Never registered for Self Assessment and now facing years of penalties?
If you started a side hustle or became self-employed and didn't realise you needed to register for Self Assessment, the responsibility to tell HMRC about untaxed income sits with you, not an employer or platform. The most useful step is to act now: penalties grow the longer a return is outstanding, so registering and filing as soon as possible stops the escalation, and coming forward voluntarily generally leads to more favourable treatment than being caught.
Written by the team behind Agent Arthur, our HMRC and tax specialist. This guide is general information about UK law, reviewed for accuracy.The duty to notify sits with you
A common and stressful situation: you started a side hustle or became self-employed, didn't realise you needed to register for Self Assessment, and now face penalties stacking up across one or more tax years. The important thing to understand is that the responsibility to tell HMRC about untaxed income sits with you, not with an employer, a platform, or anyone else.
If you earn more than £1,000 from self-employment or a side hustle in a tax year (above the £1,000 trading allowance), you generally need to register and file a return. The deadline to register is 5 October following the end of the relevant tax year, and returns are then due by the following 31 January.
Why acting now matters
The single most useful step is to act now rather than wait. Penalties grow the longer a return is outstanding: after the initial £100, a return three months late can attract £10 per day for up to 90 days (up to £900), and at six and twelve months further penalties of £300 or 5% of the tax due (whichever is greater) can apply at each stage. Registering and filing as soon as possible stops that escalation.
Appeals and voluntary disclosure
On appeals, be realistic. 'I didn't get a reminder' or 'I didn't know' is not usually accepted as a reasonable excuse by itself. But if a genuine reasonable excuse, such as serious illness or bereavement, genuinely explains part of the delay, it is still worth setting out clearly. Coming forward voluntarily through HMRC's disclosure routes, rather than being caught, generally leads to more favourable treatment on penalties.
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Whose job is it to register: mine or the platform's?
Yours. The responsibility to tell HMRC about untaxed income sits with you, not with an employer, a platform, or anyone else. If you earn more than £1,000 from self-employment or a side hustle in a tax year, you generally need to register and file a return.
When should I have registered?
The deadline to register for Self Assessment is 5 October following the end of the relevant tax year, and returns are then due by the following 31 January. If you've missed these across several years, registering and filing as soon as possible stops penalties escalating further.
Is 'I didn't know I had to register' a reasonable excuse?
Not usually on its own. 'I didn't get a reminder' or 'I didn't know' is not generally accepted as a reasonable excuse. But a genuine reasonable excuse, such as serious illness or bereavement, that explains part of the delay is still worth setting out clearly.
Is it better to come forward or wait?
Coming forward voluntarily through HMRC's disclosure routes, rather than being caught, generally leads to more favourable treatment on penalties. A well-organised voluntary disclosure that explains the timeline and any mitigating circumstances can meaningfully reduce the penalties that stick.