Self-Assessment Guide for 2023-24: Deadlines, Common Mistakes, and How to Avoid Penalties
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.