Registering for Self Assessment by 5 October: What New Arrivals Need to Do

If you need to file a UK tax return for the year ended 5 April 2026 and have never filed one, you must register for Self Assessment with HMRC by 5 October 2026. Registration produces your Unique Taxpayer Reference. Without it you cannot file, and missing the date can bring a failure-to-notify penalty.

For people who arrived in the UK during the 2025/26 tax year, 5 October is the deadline nobody mentions. Employers do not raise it, HMRC does not write to remind you, and it falls nearly four months before the 31 January filing date that everybody has heard of. This guide covers the registration step itself: who it applies to, which route to use, how long each route takes, and what the penalty looks like if you are late. Whether you need a return at all is covered in our companion guide, do I need to file a Self Assessment tax return.

When do you need to register for Self Assessment by 5 October?

You need to register by 5 October when you have to complete a tax return for the previous tax year and have not sent one before. GOV.UK puts it plainly on its registration page: you must tell HMRC by 5 October 2026 if you need a return for the previous tax year, and telling HMRC after that date could mean a penalty. The same applies if you did not need to send a return for 2024/25 but do for 2025/26.

The obligation belongs to you, not to HMRC. HMRC cannot issue a notice to file to someone it does not know has untaxed income, so the law puts the burden of telling it on the taxpayer. For a new arrival from the US, the usual triggers are income PAYE never touches: dividends and interest from a US brokerage account, rent from a property kept in the US, consulting income from US clients, or a gain on shares sold after arriving.

Why new arrivals are the people who miss it

Three features of the first UK year make the 5 October deadline easy to miss for Americans.

• The UK year does not match the US year. The 2025/26 tax year ran from 6 April 2025 to 5 April 2026. Someone who arrived in August 2025 is thinking about a US return for calendar 2025 and does not realise a UK year closed in April.
• PAYE feels complete. A UK salary is taxed at source, a payslip shows tax deducted, and nothing suggests anything else is owed. The foreign income is invisible to the payroll system.
• The deadline is early. 5 October falls six months after the year ends and well before the 31 January filing deadline, so people who plan to file in January are already late to register.

Illustrative example: an American who moved to Manchester in August 2025 on a UK employment contract and kept a US brokerage account paying quarterly dividends became UK resident during 2025/26. Her salary is taxed under PAYE, but the US dividends are foreign income HMRC has never seen. She has never filed a UK return, so she must register for Self Assessment by 5 October 2026, receive her UTR, and then file online by 31 January 2027, where she can also consider an arrival-year split-year claim.

Which registration route applies to you?

How you register depends on why you need a return and whether you already have a UK online account. HMRC's routes are summarised below.

Your situation	How to register	What to expect
Individual with a Personal Tax Account, registering yourself	HMRC's online registration service	UTR in your online account within 72 hours, per HMRC's September 2026 announcement
Not self-employed (foreign income, rent, gains, High Income Child Benefit Charge)	Form SA1, or the online route above	UTR by post, usually around 15 days; longer overseas
Self-employed or starting as a sole trader	The self-employed registration route (form CWF1 for agents)	UTR by post, and the business start date is recorded
Registered in an earlier year but did not file last year	Reactivate the existing Self Assessment record	Your existing UTR is reused
Using an accountant or tax adviser	The adviser registers you on form SA1 or CWF1	Agents cannot use the new online service

The form for people who are not self-employed is SA1, and it asks for the reason you need to register and the date that reason started. For most new arrivals from the US the reason is foreign income, and the start date is the date UK residence began. Getting that date right matters because it becomes the starting point for any split-year claim on the return.

How to register for Self Assessment before 5 October

1. Confirm you need a return. Check the categories HMRC lists and identify the income that is not taxed under PAYE. Write down the reason you will give HMRC.
2. Check whether you already have a record. If you were registered in an earlier year, reactivating is quicker than a new registration, and GOV.UK warns that filing without reactivating can delay the return.
3. Choose the route. If you have a Personal Tax Account, the online service is fastest. If you do not, or if an adviser is acting for you, use form SA1 or CWF1.
4. Give the right start date. Use the date the income source or UK residence began, not the date you are registering.
5. Keep evidence of when you registered. A screenshot or confirmation of submission is your proof that you notified HMRC before 5 October 2026.
6. Watch for the UTR. If nothing has arrived after the expected time, use HMRC's reply-time checker rather than registering a second time.

How long does it take to get a UTR?

