Each year around 12 million people in the UK are required to file a Self-Assessment tax return. For the 2023-24 tax year (ending 5 April 2024), the online filing deadline is 31 January 2025. Miss it, and you face an automatic £100 penalty, even if you owe no tax at all. Miss it by three months and daily penalties of £10 begin to accrue.
Self-Assessment is required if you have income that is not taxed at source through PAYE, or where the income has been taxed but at the wrong amount. The most common groups who need to file are the self-employed, people with rental income, higher earners with untaxed investment income, and those who received Child Benefit while earning above the relevant threshold.
Key deadlines for 2023-24
- 5 October 2024: Register for Self-Assessment if you have not filed before
- 31 October 2024: Deadline to file a paper tax return
- 30 December 2024: Deadline to request HMRC collect tax through PAYE (for amounts under £3,000)
- 31 January 2025: Online filing and payment deadline for 2023-24
Most people who use Self-Assessment will also owe a Payment on Account for 2024-25, a down-payment towards the following year's bill, calculated at 50% of the 2023-24 liability. This is also due on 31 January 2025, meaning total amounts due can be significantly higher than the outstanding 2023-24 liability alone.
The most common mistakes that cost money
Missing rental income. If you let property, even a single room on Airbnb for occasional weekends, this income must be declared. HMRC receives data from platforms including Airbnb under the DAC7 regulations and cross-checks it against returns. The Rent a Room scheme allows up to £7,500 per year tax-free from renting a room in your own home, but only if you claim it on your return.
Omitting side income. Freelance, consultancy, driving, or selling goods online that total more than £1,000/year must be declared (below £1,000 the trading allowance applies). HMRC's digital cross-referencing is increasingly sophisticated.
Forgetting Gift Aid. If you make charitable donations under Gift Aid and are a higher-rate taxpayer, you can claim the additional tax relief through your Self-Assessment return. Many people miss this, costing them up to 20p for every pound donated.
Student loan repayments. Your employer deducts Plan 1/2/4/5 loan repayments through PAYE, but if you also have self-employed income, additional repayments may be due and must be calculated on your return.
Penalties for missing the 31 January deadline
- 1 day late: £100 fixed penalty (regardless of whether tax is owed)
- 3 months late: £10/day for up to 90 days (maximum £900 in daily penalties)
- 6 months late: 5% of tax owed (or £300 if greater)
- 12 months late: A further 5% of tax owed (or £300 if greater)
Interest also accrues on unpaid tax at the Bank of England base rate plus 2.5%. With the base rate elevated since 2022, the interest charge on late payments is materially higher than it was in the low-rate era.
Conclusion
Self-Assessment is not inherently complicated, but it does require care and an awareness of which sources of income need to be declared. The 31 January 2025 deadline for 2023-24 is approaching, use the autumn months to gather your records and file early. Our self-employed take-home pay calculator can help you estimate the tax you owe before you file.