the steps from registration to payment, payments on account with a worked example, penalties, and how Making Tax Digital changes things. It is general information, not tax advice. Dates and penalties are taken from GOV.UK's Self Assessment deadlines page and related guidance; check GOV.UK before you act.

Who needs to file a Self Assessment tax return?

You normally need to file if, in the last tax year (6 April to 5 April), any of these applied:

• You were self-employed as a sole trader and earned more than £1,000 before expenses, which is the trading allowance.
• You were a partner in a business partnership.
• You had untaxed income, such as rental income, significant savings or investment income, or foreign income.
• You need to pay the High Income Child Benefit Charge, or had income over certain levels.
• HMRC sent you a notice to file.

Limited company directors may need to file too, depending on their income. If you are unsure, GOV.UK has a checker tool. If you are not yet sure what structure to use, our guide to sole trader invoices in the UK explains what a sole trader's invoices must show.

Self Assessment tax return deadlines

These are the dates for the 2025 to 2026 tax year, which ended on 5 April 2026.

Deadline	What is due
5 October 2026	Register for Self Assessment if you need to file for the first time (or after a gap)
31 October 2026	Paper tax return must reach HMRC
30 December 2026	Online return, if you want an eligible balance under £3,000 collected through your PAYE tax code
31 January 2027	Online tax return; balancing payment for 2025 to 2026; first payment on account for 2026 to 2027
31 July 2027	Second payment on account for 2026 to 2027

Each deadline is 11:59pm on that day. If you registered after 5 October, HMRC gives you a filing deadline three months from the date of its notice if that is later than 31 January, but the payment date stays 31 January.

How to file a Self Assessment tax return: step by step

1. Register. New sole traders register online for Self Assessment and Class 2 and Class 4 National Insurance. Do it as soon as you start, and no later than 5 October after the end of your first tax year. HMRC then sends your Unique Taxpayer Reference (UTR), and you set up your online account.
2. Keep records all year. Record every invoice, payment received and business expense. Our guide on how to track business expenses sets up a simple routine.
3. Choose an accounting basis. From 2024 to 2025, the cash basis is the default for most sole traders, so you count income when paid and expenses when paid. You can opt for traditional accruals accounting instead. Our explainer on cash vs accrual accounting shows the difference.
4. Total your figures after 5 April. Add up turnover, allowable expenses and any other income for the tax year. Your accounts can be simple; HMRC does not require formal financial statements from most sole traders.
5. Complete the return. Online, the self-employment section asks for turnover and expenses. If your turnover is below the VAT threshold, you can usually give a single figure for total expenses.
6. Check the calculation. HMRC's online service calculates your Income Tax and Class 4 National Insurance and shows any payments on account.
7. Submit and pay by 31 January. Keep the submission reference and pay by bank transfer, debit card or another listed method. Allow time for payments to clear.
8. Keep your records. Sole traders generally keep records for at least five years after the 31 January filing deadline. Our guide on how long to keep invoices has the details.

What you pay: Income Tax and National Insurance

Your profit is taxed alongside your other income. For 2025 to 2026 in England, Wales and Northern Ireland, the personal allowance is £12,570 and the basic rate of 20% applies to the next £37,700 of taxable income, with higher rates above that. Scotland sets its own Income Tax bands for non-savings income, so Scottish taxpayers should use Scottish rates.

Self-employed people also pay Class 4 National Insurance at 6% on profits between £12,570 and £50,270, and 2% above that, according to GOV.UK. Class 2 contributions are no longer compulsory: if profits are above the small profits threshold, they are treated as paid, and below it you can choose to pay voluntarily to protect your State Pension record.

Payments on account explained

Payments on account are advance payments towards the next year's bill. Each one is normally half of the previous year's Self Assessment liability (Income Tax plus Class 4 National Insurance). According to GOV.UK's payments on account guidance, you do not have to make them if last year's bill was under £1,000, or if you paid more than 80% of your tax at source.

Worked example: a first-year sole trader's January bill

The figures below are illustrative. Sam started as a freelance designer in England in April 2025 and has no other income. For 2025 to 2026, Sam's turnover was £52,000 and allowable expenses were £12,000.

