The 5 October Self Assessment Deadline Is Monday: Who Has to Register and What Late Costs You

Monday 5 October is a tax deadline that rarely feels like one. It is not the day you file anything and it is not the day you pay anything; it is the date by which HMRC must be told that you need to file at all, and it falls almost four months before the return itself is due on 31 January 2027. Because nothing is calculated and no money moves, the date slides past people who earned a few thousand pounds on the side during the 2025/26 tax year and assumed HMRC would work it out for itself. The penalty for telling HMRC late is based on whatever tax is still unpaid when the January bill arrives, so the cost of a missed date depends heavily on what you do next. If any of the groups below sounds like you, register this weekend rather than leaving it until Monday evening.

What the 5 October deadline actually covers

GOV.UK puts the rule plainly: you must tell HMRC by 5 October 2026 if you need to complete a tax return for the last tax year, which ran from 6 April 2025 to 5 April 2026, and you either have not sent a return before or registered in the past but did not need to send one for 2024/25. Telling HMRC means registering for Self Assessment, an online process that ends with a ten-digit Unique Taxpayer Reference, or UTR, arriving by post. The date is not arbitrary. HMRC's failure-to-notify factsheet says that people whose self-employed profits first make them chargeable to tax must tell it within six months of the end of the relevant tax year, and six months after 5 April lands on 5 October. Registering and filing are separate acts with separate dates, and mixing the two up is easy. A paper return must reach HMRC by 31 October 2026, an online return by 11:59pm on 31 January 2027, and the tax itself is due by the same January date. If you already file every year, none of this applies to you; the 5 October rule is aimed at newcomers and at anyone whose old account went quiet.

Who has to register

The test that trips up most people is that the £1,000 trading allowance is measured on gross income, meaning everything you took in before any costs come off. Take someone who buys vintage jackets for £900 and resells them for £1,400: the profit is £500, but gross trading income is £1,400, so the registration rule applies. GOV.UK lists everyone who must send a return, and the groups that catch ordinary households tend to be these:

• Sole traders and freelancers with more than £1,000 of gross self-employment income in 2025/26, including casual work such as babysitting, gardening or hiring out power tools.
• Partners in a business partnership.
• Anyone who owed Capital Gains Tax on something they sold.
• Parents who owe the High Income Child Benefit Charge and do not pay it through PAYE. The charge starts when an individual's adjusted net income passes £60,000 and takes back 1% of the Child Benefit for every £200 above that, so at £80,000 all of it goes.
• Landlords and others with untaxed income such as rent, tips, savings interest or dividends, among other sources.

Trading and property are treated differently. Gross trading income above £1,000 means registering, while for property income the line is gross rent of £2,500: between £1,000 and £2,500 you contact HMRC, and above £2,500 you register. There is also a catch in the allowance itself. It disappears altogether when the income comes from your own company, from a partnership you or someone connected to you belongs to, or from your employer or your spouse's employer. You can also register voluntarily below the thresholds, and a return is the way self-employed people prove their income for Tax-Free Childcare or Maternity Allowance, as well as the route to paying voluntary National Insurance contributions. Anyone selling online should read our guide to when Vinted, eBay and Depop income becomes taxable before deciding where they stand.

What a late registration costs

The penalty for telling HMRC late is not a flat fine; it is a slice of whatever tax ends up unpaid.

GOV.UK's penalty page says that if you register after 5 October and do not pay all of your tax bill by 31 January, you may get a failure-to-notify penalty based on the amount still left to pay, issued within 12 months of HMRC receiving your return. Read that wording closely. Someone who registers a week late and settles the whole bill on time is in a very different position from someone who registers in March with nothing set aside, though neither should treat the date as optional. HMRC's factsheet adds that the percentage depends on behaviour and on whether you came forward first: where you tell HMRC before it has any reason to think it will find the failure, the minimum is lower than if it gets there before you. The ranges, as a share of the tax lost, run like this.

