How to register for Self Assessment

Miss it and there may be a penalty. But there is a rule almost nobody knows, and it is at the bottom of this page. It could save you the penalty entirely.

Figures are correct at the date of review. Always check GOV.UK for the latest.

Who has to register

You must send a tax return if, in the last tax year (6 April to 5 April), any of these applied to you:

• You were self-employed as a sole trader and earned more than £1,000, before taking off anything you can claim tax relief on
• You were a partner in a business partnership
• You had to pay Capital Gains Tax on something you sold or disposed of
• You had to pay the High Income Child Benefit Charge and you do not pay it through PAYE
• You are an off-payroll worker repaying a student or postgraduate loan

You may also need to file if you have untaxed income, such as money from renting out a property, tips and commission, income from savings, investments and dividends, or foreign income.

The £1,000 figure is turnover, not profit. It is measured before expenses. A side business that took £1,400 and cost £900 to run has to register, even though it only made £500.

You might also want to register even if you do not have to

You can choose to file a return in order to claim certain Income Tax reliefs, to prove you are self-employed (for Tax-Free Childcare or Maternity Allowance, for example), or to pay voluntary National Insurance contributions to protect your State Pension record.

That last one matters more than people realise, and it is worth a conversation before you decide not to bother.

How to register

Register through the Self Assessment registration service on GOV.UK. HMRC then issues you a Unique Taxpayer Reference (UTR), which is a 10-digit number you will need for the rest of your filing life.

It does not arrive instantly. Register in late January and you may not have the reference in time to file, which is a genuinely bad place to be. Register as soon as you know you need to.

If you have registered before but did not file last year

This one catches people, and it is quiet about it.

If you registered for Self Assessment in the past but did not need to send a return for the 2024 to 2025 tax year, your account may need reactivating before you can file again.

If you file without reactivating it, your return may be delayed. Not rejected, not flagged. Delayed. And you may not find out until it matters.

The registration service will tell you if this applies to you. Do it early.

If you miss the 5 October deadline

Two separate things happen, and it is worth being precise about them because most guidance blurs them together.

First, your filing deadline changes. HMRC will write to you (by letter or email) with a new deadline, and it will be three months from the date on that letter, rather than the usual 31 January.

Second, your payment deadline does not change. You must still pay the tax you owe by 31 January, whatever your new filing deadline says.

The rule almost nobody knows

Here is GOV.UK's own wording, and read it carefully:

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.

The penalty is conditional on not paying.

If you register late but you pay your tax bill in full by 31 January, there is nothing left to base a failure-to-notify penalty on. The penalty is calculated from the tax still outstanding.

So if you have missed 5 October, do not panic and do not give up. Work out what you owe and pay it by 31 January. That is the single most useful thing you can do, and almost no accountant's website will tell you.

This is not permission to register late. Register on time. But if you already have, this is what to do about it.

Then the filing deadlines

Once registered, the deadlines for the 2025 to 2026 tax year are:

• 31 October 2026 if you file a paper return
• 30 December 2026 if you want the tax collected through your PAYE tax code
• 31 January 2027 if you file online, which nearly everyone does

You can file any time from 6 April 2026. Filing early does not mean paying early. The bill is still due on 31 January. Filing in May and knowing your number in May, with nine months to save for it, is one of the genuinely painless wins available in tax.

What happens if you file late

• An initial £100 penalty, even if you owe no tax at all
• After 3 months: daily penalties of £10 per day, up to £900
• After 6 months: a further 5% of the tax due, or £300, whichever is greater
• After 12 months: another 5% or £300, whichever is greater

The £100 applies even on a nil return. People find that hard to believe until it arrives.