A self assessment tax return is the form HM Revenue and Customs (HMRC) uses to collect Income Tax on money that wasn’t taxed automatically through an employer’s PAYE payroll.1GOV.UK. Self Assessment Tax Returns: Overview If you’re self-employed, receive rental income, have significant savings or investment income, or fall into certain other categories, you’re responsible for reporting what you earned and paying whatever tax is due. The UK tax year runs from 6 April to 5 April, and the main filing deadline for online returns is midnight on 31 January after the tax year ends.
Who Has to File
You need to send a return for the previous tax year if any of the following applied to you:2GOV.UK. Self Assessment Tax Returns: Who Must Send a Tax Return
- You were self-employed as a sole trader and earned more than £1,000 before expenses.3GOV.UK. Tax-Free Allowances on Property and Trading Income
- You were a partner in a business partnership, no matter how much you earned.
- You had untaxed income from renting property, savings interest above your allowance, tips, or commission not run through payroll.
- You’re a UK resident with foreign income or capital gains. The main exception is if your only foreign income was dividends and your total dividends (UK and foreign) came in under the £500 dividend allowance.4GOV.UK. Tax on Foreign Income: Reporting Your Foreign Income
- You or your partner earned more than £60,000 while claiming Child Benefit, triggering the High Income Child Benefit Charge.5GOV.UK. High Income Child Benefit Charge: Overview
- You sold or disposed of assets and need to report a taxable capital gain.
Earning above £100,000 used to be an automatic filing trigger. HMRC has been removing that blanket rule for people whose tax is handled entirely through PAYE. If you’re employed, earn between £100,000 and £150,000, and have no other income streams or reliefs to claim, HMRC may no longer require a return from you. You’ll get a letter if your filing obligation has changed. When in doubt, check your Personal Tax Account on GOV.UK or contact HMRC directly. Not telling HMRC when you should have registered can lead to penalties under Schedule 41 of the Finance Act 2008.6GOV.UK. Extending Schedule 41 Finance Act 2008 Penalties
The Deadlines That Matter
If you’ve become liable for self-assessment and haven’t filed before, you must register with HMRC by 5 October following the end of the tax year in question.7GOV.UK. Self Assessment Tax Returns: Deadlines So if you started freelancing in August 2025, you’d need to register by 5 October 2026.
Once you’re registered, the submission and payment dates are:
- Paper returns must reach HMRC by midnight on 31 October after the tax year ends.
- Online returns must be submitted by midnight on 31 January after the tax year ends.
- Any tax owed is due by 31 January as well.
There’s also a useful earlier date. If you want HMRC to collect a smaller tax bill (under £3,000) through your PAYE code over the next year, file your online return by 30 December instead of 31 January.8GOV.UK. Pay Your Self Assessment Tax Bill: Through Your Tax Code
What You Need Before You Start
Get everything together before you open the return. Your Unique Taxpayer Reference (UTR) is the 10-digit number HMRC gave you when you registered for self-assessment.9GOV.UK. Find Your UTR Number It’s on previous returns, HMRC letters, and inside your online tax account. Without it, you can’t file.
The supporting records depend on your income. Employed taxpayers need their P60 showing total pay and tax deducted, plus any P11D for benefits like a company car or private medical cover. Self-employed taxpayers need business income and expenses: receipts, invoices, bank statements, mileage logs. If you let property, keep records of rent received and allowable costs such as repairs, letting fees, and insurance.
Bank and building society statements cover savings interest. Dividend vouchers or platform statements cover investments. If you paid into a personal or workplace pension, keep the paperwork, because higher-rate and additional-rate taxpayers can claim extra relief on the return.10GOV.UK. Claim Tax Relief on Your Private Pension Payments The same is true for Gift Aid donations.
Keep your records for at least five years after the 31 January submission deadline for that tax year.11GOV.UK. Business Records if You’re Self-Employed: How Long to Keep Your Records A 2024–25 return filed by 31 January 2026 means holding records until the end of January 2031.
