What happens if I miss the 5 October deadline to register for Self Assessment?

You could get a penalty for failing to notify HMRC. It is based on tax that is still unpaid after 31 January, so if you register now and file and pay in full by 31 January 2027, you will usually not be charged one.

Can I still file my 2025–26 tax return on time if I register late?

Yes. The online filing deadline is 31 January 2027. Register as soon as possible, because you need your Unique Taxpayer Reference before you can file. If you register online it should appear in your online account within 72 hours.

Do I need to register if I earned less than £1,000?

No. If your total trading income for the tax year was £1,000 or less, it is covered by the trading allowance and you do not need to tell HMRC.

What is the penalty for filing a Self Assessment return late?

£100 straight away, then £10 a day after 3 months up to £900, then the greater of 5% of the tax due or £300 at 6 months and again at 12 months.

Hamster blog

Missed the 5 October deadline to register for Self Assessment? Here's what to do

Published 5 October 2026 · Figures are for the 2026–27 tax year

The short answer

Register now. The deadline to tell HMRC about side-hustle income from the 2025–26 tax year was 5 October 2026, and you could get a penalty for registering late. But HMRC's guidance says that penalty is based on tax still unpaid after 31 January 2027, so if you register straight away, file your return online and pay everything you owe by 31 January, you will usually not be charged one.

Which deadline you missed

If your side hustle earned more than £1,000 in the 2025–26 tax year (6 April 2025 to 5 April 2026), and you have never filed a tax return, you needed to register for Self Assessment by 5 October 2026.

That is only the registration deadline. The tax return itself and the payment are not due until 31 January 2027, so there is still time to get everything right.

Step 1: Register today

Register online on GOV.UK as self-employed. Have your National Insurance number and the date you started your side hustle ready.

You cannot file without a Unique Taxpayer Reference (UTR). Since September 2026, HMRC's improved online registration puts your UTR in your online account within 72 hours, instead of the old wait of up to 15 days for a letter.

Step 2: Gather your figures

While you wait for your UTR, pull together for 2025–26:

everything you earned from your side hustle, across all platforms
your costs: stock, materials, fuel or mileage, fees, equipment
receipts or records for those costs

Then work out whether to deduct your actual costs or the £1,000 trading allowance, whichever is bigger.

Step 3: File and pay by 31 January 2027

This is the step that decides whether you pay a penalty for registering late.

HMRC's penalty guidance says that if you register after 5 October and do not pay your full bill by 31 January, you may get a "failure to notify" penalty based on what is still unpaid. Pay it all on time and there is normally nothing left for that penalty to be based on.

If your bill is over £1,000, remember that you will also owe the first payment on account for 2026–27 on the same day.

Key dates from here

31 October 2026: paper return deadline. Most people file online instead.
30 December 2026: file online by now and, if you are employed and owe less than £3,000, HMRC can collect it through your tax code instead of in one go.
31 January 2027: online return and payment deadline.

If you miss 31 January too

The costs stack up for a late return:

£100 straight away, even if you owe no tax
£10 a day after 3 months, up to £900
the greater of 5% of the tax due or £300 after 6 months, and again after 12 months

Paying late adds 5% of the unpaid tax at 30 days, 6 months and 12 months, plus interest. If you cannot afford the bill, file on time anyway and contact HMRC about a payment plan. Filing late and paying late are penalised separately.

Did you even need to register?

Not everyone with extra income does. You do not need to register if:

your total trading income for the year was £1,000 or less
you only sold your own unwanted belongings, which is not trading

You may have read that the threshold is going up to £3,000. That has not happened yet, so the £1,000 rule still applies.

Keeping track

Most people miss 5 October because they never noticed they had passed £1,000. Hamster is a free Android app that totals your side-hustle income as you log it, and with Premium it alerts you the moment you cross £1,000, so next year's deadline does not sneak up on you.

Quick answers

What happens if I miss the 5 October deadline to register for Self Assessment?: You could get a penalty for failing to notify HMRC. It is based on tax that is still unpaid after 31 January, so if you register now and file and pay in full by 31 January 2027, you will usually not be charged one.
Can I still file my 2025–26 tax return on time if I register late?: Yes. The online filing deadline is 31 January 2027. Register as soon as possible, because you need your Unique Taxpayer Reference before you can file. If you register online it should appear in your online account within 72 hours.
Do I need to register if I earned less than £1,000?: No. If your total trading income for the tax year was £1,000 or less, it is covered by the trading allowance and you do not need to tell HMRC.
What is the penalty for filing a Self Assessment return late?: £100 straight away, then £10 a day after 3 months up to £900, then the greater of 5% of the tax due or £300 at 6 months and again at 12 months.

Sources

This is general information, not personal tax advice. Rules and rates change — check the linked GOV.UK pages or speak to an accountant about your own situation.