Missed the 5 October Self Assessment Registration Deadline? What to Do Now

If you missed the 5 October deadline to register for Self Assessment, register with HMRC now. There is usually no penalty as long as you file your return and pay all the tax you owe by 31 January. The failure-to-notify penalty is based on tax still unpaid at that date, so paying on time means nothing is lost.

What the 5 October deadline is for

You must tell HMRC you need to file a Self Assessment return by 5 October after the end of the tax year in which you first had income to report. For the 2025/26 tax year, which ended on 5 April 2026, that deadline was 5 October 2026. It catches people who started self-employment, began letting property, or had other untaxed income for the first time.

Step 1: register now

Register online with HMRC straight away. Sole traders register as self-employed, which sets up Self Assessment at the same time; landlords and people with other untaxed income use the non-self-employed route; partners register both themselves and the partnership. You'll need your National Insurance number and the date your income started.

Step 2: get your UTR in time

HMRC posts your Unique Taxpayer Reference (UTR) after you register, and it can take a few weeks — longer in January. You can't file without it, so register well before the 31 January filing deadline. Once you have the UTR, set up your online account so you can file.

Step 3: file and pay by 31 January

The online filing and payment deadline for 2025/26 is 31 January 2027. If you pay everything due by then, a late registration on its own usually costs nothing. If you owe more than £1,000, your first bill may also include a first payment on account towards next year, so budget for it. Our self-employed NI calculator helps you estimate what you'll owe.

What if HMRC's notice to file arrives late?

If HMRC issues your notice to file after 31 October, your filing deadline becomes three months from the date of that notice rather than 31 January. The payment deadline does not move, so still pay by 31 January to avoid interest.

When a penalty does apply

A failure-to-notify penalty is a percentage of the tax still unpaid at 31 January (the 'potential lost revenue'). How much depends on whether HMRC sees the failure as careless or deliberate and on whether you told them before they found out. Registering now, before HMRC contacts you, puts you in the lowest bracket — and paying on time takes the figure to nil.

• Register before HMRC contacts you: this is treated as an unprompted disclosure.

• Pay everything due by 31 January: no unpaid tax means no penalty to charge.

• Keep records of when your income started and when you registered.

Check your other deadlines

Use the tax deadline checker to see every date that applies to you and add them to your calendar. If you're a sole trader or landlord with income over £20,000, also check whether Making Tax Digital for Income Tax will apply to you from 2027 or 2028.

Want us to handle it?

Countify registers clients with HMRC, chases the UTR and files the return on a fixed fee. See our Self Assessment service or call 07515 646845.