How to Do Your Self-Assessment Tax Return Online

Your first self-assessment tax return is intimidating. The jargon, the fear of getting it wrong, the vague dread of HMRC. But here’s the secret: for most self-employed people with straightforward affairs, it’s a 1–2 hour job once you know the steps — and doing it in November beats doing it at 11pm on 31 January. Here’s the full walkthrough.

How Do I Do My Self Assessment Tax Return Online?

Register with HMRC (if you haven’t), gather your income and expense records for the tax year (6 April–5 April), log into your Government Gateway account, complete the SA100 plus the self-employment pages (SA103S or SA103F), and submit by 31 January. Pay any tax owed by the same date.

Key Dates (Put These in Your Calendar Now)

• 5 October: deadline to register for self-assessment if you’re newly self-employed
• 31 October: paper return deadline (don’t do paper — do it online)
• 31 January: online filing deadline AND payment deadline for the previous tax year
• 31 July: second payment on account (if applicable)

Miss 31 January and it’s an automatic £100 fine, plus interest on unpaid tax, plus daily penalties after three months. The fines escalate fast — file on time even if you can’t pay in full (contact HMRC about a payment plan; the penalty for late filing is separate from late payment).

Step-by-Step: Filing Online

1. Register. If this is your first time: go to gov.uk, register for self-assessment, and HMRC posts you a Unique Taxpayer Reference (UTR) — allow a few weeks. Then set up your Government Gateway account.
2. Gather your records. Total income for the tax year, total allowable expenses (by category), plus any other income: employment, rental, dividends, savings interest. If you’ve used accounting software all year, this is a ten-minute export. If you haven’t… this is the painful part. (Start using software next year.)
3. Log in and start the return. HMRC’s online system guides you section by section. Complete the main SA100, then the self-employment supplementary pages. Answer honestly; the system calculates the tax.
4. Check the calculation. HMRC shows what you owe before you submit. Sanity-check it: roughly 20% income tax on profit above £12,570, plus Class 2 NICs (£179.40/year) and Class 4 NICs (6% on profits £12,570–£50,270). If the figure looks wildly wrong, you’ve probably mistyped something.
5. Submit and save the confirmation. You’ll get a submission receipt — keep it.
6. Pay by 31 January. Bank transfer, Direct Debit, or through your Government Gateway account. Set up the payment a few days early — bank processing delays don’t excuse late payment.

Payments on Account: The Nasty Surprise

First-timers get blindsided by this: if your tax bill exceeds £1,000, HMRC assumes you’ll owe similar next year and demands payments on account — 50% of this year’s bill by 31 January, 50% by 31 July, on top of this year’s actual bill. So a £4,000 first tax bill means paying £6,000 in January. It’s not extra tax (it offsets next year’s bill), but the cash-flow shock is real. Budget for it from your first invoice — put 25–30% of every payment aside in a separate account.

Common Mistakes

• Forgetting income. That side gig, the rental room, the dividends — HMRC’s data-matching finds it. Declare everything.
• Claiming the commute. Home to your usual workplace is never allowable. (See our expenses guide for what is.)
• Not registering in time. The 5 October deadline catches new freelancers who started in spring.
• Leaving it to January. Every accountant’s horror stories start with “my client called on 30 January”. File in November. Sleep better.
• Ignoring the payment deadline. Filing ≠ paying. Both due 31 January.

Should You Use an Accountant?

For a straightforward sole trader return, many people manage fine alone — especially with decent software. Consider an accountant if: you have multiple income sources, you’re near tax thresholds, you’ve incorporated (see registering a limited company — company directors’ affairs get complex), or you simply hate it. A simple self-assessment from an accountant costs £150–£350 — see our full breakdown of accountant costs. Often they save more than they cost.

The Bottom Line

Register early, keep records all year (not in January), file in autumn, pay by 31 January, and budget for payments on account. It’s admin, not rocket science — the stress comes from leaving it late, not from the return itself. HMRC’s self-assessment pages are the official guide; bookmark them.

Self-Assessment: Questions, Answered

What if I can’t pay my tax bill by 31 January?

File on time anyway — the late-filing penalty (£100) applies regardless of payment, so don’t compound the problem. Then contact HMRC immediately about a Time to Pay arrangement; they’re pragmatic with people who engage early. Interest accrues on the unpaid amount, but a payment plan stops the escalating penalties. What you mustn’t do: ignore it and hope. HMRC’s debt collection is persistent and their powers are extensive.

Do I need to file if I earned very little?

If you’re registered for self-assessment, yes — you must file every year until HMRC tells you to stop, even for £500 of freelance income. (You can ask HMRC to remove you from self-assessment if you’ve stopped trading.) The filing obligation is separate from the tax owed: little income means little tax, but the return is still due. The £1,000 trading allowance means very small earnings may result in zero tax — but you still file to claim it properly.

Can I amend my return after submitting?

Yes — you have 12 months from the filing deadline to amend online (so for the 2024/25 return filed by 31 January 2026, until 31 January 2027). Forgot an expense? Found a missing invoice? Amend it — it happens constantly and HMRC expects it. After the amendment window, you can still write to HMRC about errors, but it’s more formal. When in doubt, file on time with your best figures and amend later.

Should I hire someone to do my return?

For a straightforward sole trader with tidy records: many manage fine alone, especially with accounting software. Consider help when: multiple income sources, rental income, capital gains, you’re near thresholds, or you simply dread it — stress has a cost too. A simple return from an accountant is £150–£350 (see our accountant cost guide), and they often find savings that cover the fee.