Self Assessment Walkthrough
Who needs to file a Self Assessment tax return, the key deadlines, penalties for late filing, and what you can claim.
Self Assessment Walkthrough
Self Assessment is the process by which you report your income and gains to HMRC and calculate the tax you owe. If your tax affairs are straightforward (just PAYE employment), you probably do not need to file. But many people are caught out.
Who Needs to File?
You must file a Self Assessment return if any of the following apply:
- You were self-employed and earned more than £1,000
- You earned more than £150,000 in the tax year (even if all PAYE)
- You had untaxed income over £2,500 (rental, dividends above the allowance, freelance work)
- You need to claim tax relief on pension contributions or Gift Aid (higher/additional-rate taxpayers)
- You are a company director (unless it is a non-profit)
- You received Child Benefit and your (or your partner's) income exceeded £60,000
- You had capital gains above the reporting threshold
- You received income from overseas
- HMRC have issued you a notice to file
Key Deadlines (2024-25 Tax Year)
| Deadline | Date |
|---|---|
| Tax year ends | 5 April 2025 |
| Register for Self Assessment (if new) | 5 October 2025 |
| Paper return deadline | 31 October 2025 |
| Online return deadline | 31 January 2026 |
| Pay tax owed | 31 January 2026 |
| Second Payment on Account | 31 July 2026 |
Penalties for Late Filing
HMRC's penalty regime is strict and escalating:
| How late | Penalty |
|---|---|
| 1 day late | £100 (even if you owe no tax) |
| 3 months late | £10 per day for up to 90 days (up to £900) |
| 6 months late | £300 or 5% of tax due (whichever is greater) |
| 12 months late | £300 or 5% of tax due (whichever is greater) — can be up to 100% in serious cases |
On top of filing penalties, there are late payment penalties and interest (currently 7.75% — Bank of England base rate + 4%) on any tax paid after 31 January.
Payments on Account
If your Self Assessment tax bill is over £1,000 and less than 80% of your tax is collected at source (PAYE), HMRC requires Payments on Account — advance payments towards next year's bill.
Each payment is 50% of your previous year's bill:
- First payment: 31 January (same date as the previous year's balancing payment)
- Second payment: 31 July
Worked Example
Hannah's 2023-24 Self Assessment bill is £6,000. She must pay:
| Date | Payment | Amount |
|---|---|---|
| 31 Jan 2025 | Balancing payment for 2023-24 | £6,000 |
| 31 Jan 2025 | First POA for 2024-25 (50% of £6,000) | £3,000 |
| 31 Jul 2025 | Second POA for 2024-25 | £3,000 |
| Total due 31 Jan 2025 | £9,000 |
This catches many people off guard. If your income has dropped, you can apply to reduce your Payments on Account — but if you reduce them too much, you will owe interest.
What Can You Claim?
Common deductions and reliefs to include on your return:
- Pension contributions — claim higher/additional-rate relief
- Gift Aid donations — extends your basic-rate band
- Trading expenses — if self-employed (stationery, software, travel, home office)
- Property expenses — letting agent fees, repairs, insurance (not improvements)
- Capital losses — set against gains
- Marriage Allowance — transfer £1,260 to your spouse if one of you earns under £12,570
- Professional subscriptions — if required for your job and on the HMRC-approved list
Filing Tips
- Keep records for at least 5 years after the 31 January deadline.
- File early — you can submit from 6 April even though the deadline is January. Filing early does not mean paying early; payment is still due 31 January.
- Use HMRC's online service or commercial software. Paper returns have an earlier deadline.
- Check your tax code. Errors in PAYE coding can mean you over- or underpay throughout the year.
This is educational content, not financial advice. If you are unsure whether you need to file, contact HMRC or a qualified accountant.
Explain Like I'm 5
Self Assessment is like a homework sheet the government gives you once a year. You write down all the money you earned and all the things you can subtract, and then you work out how much tax you owe. If you hand it in late, you get a fine — even if you do not owe anything! The good news is you can hand it in early to get it over with, and you can ask for deductions like money you gave to charity or put into your pension.
Key Takeaways
- The online Self Assessment deadline is 31 January — file early to avoid the rush (you still pay on the same date).
- A £100 penalty applies from day one of late filing, even if you owe no tax.
- Payments on Account catch people off guard — budget for 150% of your bill in January.
- Claim all eligible reliefs: pension contributions, Gift Aid, Marriage Allowance, trading expenses.
- Keep records for at least 5 years after the filing deadline.
CoreFi's Self Assessment wizard auto-populates HMRC boxes from your financial data.
Try the SA wizardEducational only - not financial advice