Self Assessment is HMRC’s system for collecting tax from people whose income isn’t fully taxed automatically through PAYE. If you’re self-employed, a landlord, have significant untaxed income, or fall into certain other categories, you may need to file a tax return each year.
This guide explains who needs to file, how the process works, the key deadlines, and how to avoid the penalties that catch thousands of taxpayers every year.
What Is Self Assessment?
Self Assessment is the annual tax return system used to report income and calculate any tax you owe. Instead of your employer deducting everything through PAYE, you tell HMRC about your income and expenses, and HMRC calculates your bill.
Common Reasons People File
| Reason | Typical Example |
|---|---|
| Self-employment | Sole trader, freelancer, side hustle income |
| Rental income | Buy-to-let landlord |
| High untaxed income | Dividends, foreign income, interest |
| High Income Child Benefit Charge | Income over £60,000 and Child Benefit claimed |
| Capital gains | Selling shares, property, crypto |
| Complex tax affairs | Multiple income streams |
Do You Need to File a Tax Return?
Not everyone with extra income needs Self Assessment. The rules depend on the type and amount of income.
Usually Need to File
| Situation | Self Assessment Needed? |
|---|---|
| Self-employed and income over £1,000 | Yes |
| Rental income over £1,000 | Yes |
| Director with untaxed income | Usually yes |
| Foreign income | Often yes |
| Capital gains above annual allowance | Yes |
| High Income Child Benefit Charge applies | Usually yes |
| HMRC sent you a notice to file | Yes |
Might Not Need to File
| Situation | Self Assessment Needed? |
|---|---|
| Employee with simple PAYE only | No |
| Savings interest within allowances | No |
| Small side income under £1,000 trading allowance | No |
| Small rental income under £1,000 property allowance | No |
Critical rule: If HMRC issues a notice to file, you must either submit the return or get HMRC to withdraw the notice. Ignoring it triggers penalties even if no tax is due.
Key Deadlines
Self Assessment runs on the tax year ending 5 April.
| Deadline | What It Covers |
|---|---|
| 5 October | Register for Self Assessment if new |
| 31 October | Paper tax return deadline |
| 30 December | Online return deadline if you want tax collected through PAYE (where eligible) |
| 31 January | Online tax return deadline and tax payment deadline |
| 31 July | Second payment on account deadline |
Example Timeline
For the tax year 6 April 2025 to 5 April 2026:
- Register by 5 October 2026
- File paper return by 31 October 2026
- File online return by 31 January 2027
- Pay tax due by 31 January 2027
- Pay second payment on account by 31 July 2027
How to Register for Self Assessment
If You’re Self-Employed
Register as self-employed with HMRC and for Self Assessment. You’ll need to do this by 5 October after the tax year you started trading.
If You’re Not Self-Employed but Need a Return
Register for Self Assessment online via gov.uk. HMRC will send you a Unique Taxpayer Reference (UTR) by post.
What You Need
| Information | Why It’s Needed |
|---|---|
| National Insurance number | Identity check |
| Personal details | Name, address, date of birth |
| Contact details | Email and phone |
| Business details | If self-employed |
| Date you started | Registration timing |
Your UTR Number
A UTR (Unique Taxpayer Reference) is a 10-digit number HMRC uses to identify your tax record. You’ll need it to:
- File your tax return
- Set up your HMRC online account
- Pay tax correctly
- Communicate with HMRC about Self Assessment
Where to Find Your UTR
| Source | Details |
|---|---|
| Welcome letter from HMRC | Sent after registration |
| Previous tax return | Printed on notices |
| HMRC online account | Usually visible once set up |
| Payment reminders | Often included |
If you’ve lost it, use our UTR guide to retrieve it.
What Information Goes on a Tax Return?
Your return may include:
| Income Type | Examples |
|---|---|
| Employment income | Salary, bonuses, benefits |
| Self-employment | Business income and expenses |
| Property income | Rent received, allowable costs |
| Savings and investments | Interest, dividends |
| Pension income | Private pension, State Pension |
| Capital gains | Share sales, property sales |
| Foreign income | Overseas rent, dividends, employment |
You can also claim deductions and reliefs:
- Business expenses
- Pension contributions
- Gift Aid donations
- Trading allowance or property allowance
- Capital allowances
Filing for the First Time
First-time filing feels more complex than it is, but you do need to be methodical.
Step-by-Step
- Register early — Don’t wait until January; UTRs and activation codes take time
- Set up your HMRC online account — Activate before the deadline rush
- Gather records — Income, expenses, bank interest, dividends, pension statements
- Work through each section carefully — Only complete sections relevant to you
- Double-check before submitting — Errors create delays, overpayments, or penalties
- Save confirmation and calculation — Keep copies of everything
Records You Should Keep
| Record Type | Keep For |
|---|---|
| Invoices and receipts | At least 5 years after 31 January deadline |
| Bank statements | Same |
| Mileage logs | Same |
| Rent records | Same |
| Tax return copies | Same |
For the 2025/26 return filed by 31 January 2027, keep records until at least 31 January 2032.
Payments on Account
One of the biggest surprises for first-time filers is payments on account. If your tax bill is over £1,000 and less than 80% of your tax is collected at source, HMRC usually asks you to pay next year’s tax in advance.
How It Works
If your first bill is £4,000:
| Payment | Due Date | Amount |
|---|---|---|
| Balancing payment for current year | 31 January | £4,000 |
| First payment on account for next year | 31 January | £2,000 |
| Second payment on account for next year | 31 July | £2,000 |
Total due on 31 January: £6,000
This catches many new self-employed people off guard. The July payment is then offset against your next final bill.
