How to File a Tax Return Online UK: First-Timers

31 January is not a suggestion. Miss it by a single day and HMRC hands you an automatic £100 penalty, no questions asked, no appeal needed on their end. If you have never filed a Self Assessment tax return online before, the process is simultaneously simpler than you fear and more bureaucratic than it has any right to be.

This post walks you through how to file your tax return online in the UK, specifically if you are a sole trader, freelancer, or tradesperson doing it for the first time or the first time without an accountant. No jargon left unexplained. No steps skipped.

Who Actually Needs to File a Tax Return Online?

HMRC's own guidance on this is characteristically vague, so here is the plain version. You must file a Self Assessment tax return if, in the last tax year, any of the following applied to you:

• You were self-employed as a sole trader and earned more than £1,000 (before expenses)
• You were a partner in a business partnership
• You earned more than £100,000 in total income
• You had untaxed income above £2,500 (rental income, savings interest, freelance work on top of a PAYE salary)
• You claimed Child Benefit and either you or your partner earned more than £60,000

For most tradespeople, the first bullet is the only one that matters. If you picked up even occasional self-employed work, such as a few weekend plumbing jobs or a handful of freelance design projects, and earned more than £1,000 gross, you are legally required to declare it.

Self Assessment

HMRC's system by which individuals report income that has not already been taxed at source (via PAYE). Sole traders, landlords, and others with complex tax affairs use Self Assessment to calculate and pay the Income Tax and National Insurance they owe each year.

Step One: Register Before You Can Do Anything Else

This is the part that catches first-timers every year. You cannot simply log in and file. You must first register for Self Assessment, and HMRC will post a Unique Taxpayer Reference (UTR) number to your home address. That letter takes up to 10 working days to arrive.

If you are registering as a sole trader for the first time, you also need to register as self-employed through the same process. Go to HMRC's online registration service (GOV.UK) and select "Register if you're self-employed."

Once you have your UTR, you then need to create or sign in to a Government Gateway account and enrol for Self Assessment. HMRC will send an activation code, again by post. Allow another five working days.

The practical upshot: if you leave registration until mid-January, you will almost certainly miss the 31 January deadline through no fault of your own, and HMRC will still fine you £100.

Register by: 5 October following the end of the tax year in which you became self-employed. So if you started working for yourself in the 2024/25 tax year (which ended 5 April 2025), your registration deadline was 5 October 2025.

Step Two: Gather What You Actually Need

Before you open the HMRC portal, assemble the following. Hunting for these mid-filing is how a 90-minute task becomes a four-hour one.

Income records:

• Total self-employment income received (not invoiced; what was actually paid to you in the tax year)
• Any employment income and your P60 or P45
• Bank interest received
• Rental income if applicable

Expense records:

• Business mileage (HMRC's approved mileage rate is 45p per mile for the first 10,000 miles, then 25p)
• Materials, tools, and equipment purchased for work
• A proportion of phone bills used for business
• Professional subscriptions and insurance
• Any use-of-home-as-office costs

If you have been tracking expenses properly throughout the year, this step takes minutes. If you have been keeping receipts in a carrier bag, block out an afternoon.

It is also worth checking whether simplified expenses apply to you. HMRC allows flat rates for mileage and working from home that can be simpler to calculate than actual costs.

Step Three: File Through HMRC's Portal (The Free Route)

Once you have your Government Gateway login and UTR, go to HMRC's Self Assessment portal. The interface has improved marginally since its 2003-era origins, though it still occasionally times out mid-session, so save your progress constantly.

Here is the filing sequence:

1. Tailor Your Return

HMRC asks a series of questions to determine which sections of the return you need to complete. As a sole trader, you will definitely need the "Self-employment" pages (SA103). Answer accurately; HMRC will only show you the relevant sections.

2. Employment Income

If you also had a PAYE job during the year, enter your figures from your P60. HMRC often pre-populates this from employer submissions, but always verify it manually.

3. Self-Employment Pages

Enter your total self-employment turnover (all money received from clients, before any expenses). Then enter your allowable expenses. HMRC gives you two options here:

• Cash basis accounting: You record income when you receive it and expenses when you pay them. Since 6 April 2024 this has been the default for sole traders, with no turnover limit.
• Traditional (accruals) accounting: You record income when invoiced and expenses when incurred, regardless of when money changes hands. This is no longer anyone's automatic position; you have to elect for it on your return.

For most tradespeople, cash basis is the right choice, and it is what you get unless you actively opt out. Less administrative complexity, same tax outcome in most cases.

4. Claim Your Allowances

Do not skip this section. The Trading Allowance (£1,000), the Personal Allowance (£12,570 in 2024/25), and any pension contributions or Gift Aid donations all reduce your tax bill. Also check whether capital allowances apply if you bought significant equipment during the year.

5. Review the Calculation

Before you submit, HMRC shows you a tax calculation. Check it. If the number looks dramatically different from what you expected, stop and check your entries before proceeding. A common error is entering turnover in the expenses box or vice versa.

6. Submit and Save Your Confirmation

Once submitted, save or print the confirmation page. HMRC will also send a confirmation to your Government Gateway inbox. This is your proof of filing.