A UTR usually arrives by post around 15 days after you register, and HMRC states on its find your UTR number page that it takes longer if you live overseas. The UTR is a 10-digit number, and once issued it also appears in your Personal Tax Account and the HMRC app.

The faster route is new. On 9 September 2026 HMRC announced an improved Self Assessment registration service for individuals with a Personal Tax Account. People using it receive their UTR in their online account within 72 hours instead of waiting up to 15 days for the post. The service pre-fills information HMRC already holds and lets you save and return to the form. Agents cannot use it, so if an adviser is registering you, the postal timetable still applies.

Timing matters because the UTR is only the first step. After it arrives you still need online access to file, and HMRC may post an activation code for the online return. For someone registering from an overseas address in late September, the post can take up much of the margin before 5 October and a good deal of the time before 31 January.

What happens if you miss the 5 October deadline?

Missing 5 October exposes you to a penalty for failure to notify, which is different from the late filing penalty. HMRC calculates it as a percentage of the potential lost revenue, which its factsheet CC/FS11 defines as the amount that arises as a result of the failure to notify. In plain terms, it is the tax that went unpaid because HMRC did not know about your income.

The percentage depends on why the failure happened, whether you told HMRC before it asked, and how long it went on. The ranges in CC/FS11 are:

Behaviour	Disclosure	Penalty range
Non-deliberate	Unprompted, within 12 months	0% to 30%
Non-deliberate	Unprompted, 12 months or more	10% to 30%
Non-deliberate	Prompted, within 12 months	10% to 30%
Non-deliberate	Prompted, 12 months or more	20% to 30%
Deliberate	Unprompted / prompted	20% to 70% / 35% to 70%
Deliberate and concealed	Unprompted / prompted	30% to 100% / 50% to 100%

Two practical lessons follow. First, a new arrival who realises late should register straight away, because an unprompted, non-deliberate disclosure within 12 months sits in the lowest range and can reduce to 0%. Second, HMRC recognises a reasonable excuse, which CC/FS11 describes as something that stopped you meeting an obligation you took reasonable care to meet. Not knowing the deadline existed is rarely accepted on its own.

Late filing is a separate charge on top. If you register late and then also miss the 31 January 2027 online deadline shown on HMRC's Self Assessment deadlines page, the late filing and late payment penalties run as well.

What to have ready once your UTR arrives

Registering by 5 October buys time to file properly. For a new arrival, most of the work on the first UK return is establishing facts that will not be on any UK document.

Your residence start date and day counts

The Statutory Residence Test decides when you became UK resident, and split-year treatment decides whether income from before your arrival stays outside UK tax. Both depend on documented travel dates. Rebuild them from passport stamps, flight confirmations and calendars while they are still easy to find.

Your US income, converted and re-dated

US brokerage statements, rental ledgers and 1099s run on the calendar year. The UK return needs the same income reassembled into the year ended 5 April 2026 and converted to sterling. Keeping the working papers helps when the same figures are needed for foreign tax credits on the US return.

A decision on the FIG regime

Arrivals from 6 April 2025 may qualify for the four-year foreign income and gains regime, which can take qualifying foreign income and gains out of UK tax. HMRC's FIG guidance says the claim is made on the Self Assessment return and that you need to register if you are not already registered. Claiming costs you the Income Tax and Capital Gains Tax tax-free allowances for that year, so for a US citizen it should be modelled against the US return before it is made. Our checklist of tax obligations for Americans living in the UK sets out where the FIG decision sits among everything else.

Where the US return fits

Registering with HMRC does nothing for your US position. The IRS continues to tax US citizens on worldwide income from all sources, with an automatic extension to 15 June for citizens living abroad and a further extension to 15 October available on Form 4868. The two calendars interact: foreign tax credits on the US return depend on UK tax actually assessed, and the UK return is not due until 31 January. Our guide to US tax filing requirements for Americans in the UK explains how the two are usually sequenced.

Getting registered in time

US/UK Cross Border Tax is US CPAs and UK tax advisers working as one team, with offices in London, Manchester, New York and San Francisco. For new arrivals we confirm whether a return is needed, register you on the right basis and with the right start date, and prepare the first UK return alongside the US one so residence dates, split-year claims and foreign tax credits agree.

If you are still working out whether you need to file Self Assessment in the UK, there is still time to settle it before 5 October 2026. Read how we work with Americans living in the UK, or contact us with your arrival date and income sources and we will tell you what to register and when.