Can You Reduce Payments on Account?
Yes, if you reasonably expect your next year’s income to fall. But reducing them too aggressively can trigger interest if you underpay.
Common Mistakes
Frequent Errors
| Mistake | Consequence |
|---|---|
| Missing the deadline | Automatic penalties |
| Forgetting income | Underpayment, penalties, interest |
| Claiming non-allowable expenses | HMRC enquiry risk |
| Mixing personal and business spending | Inaccurate return |
| Missing payments on account | Cashflow shock |
| Filing without records | Higher error risk |
Expense Rules: What Counts?
For self-employed people, expenses must be wholly and exclusively for business purposes. Mixed-use costs can usually only be claimed proportionally.
Examples:
- Laptop used 80% for business → claim 80%
- Mobile phone partly personal → business-use proportion only
- Commuting to your usual workplace → usually not allowable
- Travel to client sites → usually allowable
Correcting a Mistake
If you notice an error after filing, don’t panic. You can usually amend your tax return within 12 months of the online filing deadline.
Amendment Deadlines
| Tax Year | Amendment Deadline |
|---|---|
| 2025/26 | 31 January 2028 |
| 2024/25 | 31 January 2027 |
After the amendment window closes, you may still be able to ask HMRC for an overpayment relief claim or disclose an error directly.
What Happens If You Miss the Deadline?
Self Assessment penalties escalate quickly.
Late Filing Penalties
| Delay | Penalty |
|---|---|
| 1 day late | £100 fixed penalty |
| 3 months late | £10/day up to 90 days (max £900) |
| 6 months late | Additional £300 or 5% of tax due |
| 12 months late | Another £300 or 5% of tax due |
Late Payment Penalties
| Delay | Penalty |
|---|---|
| 30 days late | 5% of unpaid tax |
| 6 months late | Additional 5% |
| 12 months late | Another 5% |
Interest also runs on unpaid tax.
Important: You can be fined for late filing even if you owe no tax.
Self Assessment for Landlords
If you receive rental income, you may need to file a tax return. You report:
- Gross rent received
- Allowable property expenses
- Mortgage interest relief position
- Furnished holiday lets (if applicable)
The property allowance gives the first £1,000 of property income tax-free, but above that, Self Assessment is usually required.
PAYE vs Self Assessment
Many people have both PAYE income and Self Assessment obligations. For example:
- Employee with side hustle
- Employee with rental income
- Director with dividends
- Worker with untaxed foreign income
PAYE continues through your employer, while Self Assessment reconciles everything annually.
High Income Child Benefit Charge
If you or your partner receives Child Benefit and one of you has adjusted net income over £60,000, the higher earner may need to file Self Assessment to pay the charge.
The charge gradually removes Child Benefit between £60,000 and £80,000. Above £80,000, it’s fully clawed back.
How to Make Self Assessment Easier
Good Habits
| Habit | Benefit |
|---|---|
| Separate business bank account | Cleaner records |
| Monthly bookkeeping | Less January panic |
| Set aside tax money regularly | Avoid cashflow shock |
| Keep digital copies of receipts | Easier evidence |
| Review income quarterly | More accurate planning |
Tax Pot Rule of Thumb
Many self-employed people set aside 25%-30% of income for tax and National Insurance. Higher earners or those with student loans may need more.
Common Questions
Who has to do Self Assessment?
Typically self-employed people, landlords, those with significant untaxed income, people subject to the High Income Child Benefit Charge, and anyone HMRC has issued a notice to file. Employees with simple PAYE income usually do not need it.
What is the deadline for Self Assessment?
The online filing deadline is 31 January after the end of the tax year. Paper returns are due by 31 October. Payment is also due by 31 January, with a second payment on account due by 31 July where applicable.
What happens if I file my tax return late?
You get an automatic £100 penalty even if no tax is due. Further penalties apply after 3, 6, and 12 months, and interest plus late payment penalties apply if tax remains unpaid.
How do I get a UTR number?
Register for Self Assessment with HMRC. Your 10-digit UTR is then sent by post. If you’ve lost it, you can often find it in your HMRC account or on previous tax correspondence.
Can I do Self Assessment myself?
Yes. Many people file their own returns directly through HMRC’s online system. An accountant becomes more useful if you have complex business expenses, capital gains, multiple properties, foreign income, or want proactive tax planning.
What are payments on account?
They’re advance payments toward next year’s tax bill, usually due if your tax bill exceeds £1,000 and most of your tax wasn’t collected at source. They are a major cashflow issue for first-time filers.
Related Guides
Hub Connections
Self Assessment Basics
- Self Assessment Guide — Full overview
- Self Assessment Tax Return Guide — Filing process
- First-Time Self Assessment Guide — New filer help
- Do I Need to Do a Tax Return? — Filing requirement checker
Registration and UTR
- How to Get a UTR Number — Registration and retrieval
Deadlines and Penalties
- Self Assessment Deadlines — Key dates
- Tax Return Deadline Guide — Don’t miss the cutoff
- Last-Minute Self Assessment Guide — Filing close to deadline
- What Happens If You Don’t File? — Penalties explained
Fixing Problems
- How to Correct a Tax Return — Amendments
- Self Assessment Mistake: How to Fix — Common error corrections
Special Situations
- Landlord Tax Return Guide — Rental income
- PAYE vs Self Assessment — When both apply