Leader Accountancy | Accountant in Milton Keynes

What it is, who needs to file, how to do it, deadlines, penalties, Payments on Account, Student Loan and Child Benefit Charge — everything in one place

Written by the team at Leader Accountancy, an AAT Licensed Accountancy practice (Licence No. 1008787) based in Milton Keynes, serving clients across the UK. Updated July 2026.

If you’ve ever typed “do I need to file a Self Assessment tax return” into Google at 11pm, you’re not alone. Self Assessment is the system HMRC uses to collect Income Tax from people whose earnings aren’t taxed automatically through PAYE — and it trips up more people than it should, mostly because the rules are scattered across dozens of separate gov.uk pages. This guide puts everything in one place.

⚠️ Important note
This guide is for general information only. Tax rules change with each government Budget, and personal circumstances vary. Speak to a qualified accountant before making decisions — Leader Accountancy can help with a personalised consultation.

1. What is Self Assessment?

Self Assessment is HMRC’s system for collecting Income Tax from income that isn’t automatically taxed at source — self-employment profits, rental income, dividends above the allowance, capital gains, and foreign income, among others. Unlike PAYE, where your employer deducts tax before you’re paid, Self Assessment puts the responsibility on you to calculate, declare, and pay what you owe, usually once a year.

Read the official overview on gov.uk’s Self Assessment tax returns page.

2. Who needs to file a Self Assessment return

You typically need to file if, in the relevant tax year, you:

• Were self-employed as a sole trader and earned more than £1,000 (before deducting expenses)
• Were a partner in a business partnership
• Had untaxed income from renting out property
• Received dividend income above the dividend allowance
• Had savings or investment income above certain thresholds
• Made a capital gain (e.g. selling a second property, shares, or crypto) above the annual exempt amount
• Earned foreign income that needs to be reported in the UK
• Earned over £100,000 (even if fully taxed under PAYE) or need to claim certain reliefs
• Need to pay the High Income Child Benefit Charge (see below)

⚠️ Common mistake
Having tax deducted automatically through PAYE on your main job doesn’t exempt you from filing if you also have side income above £1,000, rental income, or other untaxed sources. Many people assume PAYE “covers everything” — it only covers that one income stream.

3. How to file: step by step

1. Register for Self Assessment with HMRC (if it’s your first year) and receive your Unique Taxpayer Reference (UTR)
2. Set up a Government Gateway account to file online
3. Gather your records: income, expenses, P60/P45, dividend vouchers, rental statements, and any relevant certificates
4. Complete the relevant sections of the return (main return plus supplementary pages for self-employment, property, foreign income, etc.)
5. Calculate the tax due (HMRC’s online system does this automatically once you submit figures)
6. Submit online by the deadline and pay any tax owed

Many people choose to have a qualified accountant handle the whole process — the Leader Accountancy Self Assessment service covers registration through to submission.

4. Key deadlines

Deadline	What happens
5 October	Register for Self Assessment if this is your first year needing to file
31 October	Paper return deadline
31 January	Online return deadline and payment deadline for tax owed
31 July	Second Payment on Account, if applicable

5. Penalties for late filing and late payment

• 1 day late: automatic £100 fixed penalty
• 3 months late: £10 per day (up to 90 days, max £900)
• 6 months late: a further 5% of the tax due or £300, whichever is greater
• 12 months late: another 5% or £300 — rising to 100% in cases of deliberate concealment

Late payment penalties run separately from late filing penalties, plus interest accrues daily on unpaid tax from the due date. See gov.uk’s penalties page for the full schedule.

⚠️ Common mistake
“I didn’t know” is not accepted as a reasonable excuse by HMRC. Penalties apply even if you genuinely weren’t aware of the requirement to file.

6. Payments on Account explained

Payments on Account are advance payments towards your next year’s tax bill, required when your last Self Assessment bill was over £1,000 (and less than 80% of your tax was collected at source). Each payment is normally 50% of your previous year’s tax bill, due on 31 January and 31 July.

💡 Practical example
If your 2024/25 tax bill was £4,000, you’d typically pay: £4,000 (2024/25 balance) + £2,000 (first Payment on Account for 2025/26) on 31 January 2026, then a further £2,000 on 31 July 2026. Your actual 2025/26 bill is reconciled the following January, with a balancing payment or refund.

If you expect your income to drop, you can apply to reduce your Payments on Account — but reducing them too far, if your income doesn’t actually fall, triggers interest on the shortfall.

7. Student Loan repayments through Self Assessment

If you have a student loan and file Self Assessment (because you’re self-employed, or have income not covered by PAYE), repayments are calculated and collected as part of your Self Assessment bill, based on your total income above the relevant repayment threshold for your loan plan (Plan 1, 2, 4, 5 or Postgraduate Loan).

⚠️ Common mistake
People with both PAYE employment and self-employment income sometimes have student loan repayments deducted through payroll and calculated again through Self Assessment on their total income — this is usually correct (it reconciles against total income), but it surprises people expecting to pay only once.

8. The High Income Child Benefit Charge

If you or your partner claim Child Benefit and either of you has adjusted net income above a set threshold, some or all of the benefit is clawed back as a tax charge, reported through Self Assessment. Above a higher threshold, the charge equals 100% of the Child Benefit received — effectively cancelling it out.

Many higher earners don’t realise they’re liable for this charge until HMRC contacts them, sometimes with penalties for late notification. If your income has crossed the relevant threshold and you (or your partner) claim Child Benefit, this is worth checking every tax year, since thresholds are reviewed periodically.

Frequently Asked Questions

Do I need to file Self Assessment if I’m employed and pay tax through PAYE?

Not usually — unless you have additional untaxed income (self-employment, rental, dividends above the allowance, capital gains), earn over £100,000, or are liable for the High Income Child Benefit Charge.

What happens if I miss the 31 January deadline?

You get an automatic £100 penalty the moment you’re one day late, with additional penalties accruing at 3, 6 and 12 months, plus interest on any unpaid tax.

Can I reduce my Payments on Account?

Yes, if you genuinely expect lower income — but if income doesn’t fall as expected, HMRC charges interest on the shortfall between what you paid and what you should have paid.

Do student loan repayments through Self Assessment replace payroll deductions?

No — if you have both PAYE and self-employment income, repayments may be deducted through payroll and also calculated on your total income via Self Assessment, reconciling to the correct total.

Does the Child Benefit Charge apply even if my partner claims Child Benefit, not me?

Yes — the charge is based on whoever in the household has the higher adjusted net income, regardless of who actually claims the benefit.

Self Assessment checklist

• Do you know whether you need to file this tax year?
• Have you registered with HMRC and received your UTR?
• Do you know your filing and payment deadlines (31 January, 31 July)?
• Are your records (income, expenses, dividends, rental statements) organised?
• Have you checked whether Payments on Account apply to you?
• If you or your partner claim Child Benefit, have you checked the income threshold?

Want your Self Assessment handled by a licensed accountant, start to finish?

Leader Accountancy prepares and files Self Assessment returns for individuals and business owners across the UK.Book a consultationFree 15-minute chat