Late Registration for Self Assessment: What Happens After 5 October?

The 5 October rule

You must tell HMRC by 5 October after the end of the tax year in which you became liable. Missing the date means a possible failure to notify penalty, linked to the tax you owe.

How big can the penalty be

The penalty depends on how late you are and why. It ranges from 0% for a reasonable excuse up to 30% of the unpaid tax for prompt disclosure, rising for deliberate failure.

What to do now

Register online for Self Assessment to get your UTR, which arrives by post in about 10 working days. Activate your online account as soon as the activation code arrives.

File and pay on time

Even if you registered late, the 31 January deadline still applies. Plan ahead because the UTR letter can take time.

Why 5 October matters and what happens if you miss it

If you need to complete a Self Assessment return for a tax year, you must tell HMRC by 5 October after the end of that tax year. For 2025/26, which ended on 5 April 2026, that date has now passed. The registration deadline is not the same as the filing deadline, which is 31 October for a paper return and 31 January for an online return and payment. Missing 5 October does not by itself create a penalty if you file and pay on time, but it creates risk because HMRC needs time to issue your Unique Taxpayer Reference and activation code.

The bigger problem is when someone never registers at all. HMRC can charge a failure to notify penalty. This is a percentage of the tax that was unpaid because you did not tell HMRC, and the percentage depends on behaviour: it is lower where the failure was not deliberate and where you come forward voluntarily before HMRC contacts you. Check current HMRC guidance for the ranges.

Timeline: registering after 5 October

If you register in mid-November, you should still have time to file online by 31 January, but not much room for postal delays. Registering before the end of October also keeps the paper return route open, though most people file online.

Worked example: registering two years late

This is an illustrative example. Maya started freelancing in 2024/25 and made £20,000 profit but did not register. She registers in autumn 2026, after the 2025/26 year, in which she also made £20,000.

The 15% is an assumption used only to show the arithmetic. The actual rate depends on HMRC’s assessment of her behaviour and how promptly she disclosed. In addition, interest runs on the unpaid tax, and late filing and late payment penalties may apply to the earlier year, since its 31 January deadline has passed. Maya would be wise to file the earlier return as soon as possible. Over two years the combined unpaid tax is £3,863.60, so acting quickly matters.

Common mistakes

• Assuming that income under a certain level means you need not tell HMRC, without checking the trading allowance and other rules.
• Waiting for HMRC to send a letter rather than registering.
• Thinking that registering late erases earlier unfiled years.
• Leaving the activation code letter unopened.
• Registering as self-employed when the income should be reported another way, such as rental income or capital gains.

Checklist

1. Work out which tax years you should have registered for.
2. Register online as soon as possible and diarise the UTR and code arrival.
3. Collect income and expense records for each unfiled year.
4. File the oldest year first and pay what is due.
5. If you receive a letter from HMRC, read it and reply by the date given.

Related guides: help if you have never filed before, setting up your Government Gateway account, HMRC nudge letters about undeclared income and reasonable excuse examples. If you are behind, our late Self Assessment tax return service can help.

SATR is a trading style of Accotax. Our qualified UK accountants register you, prepare the return or returns, and file online for a fixed fee from £199 + VAT.

This is general information, not personal tax advice.

Choosing the right category when you register

When you register, HMRC asks why you need to file. The common reasons are self-employment, rental income, a capital gain, foreign income, being a company director, or a Child Benefit charge. The category you choose affects the guidance and the returns pages that are set up for you, so choose the one that fits your main source of untaxed income, and tell HMRC about any others when you file. If you have started trading, remember to register for Self Assessment even if you have a full-time job, because the self-employed income is reported in addition to your employment income.

If you are registering after the deadline, it helps to explain briefly why, and to be accurate about the date your income began. Accurate dates determine which years need a return, and a clear account of events can make the difference when HMRC decides how to treat a failure to notify.

Frequently asked questions

Can I register after 5 October?

Yes, and you should do so immediately to limit penalties.

Will I be charged if I owe no tax?

Failure to notify penalties are based on tax unpaid, so if no tax is due the penalty is normally nil.

Tax rules and rates change. Figures here reflect 2025/26 guidance; always check current HMRC rules or ask us before acting.

Can I still file by 31 January if I register in November?

Often yes, but allow for post: the UTR takes around 10 working days and the activation code arrives separately. Register as early as you can.

Do I need to register if my income is below the trading allowance?

If gross self-employment income is £1,000 or less, you may not need to. Check current HMRC guidance, and register if you are unsure or if you have other reasons to file.

What if I realise I should have registered years ago?

Register and file the earlier returns as soon as you can. Coming forward before HMRC contacts you usually results in a lower penalty than being found by HMRC.

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Related guides

• Self Assessment Deadline 31 January 2027: Preparation Plan
• What to Do If HMRC Opens an Enquiry Into Your Tax Return
• How to Claim a Self Assessment Tax Refund
• Self Assessment registration