There are three dates every UK side hustler needs to know: 5 October, 31 January, and 31 July. Each carries a different obligation and a different penalty for missing it. Getting them wrong costs money — sometimes significantly more than the tax owed.
This guide sets out every relevant deadline for the 2025/26 and 2026/27 tax years, what each requires, and the specific consequences of missing each one.
For the complete UK side hustle tax overview, see our complete UK side hustle tax guide.
The Three Critical Dates
5 October: Register for Self Assessment
By 5 October following the end of the tax year in which you first had gross self-employment income above £1,000, you must be registered with HMRC for Self Assessment.
If you had gross side hustle income above £1,000 in the 2025/26 tax year (6 April 2025 to 5 April 2026), you must register by 5 October 2026.
This is the “failure to notify” deadline. Missing it triggers a separate penalty regime from the late filing penalties.
31 January: File Your Return and Pay
By 31 January following the end of the tax year, you must: submit your online Self Assessment return, and pay all tax and NIC owed for that year.
- For 2025/26 (year ended 5 April 2026): file and pay by 31 January 2027.
- For 2026/27 (year ended 5 April 2027): file and pay by 31 January 2028.
- The paper return deadline is earlier — 31 October following the year end. Almost all self-employed people use online filing.
31 July: Second Payment on Account
If HMRC requires payments on account (advance payments toward the following year’s expected tax), the second instalment is due 31 July.
For the 2026/27 payments on account: second instalment due 31 July 2027.
The 5 October Deadline — Registration in Detail
The Trigger
You must register if, in any tax year, your total gross income from self-employment (across all activities combined) exceeds £1,000.
“Gross income” means the total paid to you by customers or platforms before any deductions, costs, or fees. It is not profit. See our guide on how to actually complete the registration process for the full explanation of what counts as gross income.
The Deadline
Register by 5 October following the end of the tax year in which you first crossed £1,000 gross.
- Tax year 2025/26 (ended 5 April 2026): register by 5 October 2026.
- Tax year 2026/27 (ended 5 April 2027): register by 5 October 2027.
The Process
Register online at gov.uk/register-for-self-assessment. After registering, your Unique Taxpayer Reference (UTR) arrives by post in 10–14 working days.
Allow 20–25 working days total to have a fully activated account — there are two separate letters, two separate codes. Register early to avoid missing the filing deadline in January.
If you already have a UTR from previous self-employment: use form CWF1 to notify HMRC you have restarted self-employment, rather than registering again from scratch.
The 31 January Deadline — Filing and Payment
What Must Be Done by 31 January
Submit the Self Assessment return online (paper deadline is 31 October — earlier).
Pay all tax owed including: income tax on self-employment profit, Class 4 NICs on self-employment profit, and any tax owed on other income not collected through PAYE.
Pay the first payment on account (if applicable — see below).
Payments on Account
If your total Self Assessment bill exceeds £1,000 AND less than 80% of your total income was taxed at source through PAYE, HMRC requires advance payments on account toward the following year’s estimated bill.
The first payment on account (50% of the current year’s bill) is due at the same time as the balancing payment — both by 31 January.
This is the most common source of shock for first-year self-employed filers. If your 2025/26 tax bill is £1,200, you pay not £1,200 by 31 January 2027 but £1,200 + £600 (first payment on account) = £1,800.
The employed side hustler caveat: if your salary is taxed through PAYE and the PAYE represents more than 80% of your total income, you may be exempt from payments on account. HMRC calculates this automatically on your return.
How to Pay?
Via your HMRC online account (pay via bank transfer using your UTR as the reference), direct debit, or debit card. Do not use credit cards — HMRC charges a 0.5% surcharge for credit card payments.
The 31 July Deadline — Second Payment on Account
If payments on account apply to you, the second instalment is due 31 July, six months after the January payment.
The second payment on account is also 50% of the prior year’s Self Assessment bill.
If you believe your current year’s income will be significantly lower than the prior year, you can apply to reduce your payments on account. This is done through your HMRC online account.
If you reduce payments on account and your actual tax liability turns out to be higher, you pay interest on the shortfall.
