UK Self Assessment: the 31 January deadline explained

UK Self Assessment: the 31 January deadline explained

If you receive income that is not taxed at source through PAYE, you are generally required to file a Self Assessment tax return with HMRC. The online submission deadline is 31 January following the end of the tax year, and missing it triggers an automatic penalty of £100, with further charges accruing the longer the return remains outstanding.

Who must file a Self Assessment return?

HMRC requires a Self Assessment return from anyone who falls into one or more of the following categories during the relevant tax year:

• Self-employed individuals or sole traders with trading income above £1,000
• Partners in a business partnership
• Directors of limited companies (in most circumstances)
• Individuals with untaxed income from property, investments, or overseas sources
• Higher or additional-rate taxpayers with certain income streams not captured by PAYE
• Those who need to claim certain reliefs or repayments that cannot be processed through a tax code

If you are unsure whether you have a filing obligation, HMRC publishes a self-assessment tool on gov.uk that can help you determine your position. When in doubt, speaking with a qualified tax professional is the safest course of action.

Registering with HMRC

First-time filers must register before they can submit a return. Registration deadlines differ from submission deadlines: new Self Assessment customers should register by 5 October following the end of the tax year in which the reportable income arose. Missing this date does not remove the obligation; HMRC can still assess tax for earlier periods.

Registration is done through the Government Gateway. Once registered, HMRC issues a Unique Taxpayer Reference (UTR), which must appear on every return. The UTR can take two weeks or more to arrive by post, so early registration matters.

Online vs paper filing deadlines

The tax year runs from 6 April to 5 April. Two separate submission deadlines apply, depending on how you file:

Most people now file online. The later January deadline for online filers gives an additional three months compared with the paper route. HMRC's online portal also calculates the tax owed automatically based on the figures entered.

What you owe and when you pay it

Filing a return and paying any tax owed are two separate actions. The payment deadline for any balance due aligns with the online filing deadline: 31 January. If the preceding tax year produced a bill above a certain threshold, HMRC may also require payments on account — advance instalments due on 31 January and 31 July — to spread the liability for the current year.

Interest accrues on overdue tax from the day after the payment deadline. A separate surcharge applies if the balance remains unpaid beyond 30 days of a formal notice.

What happens if you miss the deadline?

HMRC applies an automatic £100 penalty for late filing, even if no tax is owed or the tax was already paid in full before the deadline. Further penalties follow at three months (daily charges), six months, and twelve months. Late payment attracts interest and, eventually, surcharges. HMRC does accept reasonable excuse claims in certain circumstances — illness or bereavement, for example — but these must be substantiated.

Keeping records

HMRC requires Self Assessment records to be kept for at least 22 months from the end of the relevant tax year if you are an employee or company director, and five years from the 31 January filing deadline if you are self-employed. Adequate records include income documentation, receipts for allowable expenses, and bank statements.

Where to get help

A qualified tax professional can help you determine your filing obligations, prepare your return accurately, and meet your payment deadlines. Find accredited professionals on TaxProsRated through the recognised professional bodies for the UK.

Sources

HM Revenue and Customs (HMRC) — gov.uk Self Assessment guidance: https://www.gov.uk/self-assessment-tax-returns