Self Assessment: register by 5 October or face a penalty

Most people know the Self Assessment filing deadline is 31 January. Far fewer know there is an earlier one, and that missing it carries its own penalty.

If you need to complete a tax return for the first time, you must tell HMRC by 5 October following the end of the tax year in which the liability arose. Registration is a separate step from filing, it is not instant, and you cannot file without completing it first.

At a glance

Who needs to register?

You must register for Self Assessment if any of the following applied in the tax year:

• You were self-employed and your gross trading income exceeded the trading allowance of £1,000.
• You were a partner in a business partnership.
• You received rental income above the property allowance of £1,000.
• You had untaxed income — tips, commission, income from savings or investments not taxed at source, or foreign income.
• You or your partner had income over £60,000 and you claimed Child Benefit, bringing you into the High Income Child Benefit Charge.
• You made capital gains above the £3,000 annual exempt amount, or need to report a property disposal.
• You want to claim reliefs that cannot be given another way — higher-rate pension relief, or significant work expenses.

You generally do not need to register if all your income is taxed through PAYE and you have no other liability. If you are unsure, HMRC provides an online checker, and it is worth using rather than guessing in either direction.

Why 5 October matters more than people think

Registration is not a formality you can complete on the way to filing. It triggers a chain of steps, each with its own delay:

1. You register online, or by post for some circumstances.
2. HMRC issues a Unique Taxpayer Reference — a ten-digit UTR sent by post. This commonly takes around ten working days, and longer at busy times.
3. You then need to activate your online account, which requires a separate activation code, also sent by post.
4. Only then can you actually file.

Two posted items, each taking days, before you can even begin. Someone who registers in mid-January may simply not have the credentials in time to file by the 31st — and the filing penalty applies regardless of the reason.

This is why the October deadline exists. It is not arbitrary; it is the point by which HMRC needs to know you are coming so the paperwork can reach you in time.

The failure to notify penalty

Registering late is a separate offence from filing late, with its own penalty regime.

The penalty for failure to notify is calculated as a percentage of the tax that remains unpaid, and the percentage depends on why you were late. An unprompted disclosure — where you come forward before HMRC contacts you — attracts a much lower penalty than a prompted one, and in some cases of genuine reasonable excuse it may be reduced to nothing.

The practical implication is important: if you have missed the deadline, register immediately rather than waiting. Coming forward voluntarily materially reduces what you will pay, and delay does the opposite.

Where there is no tax to pay, a failure to notify penalty is generally not charged — but do not rely on that assumption without checking your position properly.

How to register, step by step

The route depends on why you need to file, and choosing the wrong one causes delay.

If you are newly self-employed, you register as a sole trader. This does two jobs at once: it brings you into Self Assessment and it registers you for Class 2 and Class 4 National Insurance. You will need your National Insurance number, the date you started trading, and a description of your business.

If you are not self-employed — you have rental income, capital gains, or the High Income Child Benefit Charge — you register using a different form for non-self-employed taxpayers. Registering as a sole trader when you are not one creates a trading record that then has to be closed, which is a nuisance to unpick.

If you are joining a partnership, both the partnership itself and each individual partner must register separately. Missing the partnership registration is a common oversight, and the partnership return has its own penalties.

You will need a Government Gateway account. If you already have one for another service, use it rather than creating a second — duplicate accounts are a frequent cause of people being unable to see their own tax record later.

Getting the date you started right

Registration asks when you began trading, and the answer determines which tax year your first return covers. People routinely give the date they registered rather than the date they actually started, which can push a liability into the wrong year.

Trading generally begins when you start doing business — making your first sale, or offering services — not when you first thought about it, and not when you registered. If you began selling in February and register in September, your first return covers the tax year containing that February.

This matters because it can mean a return is due sooner than you expect, and occasionally that a registration deadline has already passed. If your start date falls in an earlier tax year than you assumed, say so on the registration and deal with it openly rather than adjusting the date to something more convenient.

The deadlines that follow

The 30 December date is the one most often missed and most often useful. If you owe less than £3,000 and file by then, HMRC can collect it through your PAYE code across the following year instead of demanding it in one payment. For anyone with a modest liability and a salary, that is a materially better cash-flow outcome, and it costs nothing but filing a month early.

Track all of these with our Tax Deadline Tracker.

What to prepare before you file

Registration is the trigger to start gathering paperwork, not the end of the job. Most of the January panic comes from hunting for records that could have been assembled in October.

You will typically need your P60 and any P45s, P11D details of benefits in kind, records of self-employed income and expenses, bank interest statements, dividend vouchers, details of pension contributions, records of any gift aid donations, and completion statements for any property disposals.

If you are newly self-employed, this is also the moment to decide how you will keep records for the year ahead. Making Tax Digital for Income Tax is changing the requirements for many sole traders and landlords — our guide to Making Tax Digital for Income Tax explains who is affected and when.

Budget for the bill before January

The single most common first-year shock is not the return itself but the size of the payment. A first Self Assessment bill frequently includes the tax for the year and a payment on account towards the next one — often 150% of what people expected.

Estimate it early with the Self-Employed Tax Calculator, and read our guide to payments on account so the January demand is not a surprise. Setting aside a percentage of every payment you receive from the start of the year is far easier than finding a lump sum in January.

If you no longer need to file

The obligation is not permanent. If your circumstances change — you stop trading, or your untaxed income ends — tell HMRC. Until you do, returns remain due, and a penalty accrues for each one you do not file even where no tax is owed.

People regularly discover several years of accumulated late filing penalties on a return they did not know they still had to submit. Closing the record properly takes one phone call or one online form.

A note on how to use this

This guide explains the rules as they stand for the 2026/27 tax year and is written to help you understand your own position. It is general information, not personal financial advice — your circumstances change the answer, sometimes completely. For a decision that matters, speak to a regulated adviser or check directly with HMRC. Our calculation methodology sets out where every figure on this site comes from.

Where these figures come from

Every rate and threshold on this page is checked against HMRC's published guidance for the 2026/27 tax year. If you spot a figure that looks out of date, please tell us.

• GOV.UK guidance for this topic