Self Assessment Registration Deadline: What You Need to Do Before 5 October

If you need to complete a Self Assessment tax return for the 2025/26 tax year and you are new to Self Assessment, there is an important deadline approaching:

5 October 2026.

This is the date by which you generally need to tell HMRC that you need to complete a return if you have not filed one before, or if you were previously registered but did not need to file for 2024/25. GOV.UK

With only a few days remaining, here is what you should check and do before the deadline.

First: don’t confuse 5 October with the filing deadline

The 5 October deadline is about notifying HMRC, not submitting your completed online tax return.

For the 2025/26 tax year, the main dates are:

HMRC confirms that paper returns must arrive by 31 October and online returns by 31 January. GOV.UK

So you do not need to have your entire online return completed by 5 October.

But you do need to establish whether you should be registered.

Check whether you actually need Self Assessment

Before registering, determine whether you need to complete a return at all.

HMRC says you must send a return if, during the previous tax year, circumstances included being a sole trader with more than £1,000 gross trading income, being a partner in a business partnership, having Capital Gains Tax to pay, or having certain other tax obligations.

You may also need a return because of untaxed income such as:

• Property or land income
• Tips and commission
• Savings interest
• Dividends
• Foreign income
• Certain taxable UK income if you’re non-UK resident GOV.UK

If you’re unsure, HMRC provides an online checker rather than requiring you to work it out from scratch.

Did you start working for yourself during 2025/26?

This is particularly important for new sole traders.

Suppose you started your business in September 2025.

That falls within the tax year:

6 April 2025 – 5 April 2026

If your circumstances mean you need Self Assessment, 5 October 2026 is therefore the relevant registration deadline.

The same principle applies if what began as a small side hustle grew during the year.

HMRC says sole traders generally need a return where gross trading income exceeded £1,000 before expenses. GOV.UK

Don’t forget income from side hustles

You do not have to think of yourself as a “business owner” to potentially have a Self Assessment obligation.

Perhaps you earned money from:

• Freelancing
• Tutoring
• Photography
• Social media work
• Consulting
• Selling products for profit
• Gardening
• Online services
• Other paid work outside your employment

HMRC recently highlighted that the £1,000 trading threshold looks at relevant side-hustle income combined. Its example gives someone earning £600 from photography and £500 from social-media work: together that exceeds £1,000. GOV.UK

However, simply selling your old personal possessions does not normally mean you are trading. HMRC distinguishes this from regularly selling goods for profit. GOV.UK

Check your property income

If you started receiving income from property during 2025/26, check whether it creates a reporting requirement.

This might include:

• A buy-to-let
• A second property
• Renting part of a property
• Holiday accommodation
• Renting land

Different allowances and rules can apply depending on exactly what you are doing.

But don’t assume that because HMRC hasn’t contacted you, nothing needs to be reported.

HMRC specifically lists money from renting property or land as a form of untaxed income that can require a return. GOV.UK

Check other income HMRC hasn’t taxed automatically

Go through your finances for 6 April 2025 to 5 April 2026.

Did you receive income outside your normal PAYE salary?

For example:

Savings interest

Dividends

Commission or tips

Foreign income

Property income

Receiving one of these does not necessarily mean you will need Self Assessment in every case.

But if you’ve had substantial untaxed income, it is worth checking before 5 October rather than assuming PAYE has dealt with everything.

Check whether you had taxable capital gains

Income isn’t the only thing to consider.

You may need to report gains from disposing of assets.

Examples could include:

• Shares
• Investment funds
• Cryptocurrency
• Second properties
• Certain valuable possessions

HMRC currently lists having Capital Gains Tax to pay on a disposal as one of the circumstances requiring a tax return. GOV.UK

Capital Gains Tax also has separate reporting requirements in some circumstances, particularly for UK residential property, so Self Assessment should not automatically be treated as the only deadline that matters.

What if you’ve completed Self Assessment before?

Do not automatically register for a completely new account.

If you were previously registered for Self Assessment but did not submit a return last year, HMRC says you may need to reactivate your Self Assessment account.

HMRC also warns that attempting to file without reactivating an existing account can delay the return. GOV.UK

If you’ve previously had a Unique Taxpayer Reference, check your existing position first.

How do you register?

HMRC launched an improved Self Assessment registration service in September 2026.

New Self Assessment taxpayers can use the online service through their Personal Tax Account. HMRC says the new system is intended to make the registration process more streamlined. GOV.UK

HMRC reported that more than 640,000 customers registered for Self Assessment in the 12 months to 31 March 2026. GOV.UK

Once registered, you’ll receive a Unique Taxpayer Reference (UTR), which you’ll need when dealing with your Self Assessment return. GOV.UK

What information should you start gathering?

