Every year, millions of people report their income to HMRC through Self Assessment. For first-time filers, the hardest part is often not the return itself, but knowing whether you need to file at all, how to register and which deadlines apply.
This guide works through the full process: who needs to file, how to register, what information to gather, the key deadlines and how to avoid penalties. It is a practical roadmap, so you can prepare in advance rather than rushing in January.
What Is Self Assessment?
Self Assessment is the system HMRC uses to collect Income Tax and National Insurance from people whose tax is not taken automatically at source. You report your income, gains and allowable expenses for the tax year, which runs from 6 April to 5 April, and HMRC uses this to work out your tax bill.
Most employees and pensioners pay tax through PAYE (Pay As You Earn), where tax is deducted before they are paid. Self Assessment covers income that PAYE cannot reach, such as self-employed profits, rental income, dividends and capital gains.
Filing a return does not always mean paying more tax. Some people file and find they are due a tax refund.
Who Needs to File a Self Assessment Tax Return?
Whether you need to file depends on your individual circumstances. Whatever your situation, if HMRC sends you a notice to file a return, you must file one.
Sole Traders
You must file if you were self-employed as a sole trader and earned more than £1,000 in gross income during the tax year. Income up to £1,000 is covered by the trading allowance. For example, a self-employed electrician earning £28,000 must file each year, even if they also have a part-time PAYE job.
Company Directors
Directors whose income is all taxed through PAYE are not automatically required to file. Many owner-managed company directors still need to, because they take much of their income as dividends. A director paying themselves a modest salary and taking dividends of more than £10,000, for example, will need to file a return.
Landlords
Rental income up to £1,000 is covered by the property allowance. You must file a return if your rental income is £10,000 or more before expenses, or £2,500 or more after expenses. Below these levels, you may be able to pay through your tax code instead.
Individuals with Additional Income
You may need to file if, alongside a job or pension, you have:
• untaxed income over £2,500, such as commission, tips or income from casual work
• dividends over £10,000
• savings or investment income over £10,000
• income from overseas
• capital gains to report, such as from selling shares or a second property
UK residential property sales with tax to pay also need a separate Capital Gains Tax return within 60 days.
High Earners and Other Common Scenarios
Other common reasons for filing include:
• you or your partner received Child Benefit and either of you had adjusted net income over £60,000, unless the High Income Child Benefit Charge is paid through your tax code
• you are a partner in a business partnership
• you want to claim higher-rate tax relief on personal pension contributions or Gift Aid donations
High earners with income over £100,000 often have more complex tax affairs, including the gradual loss of the Personal Allowance. If you are unsure, HMRC’s online checker can confirm whether you need to file.
Once you know you need to file, the process follows five stages:
Self Assessment Timeline
| Stage | What You Need to Do |
| Register for Self Assessment | Apply for a UTR and set up your HMRC online account |
| Gather financial records | Collect income and expense information |
| Complete your tax return | Submit online or by paper (where applicable) |
| Pay your tax bill | Meet the payment deadline to avoid penalties |
| Keep records | Retain records for HMRC compliance |
How to Register for Self Assessment
First-time filers need to register with HMRC in four steps:
• check that you need to file
• register online as self-employed or not self-employed
• receive your Unique Taxpayer Reference (UTR) by post
• activate online Self Assessment using the activation code HMRC sends you
Getting a UTR Number
When you register, HMRC issues a UTR number, a 10-digit reference that identifies you for Self Assessment.
Setting Up a Government Gateway Account
To file online, you need an HMRC online account, accessed through a Government Gateway user ID or GOV.UK One Login as HMRC moves services across. After you register, HMRC posts an activation code, which you use to switch on online Self Assessment.
Registering Before the Deadline
You must register by 5 October after the end of the tax year in which you first need to file. For income in the 2025 to 2026 tax year, the deadline is 5 October 2026. If you miss it, register as soon as possible, as late registration can lead to a penalty if tax is unpaid.
What Information Do You Need to Complete a Tax Return?
Depending on your circumstances, you may need:
• your UTR and National Insurance number
• P60, P45 or P11D forms from any employment
• business income and expense records, such as invoices, receipts and bank statements
• rental income and costs, including letting agent statements and mortgage interest
• dividend vouchers and interest statements from banks and building societies
• purchase and sale details for any assets sold
• pension contributions, Gift Aid donations and your student loan plan type, if relevant
For sole traders and landlords, allowable expenses are costs incurred wholly and exclusively for the business or rental, such as stock, travel, insurance and accountancy fees. Claiming the right expenses reduces your taxable income.
Key Self Assessment Deadlines
The deadlines fall on the same dates each year. Here is how they apply to the 2025 to 2026 tax year:
| Deadline | What It Is For | 2025 to 2026 Tax Year |
| 5 October | Registering for Self Assessment if you are filing for the first time | 5 October 2026 |
| 31 October | Paper tax returns | 31 October 2026 |
| 31 January | Online tax returns, paying the balance owed and any first payment on account | 31 January 2027 |
| 31 July | Second payment on account | 31 July 2027 |
If your last tax bill was more than £1,000, HMRC usually asks for payments on account: two advance payments towards the next year’s bill, each half of the previous year’s total. You do not need to make them if more than 80% of your tax was collected at source, such as through PAYE.