• Profit: £52,000 - £12,000 = £40,000
• Taxable income: £40,000 - £12,570 personal allowance = £27,430
• Income Tax: £27,430 x 20% = £5,486.00
• Class 4 National Insurance: (£40,000 - £12,570) x 6% = £27,430 x 6% = £1,645.80
• Total 2025 to 2026 bill: £5,486.00 + £1,645.80 = £7,131.80

Because the bill is over £1,000 and none of it was taxed at source, payments on account apply for 2026 to 2027. Each is half of £7,131.80, which is £3,565.90.

Date	Payment	Amount
31 January 2027	Balancing payment for 2025 to 2026	£7,131.80
31 January 2027	First payment on account for 2026 to 2027	£3,565.90
31 January 2027	Total due	£10,697.70
31 July 2027	Second payment on account for 2026 to 2027	£3,565.90

The arithmetic for January: £7,131.80 + £3,565.90 = £10,697.70. That is roughly 150% of one year's tax in a single month, which is why first-year sole traders should set money aside from every paid invoice. If Sam's 2026 to 2027 bill turns out to be £8,000, Sam pays the difference of £8,000 - (£3,565.90 x 2) = £8,000 - £7,131.80 = £868.20 by 31 January 2028.

Self Assessment penalties

Under the standard Self Assessment rules on GOV.UK's penalties page:

Lateness	Late filing penalty	Late payment penalty
1 day	£100 fixed penalty	Interest starts
30 days	No change	5% of tax unpaid
3 months	£10 a day, up to £900	Interest continues
6 months	5% of tax due or £300, whichever is greater	Further 5% of tax unpaid
12 months	Another 5% or £300, whichever is greater	Further 5% of tax unpaid

The £100 penalty applies even if you owe no tax. Interest is charged on late tax at a rate HMRC sets in line with the Bank of England base rate. You can appeal if you had a reasonable excuse. If you are within Making Tax Digital for Income Tax, a newer points-based system for late submissions and a different late payment regime apply, so check the current GOV.UK rules.

Making Tax Digital and your Self Assessment

From 6 April 2026, sole traders and landlords with qualifying income over £50,000 must use Making Tax Digital for Income Tax. They keep digital records, send quarterly updates through compatible software and submit their return through that software by 31 January. The threshold drops to £30,000 from April 2027 and is due to fall to £20,000 from April 2028. Our guide to Making Tax Digital for Income Tax explains who is in scope and what software you need.

Common mistakes with the Self Assessment tax return

• Registering late. Missing 5 October can lead to a failure to notify penalty if tax is unpaid at 31 January.
• Forgetting payments on account. The first January bill often includes the next year's first instalment.
• Leaving it until January. HMRC's systems and helplines are busiest near the deadline, and you need your UTR and login ready.
• Claiming personal costs. Only costs wholly and exclusively for the business are allowable; split mixed-use costs fairly.
• Using the wrong tax year. The UK tax year runs 6 April to 5 April, not January to December.
• Ignoring VAT. If turnover passes the UK VAT registration threshold, VAT registration is a separate obligation from Self Assessment.
• Weak records. Missing invoices make income hard to prove. Our bookkeeping basics guide covers a simple system.

Self Assessment checklist for sole traders

• Registered with HMRC and UTR received
• Government Gateway or HMRC online account login working
• All invoices and payments for 6 April to 5 April listed
• Allowable expenses totaled, with receipts kept
• Accounting basis (cash or accruals) confirmed
• Other income, such as interest or rent, gathered
• Payments on account checked and money set aside
• Return filed and paid by 31 January
• Making Tax Digital status checked for the next tax year

Where invoices fit in

Every invoice you issue is part of the evidence behind your turnover figure. Our free UK invoice generator creates PDF invoices in your browser with your details, dates, line items and sequential numbers, and you can choose a clean layout such as the minimal invoice template. It does not file your return, track income or send invoices for you, so keep each PDF and note when it was paid.

This article is general information, not tax advice. Tax rates, deadlines and penalties change, and your circumstances matter; check GOV.UK or speak to a qualified accountant before acting.