How to File Your Return Online
Filing happens through HMRC’s online services. You can sign in with Government Gateway (user ID and password) or GOV.UK One Login (email and password).12GOV.UK. HMRC Online Services: Sign In or Set Up an Account If you don’t have login credentials, set them up the first time you access the service. Have your UTR ready, and expect HMRC to ask you to verify your identity with photo ID such as a passport or driving licence.13GOV.UK. File Your Self Assessment Tax Return Online
Once you’re in, go to the self-assessment section and open your personalised return. The form is split into sections for employment, self-employment, property, investments, pensions, and other income, and you only fill in the ones that apply. The system calculates your bill automatically as you go. Review the summary, submit through the declaration page, and save the confirmation number that appears on screen.
Paying What You Owe
Whatever tax you owe is due by 31 January. You can pay by bank transfer, debit card, online or telephone banking, or in person at your bank or building society. If your bill is under £3,000 and you filed online by 30 December, HMRC can collect it through your PAYE tax code over the following year.8GOV.UK. Pay Your Self Assessment Tax Bill: Through Your Tax Code
Payments on Account
If last year’s self-assessment bill was £1,000 or more, and less than 80% of your total tax was collected at source, HMRC requires payments on account: advance payments toward next year’s bill.14GOV.UK. Understand Your Self Assessment Tax Bill: Payments on Account Each payment is half of your previous year’s tax bill, and they fall due on 31 January and 31 July.15GOV.UK. Pay Your Self Assessment Tax Bill: Overview
These are estimates. If your income drops, you can apply to reduce them. If your income rises, expect a balancing payment the following January on top of your first payment on account for the new year. That combined January bill is what catches first-time filers by surprise.
Budget Payment Plan
To spread the cost, HMRC offers a Budget Payment Plan that lets you make weekly or monthly Direct Debit payments toward a future bill.16GOV.UK. Pay Your Self Assessment Tax Bill: Pay Weekly or Monthly You choose the amount and frequency. You must be up to date on previous self-assessment payments to set one up. If what you’ve paid in doesn’t cover your bill, you pay the difference by 31 January. If you’ve overpaid, you can request a refund. Payments can be paused for up to six months.
Late Filing and Payment Penalties
The penalty structure escalates fast. For late filing:17GOV.UK. Self Assessment Tax Returns: Penalties
- Immediately after the deadline: £100 fixed penalty, even if you owe no tax.
- After 3 months: an additional £10 per day, up to £900.
- After 6 months: the greater of £300 or 5% of the tax due.
- After 12 months: a further charge of the greater of £300 or 5% of the tax due.
A return that’s a full year late can rack up over £1,600 in fixed penalties before you add tax and interest. Late payment carries its own charges: 5% of the unpaid tax at 30 days, another 5% at 6 months, and a further 5% at 12 months, with interest running from the original due date.17GOV.UK. Self Assessment Tax Returns: Penalties
HMRC will cancel a penalty if you had a reasonable excuse, but the bar is high. Accepted reasons include the death of a partner or close relative shortly before the deadline, an unexpected hospital stay or serious illness, a fire, flood, or theft that stopped you completing the return, computer or software failure while preparing an online return, problems with HMRC’s own online services, unpredictable postal delays, and a disability or mental health condition that affected your ability to deal with tax affairs.18GOV.UK. Reasonable Excuses Even with a valid excuse, you have to file or pay as soon as you’re able. The excuse covers the initial delay, not an ongoing one.
Fixing a Mistake After You File
If you notice an error after submitting, you can amend the return online within 12 months of the self-assessment deadline for that tax year.19GOV.UK. Self Assessment Tax Returns: If You Need to Change Your Return For the 2023–24 tax year, the amendment window closes on 31 January 2026. Wait at least 72 hours after your original submission, then sign in, open the relevant tax year, make the corrections, and resubmit. The updated bill appears immediately, and your statement will show the difference and any interest within three days.
Miss the 12-month window, or need to correct a return from an earlier year, and you’ll have to write to HMRC directly rather than use the online system.