For example, if your 2025 to 2026 bill is £4,000, you pay that £4,000 by 31 January 2027, plus a first payment on account of £2,000 towards 2026 to 2027. A further £2,000 is due by 31 July 2027.
Making Tax Digital for Income Tax began on 6 April 2026 for sole traders and landlords with qualifying income over £50,000. The threshold is due to fall to £30,000 from April 2027 and £20,000 from April 2028. Those affected keep digital records, send quarterly updates through compatible software and submit a final declaration after the tax year ends.
What Happens If You Miss the Deadline?
Missing the filing deadline triggers a penalty even if you owe no tax. Late filing penalties build up as follows:
| How Late | Late Filing Penalty |
| 1 day | £100 fixed penalty |
| 3 months | £10 a day, up to a maximum of £900 |
| 6 months | £300 or 5% of the tax due, whichever is higher |
| 12 months | A further £300 or 5% of the tax due, whichever is higher (more in some cases) |
Interest is charged on late payments from the day after the deadline, at the Bank of England base rate plus 4%. Late payment penalties of 5% of the unpaid tax are added at 30 days, six months and 12 months. Different penalty rules apply to people within Making Tax Digital for Income Tax.
If you cannot pay, contact HMRC before the deadline. You may be able to set up a Time to Pay arrangement, which can often be done online if you owe up to £30,000.
Common Self Assessment Mistakes to Avoid
• Missing income: forgetting bank interest, dividends, a side income or an asset sold during the year
• Incorrect expense claims: claiming personal costs, or missing expenses you are entitled to
• Poor record-keeping: relying on memory or bank statements alone
• Forgetting payments on account: budgeting only for the January balance, then being caught out by advance payments
• Landlord finance costs: deducting residential mortgage interest as an expense rather than claiming the basic rate tax credit
• Leaving it to the last minute: rushing in late January increases errors and leaves no time to fix login problems
Tips for Making Self Assessment Easier
Keep Records Throughout the Year
Set aside time each month to file receipts, invoices and statements. Sole traders and landlords must keep records for at least five years after the 31 January submission deadline.
Use Accounting Software
Accounting software links to your bank account, categorises transactions and gives a running view of your profit and tax position. If you fall within Making Tax Digital for Income Tax, compatible software is a requirement rather than an option.
Work with an Accountant
An accountant can register you with HMRC, check you are claiming all allowable expenses, prepare and file your return and help you plan for payments on account. Support is especially worthwhile in your first year, with several income sources or when moving into Making Tax Digital.
Frequently Asked Questions
What is Self Assessment?
Self Assessment is the system HMRC uses to collect tax on income that is not taxed automatically through PAYE. You report your income and gains on a tax return, and HMRC works out the Income Tax and National Insurance you owe, or any refund due.
Who needs to complete a Self Assessment tax return?
You usually need to file if you are self-employed with income over £1,000, receive rental income above certain levels, have untaxed income over £2,500, receive dividends over £10,000, have capital gains to report or have been sent a notice to file by HMRC. Requirements depend on your circumstances.
How do I register for Self Assessment?
Register online through GOV.UK by 5 October after the end of the tax year in which you need to file, choosing the self-employed or not self-employed option. HMRC will then send your UTR and an activation code by post, so you can set up online filing.
What is a UTR number?
A Unique Taxpayer Reference is a 10-digit number HMRC gives you when you register for Self Assessment. It does not change, and it appears on HMRC letters, in your HMRC online account and in the HMRC app. You need it to file your tax return.
When is the Self Assessment deadline?
The online filing and payment deadline is 31 January after the end of the tax year. For the 2025 to 2026 tax year, that is 31 January 2027. Paper returns are due earlier, by 31 October 2026, and any second payment on account is due by 31 July 2027.
What happens if I file my tax return late?
You receive an automatic £100 penalty, even if you owe no tax. Further penalties apply at three, six and 12 months, and interest is charged on any tax paid late. If you have a reasonable excuse, such as serious illness, you can appeal the penalty.
Can I complete my Self Assessment online?
Yes. Most people file online through their HMRC online account or compatible accounting software. Filing online gives you three months longer than a paper return, calculates your tax automatically and lets you save your progress as you go, so you can complete it in stages.
What expenses can I claim?
Sole traders can claim costs incurred wholly and exclusively for the business, such as stock, equipment, business travel, insurance and accountancy fees. Landlords can claim costs such as letting agent fees, repairs and insurance. Alternatively, you can use the £1,000 trading or property allowance instead of claiming actual expenses.
Do I need to file if I already pay tax through PAYE?
Not if PAYE is your only income and HMRC has not asked you to file. You may still need to if you have rental income, self-employed income, dividends over £10,000 or capital gains, or if you owe the High Income Child Benefit Charge and are not paying it through your tax code.
Can an accountant complete my Self Assessment for me?
Yes. With your authorisation, an accountant can register you, prepare and submit your return and deal with HMRC on your behalf. You remain legally responsible for the figures, but professional support reduces the risk of errors, missed claims and penalties.
Conclusion
Self Assessment is manageable when you know whether you need to file, register on time and keep good records throughout the year. Registering by 5 October, gathering documents early and filing well before 31 January will help you avoid penalties and plan for payments on account.
Need help with your Self Assessment? Our team can register you with HMRC, prepare and file your tax return and make sure you claim every allowable expense. Get in touch to discuss your return.
