At a Glance
- Self Assessment is how HMRC collects Income Tax from people who are not taxed through PAYE
- The deadline to register is 5 October after the end of your first tax year
- The deadline to file online is 31 January after the tax year ends
- The deadline to pay any tax owed is also 31 January
- Payments on account may mean you pay roughly 50% more in your first year
- Penalties for late filing start at £100 and increase over time
Who Needs to File a Self Assessment?
You must file a Self Assessment tax return if any of the following apply:
- You are self-employed (sole trader) with turnover above the £1,000 trading allowance
- You are a partner in a business partnership
- You are a landlord receiving rental income above the £1,000 property allowance
- You have income from savings, investments, or dividends above certain thresholds
- You have foreign income
- You need to claim certain tax reliefs (e.g., reliefs on charitable donations)
- You are a higher earner (over £100,000) whose tax affairs are not fully covered by PAYE
- You received income from selling assets that might be liable for Capital Gains Tax
If you are employed and your only income is your salary (taxed through PAYE), and you have no other income sources, you probably do not need to file a Self Assessment.
If you are unsure, HMRC has a handy tool on their website that tells you whether you need to file.
Registering for Self Assessment
If this is your first time filing, you need to register with HMRC. You can do this online at GOV.UK.
The deadline to register is 5 October after the end of the tax year in which you started receiving untaxed income.
For example, if you started freelancing in June 2025 (during the 2025/26 tax year), you must register by 5 October 2026.
What you will need:
- Your National Insurance number
- Your address and contact details
- The date you started receiving self-employed income
- Details of your business (name, address, type of work)
After registering, HMRC will send you a Unique Taxpayer Reference (UTR) by post. This usually takes about 10 working days (21 days if you are abroad). You cannot file your return without your UTR, so register early.
You will also need a Government Gateway user ID and password to access HMRC’s online services.
If you miss the 5 October registration deadline, HMRC can charge a penalty. The penalty increases the longer you delay. Register as soon as you know you need to — do not leave it until January.
What You Need Before You Start
Gather these documents before you sit down to complete your return. It will make the process much faster:
For self-employment income:
- Your bookkeeping records or cash book spreadsheet for the tax year (6 April to 5 April)
- Bank statements for your business account
- Invoices you issued and payments you received
- Receipts for all business expenses
For employment income (if applicable):
- Your P60 form (shows total salary and tax paid)
- Your P11D form (if you received benefits in kind)
- Any P45 forms from jobs you left during the year
For property income:
- Records of rent received and expenses paid
- Mortgage interest statements
- Letting agent statements
For other income:
- Interest certificates from banks and building societies
- Dividend vouchers
- Pension statements
- Capital gains calculations (if you sold assets)
General:
- Your UTR number
- Your National Insurance number
- Details of any charitable donations (for Gift Aid claims)
- Student loan statements (if applicable)
Most people find that having their records well organised means the actual form-filling takes about an hour.
How to Complete the Return
The Self Assessment form (SA100) has several sections. Most people only need to complete the main form plus one or two supplementary pages.
SA100 (Main Tax Return): This covers your personal details, income from employment, pensions, savings, and investments. It also has sections for Gift Aid donations and Student Loan repayments.
SA103 (Self-Employment): If you are self-employed, you complete the self-employment pages. The short version (SA103S) is for businesses with turnover under £85,000. The full version (SA103F) is for those above. You enter your total turnover, expenses by category, and capital allowances.
SA105 (Property): If you receive rental income, you complete the property pages. You enter total rental income, allowable expenses, mortgage interest (for the tax credit calculation), and any losses brought forward.
Other supplementary pages: SA106 for foreign income, SA108 for capital gains, SA109 for residence and remittance basis, etc. Most people do not need these.
The online system calculates your tax automatically as you enter figures. You can see your estimated tax bill before you submit. This is useful — if the number looks wildly wrong, you can go back and check your entries before finalising.
You can save your progress and come back later. Do not feel you need to complete it in one sitting.
Important Deadlines
| Deadline | What is Due |
|---|---|
| 5 October | Register for Self Assessment (after your first tax year) |
| 31 October | Paper tax return deadline |
| 31 January | Online tax return deadline |
| 31 January | Balancing payment (any tax owed for the previous tax year) |
| 31 January | First payment on account (advance payment for next year) |
| 31 July | Second payment on account |
Penalties for Late Filing
Missing the 31 January deadline triggers an automatic penalty system:
- 1 day late: £100 fixed penalty (even if you owe no tax)
- 3 months late: £10 per day (up to £900 maximum)
- 6 months late: Additional £300 or 5% of the tax due (whichever is higher)
- 12 months late: Another £300 or 5% of the tax due (whichever is higher). In serious cases, HMRC can charge up to 100% of the tax owed
These penalties apply to the filing, not the payment. There are separate penalties and interest for late payment of tax.
If you have a reasonable excuse for missing the deadline (serious illness, bereavement, system failure at HMRC), you can appeal the penalty. But ‘I forgot’ or ‘my accountant missed it’ are not considered reasonable excuses.