What Filing Online Actually Costs You

Filing directly through HMRC's Self Assessment portal is free. There is no charge to use it. You pay only the tax you owe.

Where costs creep in is if you use third-party software or an accountant. Accounting software such as FreeAgent, Xero, or QuickBooks can cost between £12 and £40 per month. An accountant filing on your behalf will typically charge £200 to £600 for a straightforward sole trader return, depending on complexity and location.

For most sole traders with clean records and a single income source, filing directly through HMRC's portal is entirely manageable. The software products earn their cost when your finances are more complex, when you have multiple income streams, or when you want to be ready for the changes coming in April 2026.

On that subject: cloud tax software is worth understanding before you commit to a subscription. The pricing models are less transparent than they appear.

The MTD Complication Starting April 2026

If you are a sole trader earning above £50,000, the way you file tax is about to change significantly. From April 2026, Making Tax Digital for Income Tax Self Assessment (MTD for ITSA) requires you to:

1. Keep digital records of all income and expenses throughout the year
2. Submit quarterly updates to HMRC via MTD-compatible software (not the current portal)
3. File a Final Declaration to close out each tax year

This is not optional and it is not a minor administrative tweak. It replaces the single annual Self Assessment return with five separate submissions per year. The annual tax return you file today will cease to be available for those above the threshold.

From April 2027, the threshold drops to £30,000, catching a much larger proportion of the UK's sole traders.

The free HMRC portal will not support MTD submissions. You will need approved software. Linking MTD software to HMRC is a separate process that also takes time to set up, so doing it at the last minute is a familiar and costly mistake.

People also ask

What is the deadline for filing a tax return online in the UK?

The online Self Assessment deadline is 31 January each year, covering the previous tax year which ran from 6 April to 5 April. The paper filing deadline is earlier, on 31 October. Miss the online deadline by even one day and HMRC issues an automatic £100 penalty.

How long does it take HMRC to process an online tax return?

HMRC typically processes online Self Assessment returns within 72 hours of submission. If you are due a refund, it usually arrives within 5 working days of the return being processed, though it can take longer during the January peak period.

Can I file my Self Assessment tax return for free?

Yes. HMRC's own Self Assessment portal is free to use. You pay only the tax you owe, not to file. Third-party accounting software charges a monthly subscription but is not required until Making Tax Digital for Income Tax becomes mandatory from April 2026.

What happens if I make a mistake on my online tax return?

You can amend a submitted Self Assessment return up to 12 months after the original filing deadline. Log back into your Government Gateway account and select 'Amend return.' If HMRC has already processed a repayment based on incorrect figures, they will adjust accordingly.

Common Mistakes That Cost Sole Traders Money

Forgetting payments on account. Your January bill is not just the tax for the year just ended. It also includes the first payment on account for the current year, which is 50% of last year's bill. Many first-time filers are blindsided by this. A sole trader who owes £3,000 in tax for 2023/24 will pay £4,500 in January (£3,000 plus £1,500 first payment on account) and another £1,500 the following July. If this is news to you, read our post on sole trader tax payment dates.

Mixing up income and profit. You declare your turnover (total income received) and then deduct expenses to arrive at your taxable profit. Many first-timers enter only their profit in the turnover box, which under-declares income and can trigger an HMRC enquiry.

Missing the registration deadline and blaming the filing deadline. These are different dates. If you were not registered in time to receive your UTR before 31 January, HMRC will still levy the £100 late filing penalty, even though the delay was in their own postal system.

Ignoring your tax code if you also have PAYE income. If you had an employer during the tax year as well as self-employment income, your tax code matters. An incorrect code may mean you have already overpaid or underpaid tax through PAYE, which the Self Assessment process will then reconcile. If your tax code looks unfamiliar, understanding what it means is worth five minutes of your time before filing.

After You File: What Happens Next

Once submitted, HMRC processes your return and sends a Self Assessment statement showing what you owe and when. Payment is due by 31 January (for the balancing payment) and 31 July (for the second payment on account).

HMRC accepts payment by bank transfer, debit card, or through its app. It does not accept personal credit cards. Allow three working days for bank transfers to clear; do not leave payment until 31 January evening.

If you cannot pay in full, HMRC's Time to Pay arrangement allows you to spread the bill over monthly instalments. You must set this up before the deadline, not after. Call HMRC's Self Assessment helpline or apply online through your Government Gateway account.

The January Panic Is Preventable

The reason 31 January feels catastrophic for so many sole traders is not that the process is impossibly hard. It is that registration, record-gathering, and the filing itself all collide at once because everything was left until the last fortnight. The single most effective tax admin habit you can build is filing by the end of November, when your records are fresher, HMRC's phone lines are shorter, and you have two months to fix any errors before the deadline.

From April 2026, that annual scramble becomes structurally impossible for those above the MTD threshold. Quarterly submissions mean there is no single January deadline to dread; there are four quarterly ones plus a final declaration. Whether that is better or worse depends entirely on how organised you are willing to become.

If you started reading this post because 31 January is approaching and you have not filed yet: stop reading, open a new tab, and go to GOV.UK to check whether you are registered. That is the only step that matters in the next five minutes.

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