Self Assessment runs to fixed dates. You must tell HMRC you need to file by 5 October after the tax year ends, send a paper return by 31 October or an online return by 31 January, and pay any tax owed by 31 January too. Miss the filing date and you face an immediate £100 penalty, then daily charges, then larger penalties at six and twelve months. Paying late brings further penalties plus interest on the balance.
Tax investigation insurance from £108.75 a year (£9.06 a month).Pays up to £100,000 of your accountant's fees if HMRC opens an enquiry. No excess. Buy online in a few minutes.
Self Assessment is how HMRC collects tax that is not taken automatically through PAYE. If you are a sole trader, a landlord, a higher earner or someone with untaxed income, the responsibility to register, file and pay sits with you. The dates rarely move, and the penalty regime is largely automatic, so knowing exactly what falls due and when is the simplest way to keep your costs down. This guide sets out each deadline and every penalty in plain terms.
When do you need to register for Self Assessment?
If you have not filed before and you need to complete a return for the previous tax year, you must tell HMRC by 5 October following the end of that year. The UK tax year runs to 5 April, so for income earned in 2025/26 you would register by 5 October 2026. Registering late does not automatically trigger a penalty on its own, but it can leave you short of time and can lead to a failure to notify charge if tax goes unpaid.
Not everyone needs to file. If your self employment income is below the £1,000 trading allowance, that allowance can remove the need to register at all. The wider Self Assessment thresholds have also shifted in recent years, so if your circumstances are borderline it is worth checking whether a return is genuinely required. Our accountancy and tax guide covers who falls inside the system.
What are the filing and payment deadlines?
There are two filing routes. A paper return must reach HMRC by 31 October. An online return has until 31 January, which is why the overwhelming majority of people file online. The balancing payment for the tax you owe is also due by 31 January. If you make payments on account, a second instalment falls due on 31 July, and you can read how those advance payments work in our guide to payments on account.
So for the 2025/26 tax year the online return and the payment are both due by 31 January 2027. Whether you trade as a sole trader or through a company changes how and what you report, and our comparison of sole trader versus limited company explains those differences.
What are the late filing penalties?
Late filing penalties build up in stages, and they apply even if you have no tax to pay:
- Immediately after the deadline: a fixed £100 penalty, charged as soon as the return is a single day late.
- After 3 months: daily penalties of £10 per day, running for up to 90 days, so a maximum of £900 on top of the £100.
- After 6 months: a further penalty of 5% of the tax due or £300, whichever is greater.
- After 12 months: another 5% of the tax due or £300, whichever is greater.
Left to run, a return that is a full year late can therefore attract well over £1,600 in penalties before the tax bill itself is even considered.
What are the late payment penalties and interest?
Paying late is charged separately from filing late. HMRC applies a penalty of 5% of the tax still unpaid at each of three points: 30 days after the due date, six months, and twelve months. On top of those penalties, interest is charged on the outstanding amount from the day after it was due until the day it is paid. Because interest accrues daily, even a short delay adds cost, so paying something rather than nothing usually reduces the damage.
How can you reduce the risk of a penalty?
Good record keeping is the single biggest safeguard. Knowing which costs you can offset keeps your figures accurate and your bill correct, and our guide to allowable business expenses is a useful starting point. From April 2026 many sole traders and landlords also move onto digital record keeping and quarterly updates under Making Tax Digital for Income Tax, which changes the rhythm of reporting, so it pays to prepare early. If you genuinely cannot meet a deadline for reasons outside your control, you may have a reasonable excuse and can appeal a penalty through HMRC.
HMRC opens enquiries into Self Assessment returns routinely, and even a fully correct return can be selected for review. Responding properly takes an accountant’s time, and those fees can mount quickly. A Solar Protect tax investigation insurance policy covers up to £100,000 of your own accountant’s fees with no excess, so you can defend an enquiry without worrying about the cost. Get a quote today and file with confidence.
Related guides: what counts as a reasonable excuse, how to amend a tax return, UK tax deadlines calendar, HMRC penalties table.
Reviewed by Matthew Bartlett, Director, Solar Protect. This guide is general information, not personalised tax or financial advice. Figures are correct for the 2025/26 and 2026/27 UK tax years at the time of writing.