On an unpaid bill of £800, the ceiling for an ordinary slip is therefore £240, and HMRC then settles where in the range to land according to how much help you give it. The factsheet also says no penalty is charged where you have a reasonable excuse, the failure was not deliberate and you came forward without unreasonable delay once the excuse ended. A reasonable excuse is defined as something that stopped you meeting the obligation even though you took reasonable care to meet it, and forgetting the date sits uncomfortably far from that.

Registering late also changes the filing clock. If you register after 5 October, HMRC sends a letter or email with its own return deadline, three months from the date on it, but the payment date does not move: the tax is still due by 11:59pm on 31 January 2027, with a 5% penalty at 30 days, six months and twelve months if it is late, plus interest. A late return starts at £100, adds £10 a day after three months up to £900, then a further 5% of the tax due or £300, whichever is greater, at six months and again at twelve.

Registering this weekend

Start by running GOV.UK's check for whether you need to send a tax return, then register online through the Self Assessment service linked above. The UTR usually arrives by post around 15 days after you register, and longer if you live overseas. If you registered in an earlier year but did not need a return for 2024/25, the same service takes you through reactivating the old account, and HMRC warns that a return filed without reactivation may be delayed. Register online, today, and keep the confirmation.

While the UTR is in the post, gather the 2025/26 numbers: everything that came in from the side work between 6 April 2025 and 5 April 2026, platform statements, invoices, bank statements and any costs you paid. GOV.UK lists a spreadsheet of income receipts, emails confirming payment and bank statements among the records to keep, and HMRC can charge a penalty for records that are inaccurate, incomplete or unreadable. You can deduct either the £1,000 allowance or your actual expenses, not both, so add up the costs before choosing. Skip the paper return unless there is no alternative, because its 31 October 2026 deadline is three months earlier than the online one.

What the first return will actually cost

Most first-time filers are employees with side income, and the sums are gentler than the deadline suggests. Take someone on a £30,000 salary who made £4,000 of profit from freelance work in 2025/26. That profit sits inside the basic-rate band, so Income Tax at 20% comes to £800, and because Class 4 National Insurance only starts once self-employed profits pass £12,570, there is none to pay. The same arithmetic assumes English, Welsh or Northern Irish rates, since Scottish Income Tax bands differ. An £800 bill is also below the £1,000 line at which payments on account normally begin, so January 2027 brings one payment.

Raise the profit to £10,000 and the numbers shift. Income Tax becomes £2,000, which is over that £1,000 line, so the first January brings the £2,000 plus a first instalment towards the next year equal to half the bill, £1,000, making £3,000 in one go, with a further £1,000 due on 31 July 2027. GOV.UK's own worked example for a first-time filer has the same shape, and our guide to the 31 July payment on account explains the instalments. Set aside 20% of what the side work earns, or 40% if you are a higher-rate taxpayer, in a separate easy-access savings account as the money comes in, and the January bill stops being a surprise.

Your first return also sets your Making Tax Digital date

HMRC reviews each Self Assessment return and checks your qualifying income, which is gross self-employment and property income before expenses. On GOV.UK the thresholds are £50,000 for 2024/25, with digital record-keeping from 6 April 2026, £30,000 for 2025/26, from 6 April 2027, and £20,000 for 2026/27, from 6 April 2028. The 2025/26 return you are registering to file is therefore the one HMRC will look at for the April 2027 start, and if your qualifying income topped £30,000 it says it will write to confirm. The guidance also states that you still have to submit a Self Assessment return for the tax year before you start using Making Tax Digital, so the January 2027 return is needed whatever your income turns out to be.

Between now and January

Once the registration is in, two dates are worth writing down. Filing online by 30 December 2026 is the cut-off if you want HMRC to collect the bill through your tax code, though other conditions apply, so check GOV.UK before relying on it. The final deadline is 31 January 2027 for both the return and the payment, and that day is a Sunday, so aim to file and pay by Friday 29 January instead of at 11pm on the last night.