The Complete Deadline Calendar
2025/26 Tax Year (6 April 2025 to 5 April 2026)
- 5 October 2026: register for Self Assessment if gross income exceeded £1,000 in 2025/26.
- 31 October 2026: paper return filing deadline.
- 31 January 2027: online return deadline + balancing payment + first 2026/27 payment on account.
- 31 July 2027: second 2026/27 payment on account.
2026/27 Tax Year (6 April 2026 to 5 April 2027)
- 5 October 2027: register if gross income exceeded £1,000 in 2026/27.
- 31 October 2027: paper return deadline.
- 31 January 2028: online return deadline + balancing payment + first 2027/28 payment on account.
- 31 July 2028: second 2027/28 payment on account.
What Happens if You Miss Each Deadline
Missing 5 October (Failure to Notify)
The penalty for not registering by 5 October when required is a “failure to notify” charge under Finance Act 2008, Schedule 41. The penalty is calculated as a percentage of the “potential lost revenue” (the tax you should have paid from the date you should have registered).
- Non-deliberate failure, voluntary disclosure: 0–30% of potential lost revenue.
- Non-deliberate failure, prompted disclosure (after HMRC contact): 10–30%.
- Deliberate failure: 20–70%.
- Deliberate and concealed: 30–100%.
Voluntary disclosure — registering before HMRC contacts you — consistently results in the lowest available penalty. The difference between voluntary (0–30%) and being caught (potentially 100%) is substantial.
Missing 31 January (Late Filing)
Automatic £100 penalty from day one.
£10/day from 3 months late (maximum £900).
5% of tax owed or £300 (whichever is higher) at 6 months.
5% of tax owed or £300 again at 12 months.
These apply even if you owe no tax.
Missing 31 January (Late Filing)
Interest at 7.75% per year from 1 February.
5% surcharge on unpaid tax at 30 days late.
5% surcharge at 6 months.
5% surcharge at 12 months.
For the full detail on filing and payment penalties, see our guide on what happens if you miss the filing deadline.
If You Have Already Missed a Deadline
Missed 5 October
Register immediately. The failure to notify penalty depends on “potential lost revenue” (estimated unpaid tax) and on whether you came forward voluntarily or were prompted. Every day of additional delay after the threshold increases the potential penalty.
Missed 31 January (Return Not Filed)
File immediately. The daily £10 penalty (from 3 months late) accrues every day the return is outstanding. Filing now stops the clock, even if you cannot pay immediately.
Missed 31 January (Tax Not Paid)
Contact HMRC to set up a Time to Pay arrangement. Once in place, further surcharges are suspended. The Time to Pay arrangement can be set up online if your bill is under £30,000 — gov.uk/pay-self-assessment-tax-bill. For larger bills, call HMRC’s Self Assessment payment helpline.
Frequently Asked Questions
Do I need to register by 5 October even if I will not owe any tax?
Yes. The 5 October registration deadline applies when gross income exceeds £1,000, regardless of whether you ultimately owe any tax. A sole trader whose profit is below the personal allowance owes zero income tax — but still must register and file a return.
What if I missed 5 October 2026 but have not heard from HMRC?
Register now. Voluntary disclosure before HMRC contacts you is always better than waiting.
The fact that HMRC has not contacted you does not mean they do not have your data — platform reporting means they may well have your earnings information already.
Can I request an extension to the 31 January deadline?
Not generally. HMRC only accepts late filing without a penalty in specific circumstances — “reasonable excuse” such as serious illness, bereavement, or HMRC’s own system failures. Financial difficulty and forgetting the deadline are not accepted as reasonable excuse.
I am late but want to minimise penalties — what is the priority order?
First, file the return immediately (stops the daily £10 penalty from 3 months). Second, pay as much of the tax owed as you can (reduces the basis for payment surcharges).
Third, set up Time to Pay for the remainder if you cannot pay in full. Fourth, do not ignore any HMRC letters — responding promptly is treated more favourably.
What to Read Next?
For how to file your return once registered, see our guide on how to file your Self Assessment once registered.
Verified against HMRC published guidance as of 24 June 2026.
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