What is the deadline for filing a Self Assessment tax return?

The online filing deadline is 31 January following the end of the tax year. For the 2024/25 tax year (ending 5 April 2025), the online deadline is 31 January 2026. Paper returns must be filed by 31 October.

What happens if I file my Self Assessment late?

A £100 fixed penalty applies immediately if you miss the 31 January deadline, even if you owe no tax. Further daily penalties of £10 per day apply after 3 months (up to £900), and additional penalties after 6 and 12 months.

What expenses can I claim on Self Assessment?

You can claim allowable business expenses including office costs, travel (not commuting), clothing (uniforms only), staff costs, stock, financial costs, marketing, and training. HMRC publishes a full list at gov.uk/expenses-if-youre-self-employed.

Do I need to pay tax before I file my return?

Your first payment on account is due by 31 January alongside your tax return. If your bill exceeds £1,000, HMRC will also ask for a second payment on account by 31 July. These are advance payments towards next year's bill.

Can I amend a Self Assessment return after filing?

Yes. You can amend your return online within 12 months of the original filing deadline. After that, you need to write to HMRC. Amendments can result in a refund or an additional tax bill.

How to File a Self Assessment Tax Return (UK, 2026)

Last updated May 2026Reviewed against UK gov.uk sources

Self Assessment is HMRC's annual reconciliation: you tell them everything you earned that wasn't already taxed at source, claim the expenses and allowances you're entitled to, and pay the difference. The form looks long because it covers everyone (landlords, partners, high earners, foreign income) but most sole traders only need 4–5 sections.

Direct answer

The first one is intimidating; the second is half an hour. This guide takes you screen-by-screen through HMRC's online form, what to gather first, and the figures most people miscalculate. Use the key facts, step list and official source links on this page to confirm the decision before you spend money or register anything.

Online deadline: 31 January
Paper deadline: 31 October
Late filing fine: £100 + daily
Records to keep: 5 years

Checklist

Quick checklist

Gather UTR, Government Gateway login, NI number
Total all self-employed income for 6 April–5 April
List allowable expenses with receipts to back them up
Collect P60, P45, interest summaries, dividend statements
Add pension contributions and Gift Aid
Submit by 31 January
Diarise the 31 July payment on account

Section 01

Before you log in — gather these

Your UTR (10-digit Unique Taxpayer Reference) — on any HMRC letter.
Your Government Gateway login.
All self-employed income — total invoiced or paid in the tax year (6 April–5 April).
All allowable expenses — broken down by category (see below).
P60 / P45 if you had any employed income.
Interest from UK banks — most banks post a summary in your online statements.
Pension contributions and Gift Aid donations — both reduce tax.
Student Loan plan type if applicable.

Section 02

The form — section by section

01
Tell us about you
Confirm address, marital status, residency. Tick 'self-employed' to unlock the SA103 self-employment pages.
02
Self-employment
Trading name, description, accounting dates. Then turnover (gross income) and either: claim the £1,000 trading allowance, OR list expenses (rent, phone, software, travel, materials, use of home, capital allowances on equipment).
03
Other income
Bank interest, dividends (use the £500 dividend allowance), rental income, foreign income.
04
Tax reliefs
Pension contributions, Gift Aid, marriage allowance transfer if applicable.
05
Calculate
HMRC's system computes income tax + Class 2/4 NI. Check the figure against your own estimate — a 10%+ gap usually means you missed a section.
06
Submit and pay
Pay by 31 January via bank transfer, debit card, or direct debit. If your bill is over £1,000 you'll also pay 50% on account towards next year.

Section 03

Expenses sole traders forget to claim

Use of home as office — £6/week flat or a proportion of bills.
Mileage — 45p/mile for the first 10,000 business miles, 25p after.
Mobile phone — proportion of contract used for business.
Software subscriptions, domain names, hosting.
Accountant or bookkeeper fees.
Bank charges on a business account.
Training that maintains existing skills (not new qualifications).
Equipment under £1,000 — usually fully deductible via the Annual Investment Allowance.

Section 04

Payments on account — the bit no one warns you about

If your tax bill exceeds £1,000 and less than 80% of your tax is collected at source, HMRC asks for two 'payments on account' towards the following year — 50% on 31 January and 50% on 31 July. So your first big bill is effectively 150% in one go. Set aside cash from day one.

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Common questions

How to file a Self Assessment tax return