Once you’ve dealt with registration, start preparing for the return itself.

Depending on your circumstances, this might include:

• P60 or P45
• Self-employed income
• Business expenses
• Bank statements
• Receipts
• Property income and costs
• Savings interest
• Dividend information
• Pension contributions
• Investment transactions
• Foreign income
• Gift Aid donations

HMRC says taxpayers need to keep appropriate records, such as bank statements and receipts, so their returns can be completed correctly. GOV.UK

You don’t need everything neatly completed by 5 October.

But starting now means you are less likely to spend January trying to reconstruct an entire year’s finances.

Should you wait until January to submit the return?

There is generally no advantage in waiting purely because the online deadline is 31 January.

HMRC says that once registered, taxpayers can submit their return any time after the end of the tax year. Filing earlier means you find out what you owe sooner. GOV.UK

Knowing your bill in October or November can give you considerably more time to budget than discovering it a few days before payment is due.

For a business owner or sole trader, that can be particularly valuable for cashflow planning.

You can start putting money towards the bill

If you’re worried about receiving one large tax bill in January, you can start planning before you’ve reached the payment deadline.

HMRC says that after registering you can estimate the amount you may need to pay and set up weekly or monthly payments towards your next Self Assessment bill. GOV.UK

This doesn’t change the amount of tax you owe.

It simply allows you to spread the cash leaving your bank account rather than treating tax as a single unexpected January expense.

What happens if you miss 5 October?

If 5 October passes and you realise you should have registered, don’t wait until January.

Register as soon as possible.

HMRC says that where someone registers after 5 October 2026, it will send a letter or email providing a different filing deadline, normally three months from the date of that communication.

However, the tax itself must still normally be paid by 11:59pm on 31 January 2027. GOV.UK

HMRC also warns that registering late can potentially result in a penalty. GOV.UK

What should you do before 5 October?

If you’re reading this before the deadline, focus on four things.

Check whether you need a return.
Don’t register unnecessarily, but don’t assume PAYE means you’re automatically exempt either.

Register or reactivate your account.
Use the correct HMRC process for your circumstances.

Start gathering your records.
You don’t need to finish the return immediately, but you can get the information organised.

Start planning for the bill.
If tax is likely to be due in January, work out roughly how much cash you’ll need.

That puts you in a considerably better position than starting everything after Christmas.

What if you’re unsure whether you need to register?

This is where people’s circumstances matter.

For example, you might have:

• PAYE employment and freelance income
• Several different side hustles
• Property and employment income
• Dividends from your own company
• Overseas income
• Capital gains
• Income that began partway through the year

HMRC provides an online checker if you’re uncertain.

Alternatively, an accountant can review the different sources together and establish whether a return is required.

Frequently Asked Questions

When is the Self Assessment registration deadline?

For someone who needs to enter Self Assessment for 2025/26 and meets the registration conditions, the deadline is 5 October 2026. GOV.UK

Does my tax return need to be finished by 5 October?

No. The normal online filing deadline for the 2025/26 return is 31 January 2027. GOV.UK

What if I have already completed Self Assessment before?

If you were registered previously but did not submit a return last year, you may need to reactivate your existing account rather than registering again. GOV.UK

Do I need to register if my side hustle made less than £1,000?

The £1,000 trading allowance and reporting rules need to be considered in the context of your circumstances. HMRC’s mandatory Self Assessment list includes sole traders whose gross trading income exceeded £1,000. GOV.UK

Is the £1,000 threshold based on profit?

For this test, HMRC refers to what you earned before taking off expenses, so it is the gross trading-income figure that matters. GOV.UK

What happens if I register after 5 October?

Register as soon as possible. HMRC can issue you with a later filing date, but the normal 31 January 2027 tax-payment deadline remains. GOV.UK

Final thoughts

If you need Self Assessment for 2025/26, the next few days matter.

The immediate priority isn’t necessarily completing the entire return.

It’s establishing whether you need one and, where required, telling HMRC by 5 October 2026.

Once that’s dealt with, you can organise your records, prepare the return and establish how much tax you’ll need to pay well before January.

At Merranti Accounting, we help individuals, sole traders and business owners with Self Assessment, from establishing whether a return is required through to preparing and submitting it.

Contact Merranti Accounting →

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Written by Stephen Watts ACMA

Accounting Director, Merranti Accounting

Stephen is a CIMA-qualified accountant with more than 20 years of commercial finance and accounting experience. His background includes financial management, tax, business planning and supporting business owners with practical financial decisions, giving him extensive experience of helping clients understand and manage their accounting and tax obligations.

Meet Stephen and the Merranti team →