Around 12 million people in the UK file a self-assessment tax return each year, and for most of them it’s somewhere between mildly stressful and absolutely dreaded. If you’re self-employed, a landlord, a company director, or earn over £150,000, chances are you need to file one. Here’s how to do it without losing your mind.
Who Needs to File?
You must file if any of these apply to you:
- You’re self-employed and earned more than £1,000 (the trading allowance)
- You’re a partner in a business partnership
- You’re a company director (unless your only income is PAYE salary with no benefits)
- You have untaxed income — rental income, savings interest above your allowance, foreign income
- Your total taxable income tops £150,000
- You need to claim tax relief on pension contributions or charitable donations
- You sold something and owe Capital Gains Tax
- You receive child benefit and you or your partner earns over £60,000 (the High Income Child Benefit Charge)
Key Deadlines
The UK tax year runs 6 April to 5 April. For the 2024/25 tax year:
- 5 October 2025 — deadline to register if you’re new to self-assessment
- 31 October 2025 — paper returns (almost nobody does this now)
- 31 January 2026 — online returns AND payment deadline
- 31 July 2026 — second payment on account (if applicable)
Miss the 31 January deadline and it’s an automatic £100 fine, even if you owe nothing. After 3 months, daily penalties of £10/day kick in (up to 90 days). After 6 months, they add 5% of the tax due. After 12 months, another 5%. It compounds fast — don’t be late.
Allowable Expenses
This is where self-assessment actually works in your favour. Legitimate business expenses reduce your taxable profit. Common ones include:
- Office costs — stationery, phone, internet, postage, software subscriptions
- Travel — 45p/mile for the first 10,000 business miles, then 25p, or actual fuel costs plus capital allowances on the vehicle
- Working from home — HMRC’s simplified rate of £6/week (no receipts needed), or the actual proportion of household bills
- Professional fees — accountant, professional subscriptions, trade body memberships
- Insurance — professional indemnity, public liability, business contents
- Marketing — website hosting, advertising, business cards
- Training — courses that update existing skills (not training for a completely new career)
Payments on Account — The Nasty Surprise
If your tax bill is over £1,000 and less than 80% was collected through PAYE, HMRC requires payments on account. These are advance payments towards next year’s bill, each equal to half of this year’s liability. Due on 31 January and 31 July.
This catches first-time filers off guard every single time. In your first year, you might owe the full year’s tax plus 50% of next year’s estimated bill. That’s effectively 150% of one year’s tax in a single payment. Set money aside from day one.
Filing Tips
Keep records as you go, not in a panic on 30 January. Use accounting software — FreeAgent, Xero, or QuickBooks — to track income and expenses in real time. Photograph receipts immediately (paper fades and gets lost). Open a separate business bank account to make everything cleaner.
And file early. You can submit from 6 April but still have until 31 January to pay. Filing early gives you certainty on your bill, time to budget, and avoids the January rush when HMRC’s phone lines are basically unreachable.
Estimate Your Bill
Our free self-assessment calculator estimates your income tax, National Insurance (Class 2 and 4), and student loan repayments based on your self-employed profits and other income. Plug in your numbers and see where you stand.